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Phosphorus and Nitrogen Centers in Doped Graphene and Carbon Nanotubes Analyzed through Solid-State NMR: Graphene and carbon nanotubes (CNTs) have been investigated closely in recent years because of their apparent positive effect on the electrochemical performance of new fuel cell and battery systems as catalyst stabilizers, supports, or metal-free catalysts. This is particularly true for doped graphene and CNTs, where only a small amount of doping with nitrogen and/or phosphorus can have a remarkable effect on the material performance. A direct link between structure and function in these materials is, as of yet, unclear. Doped graphene and CNTs were synthesized using varied chemical vapor deposition-based methods, and ssNMR was used to unambiguously identify dopant atom sites, revealing that these particular synthesis methods result in highly homogeneous populations of installed phosphorus and nitrogen atoms. We present the first experimental N-15 spectrum for graphitic nitrogen in N-doped graphene. N-15-labeled nitrogen-doped graphene synthesized as reported here produces mainly graphitic nitrogen sites located on the edges of sheets and around defect sites. H-1-H-1 and H-1-N-15 correlations were also used to probe dopant nitrogen sites in labeled and unlabeled N-doped graphene. A nearly homogeneous population of phosphorus in P-doped graphene is found, with an overwhelming majority of graphitic phosphorus and a small amount of phosphate oligomer. Similar findings are noted for the phosphorus sites in phosphorus and nitrogen codoped CNTs with a minor change in chemical shift, as would be expected from two chemically similar phosphorus sites in carbon allotropes (CNTs vs graphene sheets) with significantly different electronic structures.
Quantifying Site-Specific Proton Dynamics in Phosphate Solid Acids by H-1 Double Quantum NMR Spectroscopy: Solid-state magic angle spinning (MAS) NMR was used to investigate changes in proton dynamics in phosphate solid acids that exhibited increased proton conductivity between room temperature and 110 degrees C. Double quantum dipolar recoupling methods were used to quantify site-specific changes in proton-proton dipolar coupling as a function of temperature. The static dipolar coupling and moqonally induced changes to it were compared. This was accomplished by calculating (from crystal structures) and measuring (from the initial parts of the DQrecoupling curves) the root-sum-square of the dipolar coupling, a geometry-independent measure of dipolar coupling strength referred to as the ``apparent dipolar coupling'', D-app. The analysis of KH2PO4 and RbH2PO4 showed that the experimentally determined apparent dipolar couplings were reduced from the calculated values at increased temperatures in dynamic systems. Higher proton conductivity was associated with greater reduction of the apparent dipolar coupling as measured by dipolar recoupling NMR methods. Most interestingly, in its monoclinic phase, RbH2PO4 has two chemically distinct proton environments, one disordered and one ordered, which are resolved by H-1 MAS NMR. These sites exhibit different dipolar coupling responses as a function of temperature, revealing that proton conduction in this temperature range arises from motions involving only one of the sites. This site-specific dynamics is measured directly for the first time, using a combination of MAS to resolve the H-1 sites and dipolar recoupling experiments to probe the temperature dependence of the H-1-H-1 dipolar interactions.
How to Control the Discharge Products in Na-O-2 Cells: Direct Evidence toward the Role of Functional Groups at the Air Electrode Surface: Sodium-oxygen batteries have received a significant amount of research attention as a low-overpotential alternative to lithium-oxygen. However, the critical factors governing the composition and morphology of the discharge products in Na-O-2 cells are not thoroughly understood. Here we show that oxygen containing functional groups at the air electrode surface have a substantial role in the electrochemical reaction mechanisms in Na-O-2 cells. Our results show that the presence of functional groups at the air-electrode surface conducts the growth mechanism of discharge products toward a surface-mediated mechanism, forming a conformal film of products at the electrode surface. In addition, oxygen reduction reaction at hydrophilic surfaces more likely passes through a peroxide pathway, which results in the formation of peroxide-based discharge products. Moreover, in-line X-ray diffraction combined with solid state Na-23 NMR results indicate the instability of discharge products against carbonaceous electrodes. The findings of this study help to explain the inconsistency among various reports on composition and morphology of the discharge products in Na-O-2 cells and allow the precise control over the discharge products.
Structure Solution of Metal-Oxide Li Battery Cathodes from Simulated Annealing and Lithium NMR Spectroscopy: Discerning the arrangement of transition metal atoms in Li[NixMnyCoz]O-2 cathode materials has remained an open problem for many years despite the commercial importance of some stoichiometries and the even more promising characteristics of others. We present a method for structural determination in this class of cathode materials. A simple definition of the total energy, based on the chemical principle of electroneutrality, is used in combination with a simulated annealing algorithm to generate model structures. The method reproduces the well-known structure of Li-[Li1/3Mn2/3]O-2 and produces structures of the disordered Li[NixMnxCo1-2x]O-2 phases (where x = 0.02, 0.1, 0.33) that are verified by detailed Li-7 NMR spectra. For each Li[NixMnxCo1-2x]O-2 phase, the solution is found to be heavily disordered, yet retaining significant ion pairing. Since the underlying notion of favoring charge-neutral regions is generic, we anticipate its utility in a much broader family of materials.
Detection of Electrochemical Reaction Products from the Sodium-Oxygen Cell with Solid-State Na-23 NMR Spectroscopy: Na-23 MAS NMR spectra of sodium-oxygen (Na-O-2) cathodes reveals a combination of degradation species: newly observed sodium fluoride (NaF) and the expected sodium carbonate (Na2CO3), as well as the desired reaction product sodium peroxide (Na2O2). The initial reaction product, sodium superoxide (NaO2), is not present in a measurable quantity in the Na-23 NMR spectra of the cycled electrodes. The reactivity of solid NaO2 is probed further, and NaF is found to be formed through a reaction between the electrochemically generated NaO2 and the electrode binder, polyvinylidene fluoride (PVDF). The instability of cell components in the presence of desired electrochemical reaction products is clearly problematic and bears further investigation.
Determination of Mass Transfer Parameters and Ionic Association of LiPF6: Organic Carbonates Solutions: We report herein on the determination of several mass transport properties (diffusion coefficients for cations, anions, and neutral ionic aggregates; cation transference number) and degree of salt association in electrolyte solutions of practical interest (LiPF6 solutions in binary organic carbonate mixtures), through an analysis of pulsed field gradient NMR and specific conductivity data. Results were obtained as a function of LiPF6 salt concentration in a binary ethylene carbonate (EC) - dimethyl carbonate (DMC) mixture at 1: 1 volume ratios and 30 degrees C, also from 5 degrees C to 35 degrees C for 1 M LiPF6 dissolved in binary mixtures of EC with DMC and EMC at 3: 7, 1: 1 and 7: 3 volume ratios. Li+ transference numbers have significantly lower values than the transport numbers, ranging from 0.31 to 0.35, and display a correct (decreasing) variation with salt concentration, contrary to the trend observed for transport numbers. A remarkably high degree of ion pairing (from 36\% to 67\%) was observed for the solutions investigated in the present work. (C) 2017 The Electrochemical Society. All rights reserved.
Ex Situ Na-23 Solid-State NMR Reveals the Local Na-Ion Distribution in Carbon-Coated Na2FePO4F during Electrochemical Cycling: The potential Na-ion cathode material Na2FePO4F is investigated here by ex situ Na-23 solid-state nuclear magnetic resonance (ssNMR) in order to characterize the structure and ion mobility as a function of electrochemical cycling. The use of fast magic angle spinning (MAS) speeds of 65 kHz allows for the collection of high-resolution Na-23 NMR spectra that reveal two unique peaks at +450 and -175 ppm, corresponding to the two crystallographically unique Na sites in the material of interest. Two-dimensional NMR exchange spectroscopy results reveal that chemical exchange between the Na ions residing in distinct environments has a maximum hopping rate of similar to 200 Hz. The collection of one-dimensional NMR spectra as a function of electrochemical cycling reveals the reproducible formation of a new peak at +320 ppm in the Na-23 NMR spectrum at all intermediate states of charge. The appearance of this resonance at +320 ppm is attributed to the fully oxidized (NaFePO4F) phase that is present even upon initial electrochemical oxidation. The simultaneous existence of both the pristine and oxidized phases suggest formation of two distinct phases upon charging, consistent with a two-phase desodiation mechanism. This two-phase arrangement of Na ions persists for multiple charge/discharge cycles and is congruent with high reversibility of Na (de)intercalation in Na2FePO4F cathodes. These findings imply that the Na2FePO4F framework is incredibly structurally stable with a robust intercalation process, despite a lack of ideal sodium-ion kinetics.
F-19 Double Quantum NMR Spectroscopy: A Tool for Probing Dynamics in Proton-Conducting Fluorinated Polymer Materials: Solid-state NMR spectroscopy is an important technique for probing the structure and local dynamics of materials at the molecular level. For example, H-1 double quantum (DQ) NMR is a well-established probe of local dynamics. Here, this concept has been extended to characterize fluorinated ionomer materials for the first time. F-19 DQ recoupling NMR experiments are applied to investigate the site-specific local dynamics of the polymer electrolyte material, Nafion 117, under various conditions with respect to temperature and hydration level. The initial rise of the normalized double quantum (nDQ) build-up curves generated from NMR dipolar recoupling experiments is compared as a measure of the motionally averaged F-19-F-19 dipolar couplings for spectroscopically resolved domains of the polymers. Since the side-chain and backbone fluorines can be distinguished by their chemical shifts, it was possible to demonstrate a difference between the side-chain and backbone local dynamics profiles. The side chain is shown to be more sensitive toward the temperature and relative humidity (\%RH) changes, and generally the side chain exhibits greater local dynamics as compared to the hydrophobic backbone, which is consistent with subsegmental motion known as beta-relaxation. Elevated temperature and increased relative humidity give rise to increased local dynamics, which is reflected by the slower initial increase of the nDQ build-up curves. This NMR technique has been validated as a comparative analysis tool, suitable for a range of perfluorinated ionomers.
H-1-H-1 Double Quantum NMR Investigation of Proton Dynamics in Solid Acids: Currently, the most popular proton exchange membrane (PEM) for fuel cell applications is Nafion. However, Nafion does not retain its high conductivity at high temperatures due to its dependence on water for proton transport. Because operational temperatures higher than the evaporation point of water are desirable, a family of solid acids was investigated. Cations known to transport protons were paired with anions to make acidic salts. Solid acids discussed here include imidazole paired with trifluoromethanesulfuric acid as well as imidazole, benzimidazole and adenine paired with methanesulfonic acid. Solid-state NMR was utilized to show the relative mobility of protons through double-quantum filter (DQF) experiments. The POST-C7 homonuclear dipolar-recoupling scheme was paired with DUMBO homonuclear decoupling, to produce H-1 double-quantum coherence buildup curves for the hydrogen-bonded protons of interest. Experimental buildup curves, which reflect both local structure as well as dynamics, are compared to theoretical curves of the static system. The SPINEVOLUTION-simulated curves utilized up to eight pairs of homonuclear dipolar couplings within a sphere of 7 angstrom diameter centered on the proton of interest. Steep buildup of the DQ curve and maxima at short recoupling times in the buildup curves indicate strong dipole-dipole coupling and are interpreted to indicate limited dynamics of the H-bonded protons. In contrast, shallower buildup curves and maxima at longer recoupling times imply that H-bonded protons (in an otherwise similar structure) are associated with local mobility, which reduces their local dipolar coupling and may facilitate proton transport. Bulk proton conductivities, measured via electrochemical impedance spectroscopy were compared to DQF measurements to understand proton conduction within these materials.
Three-dimensional investigation of cycling-induced microstructural changes in lithium-ion battery cathodes using focused ion beam/scanning electron microscopy: For vehicle electrification, one of the biggest issues for lithium ion batteries is cycle life. Within this context, the mechanisms at the source of capacity degradation during cycling are not yet to be fully understood. In this work, we use state-of-the-art FIB-SEM serial sectioning and imaging techniques to determine the effect of cycling on lithium-ion battery cathodes. The three-dimensional (3D) micro structural study was performed on both pristine and cycled LiNixMnyCo1-x-yO2 (NMC) and Li(L-i(0.2)Ni(0.13)Mn(0.54)Co(0.13))O-2 (HE-NMC) cathodes. The spatial distribution of active material, carbon-doped binder and pore spaces were successfully reconstructed by appropriate image processing. Comparisons of NMC and HE-NMC cathodes after different number of cycles showed only minor increases in the number of smaller active particles, possibly negligible, considering the intrinsic microstructure variation within the cathodes. However, the connectivity between carbon-doped binder additives and active particles in NMC and HE-NMC cathodes, assessed using a ``neighbor counting'' method, showed an appreciable decrease after cycling which indicates a detachment of carbon-doped binder from active particles. This significant cycling-induced detachment effect between the two phases (e.g., similar to 22\% for HE-NMC) could indicate a loss in electrical connectivity, which may partially explain the capacity fade in the cells. (C) 2015 Elsevier B.V. All rights reserved.
NMR Determination of the Relative Binding Affinity of Crown Ethers for Manganese Cations in Aprotic Nonaqueous Lithium Electrolyte Solutions: Polymeric chelating agents placed in the interelectrode space of a Li-ion battery (LIB) have been suggested as a means of sequestering Mn cations dissolved from positive electrodes of LIBs to prevent their migration to, and deposition onto, negative electrodes and thus mitigate the associated degradation of LIB performance and life. In order to select the most effective chelating agent and optimize its polymeric form, it is desirable to determine the binding affinity of various chelating agents for manganese cations. The present study evaluates the relative binding affinity of crown ethers for manganese cations in a lithium-containing environment through the detection of the Li-7 nucleus chemical shift. Results are presented for the relative binding affinities of 15-crown-5 and 1-aza-15-crown-5 ethers for Mn2+ and Mn3+. Significant differences in relative binding affinity were discovered with particular crown ether manganese oxidation state combinations. Additionally, a substantial decrease in binding affinity was observed for the polymeric crown ether relative to its molecular form. These results indicate that the NMR titration technique is a useful screening tool, which will inform and assist the development of more effective manganese cations trapping materials. Quantification of the Mn trapping efficiency will permit the screening of trapping groups at the molecular level (before attachment to a polymer), as well optimization of their polymeric forms (type of backbone, morphology, length of linker, amount of cross-linking, etc.). The methodology developed in the present work will therefore accelerate the development of the Mn trapping technology.
An Unexpected Pathway: Li-6-Exchange NMR Spectroscopy Points to Vacancy-Driven Out-of-Plane Li-Ion Hopping in Crystalline Li2SnO3: The development and engineering of new materials for modern electrochemical energy-storage systems requires an in-depth understanding of Li-ion dynamics, not only on the macroscopic length scale but also from an atomic-scale point of view. Hence, the study of suitable model systems is indispensable to understand the complexity of nonmodel systems already applied, for example, as active materials in rechargeable batteries. Here, Li2SnO3 served as such a model system to enlighten the elementary steps of ion hopping between the three magnetically distinct Li sites. Through high-resolution 1D and 2D NMR spectroscopies, we probed the favored exchange pathway. Both 1D and 2D NMR spectroscopies point to nonuniform ion dynamics and two independent exchange processes perpendicular to the ab plane, namely, between the sites 4e [Li(3)] and 8f [Li(1)] and between 4e and 4d [Li(2)]. Li-6 selective inversion NMR spectroscopy confirmed extremely slow Li exchange and yielded hopping rates on the order of 3 s(-1) for 4e-8f and 0.7 s(-1) for 4e-4d. Altogether, the findings provide evidence for a three-site, two-exchange model describing Li hopping along the c axis rather than in the Li-rich ab plane as one would expect at first glance. This unexpected result can, however, be understood when the site preference of Li vacancies is considered. Recent theoretical calculations predicted the preferred formation of Li vacancies at the Li(3) sites. This allows for localized Li-ion exchange involving Li(3), thus, perfectly corroborating the present findings obtained by Li-6 MAS NMR spectroscopy.
A Search for Low-Irreversible Capacity and High-Reversible Capacity Positive Electrode Materials in the Li-Ni-Mn-Co Pseudoquaternary System: A comprehensive search for Li-ion battery positive electrode materials that can simultaneously exhibit low irreversible capacity loss (IRC) (similar to 10\% or less) and high reversible capacity (>240 mAh/g) was performed in the Li-Ni-Mn-Co-O psendoquaternary system. An array of high-capacity Li-rich layered oxides, most of which show an ``oxygen release'' plateau during the first charge, were synthesized with a wide range of Ni, Mn, and Co compositions, and their first-cycle electrochemical properties were investigated. Low-IRC materials could be synthesized at many Ni Mn Co combinations by synthesizing with an amount of lithium lower than that required by site occupation and oxidation state rules. Many of these ``Li-deficient'' low-IRC materials were found to be single-phase layered materials with inherent metal-site vacancies in their pristine state. For such single-phase materials, the amount of IRC depends on the concentration of metal-site vacancies in their pristine state. Increasing the Li deficiency eventually caused the appearance of the spinel phase, which, when it appears, lowers the IRC, irrespective of the Ni Mn Co precursor composition. The number of metal-site vacancies that can be incorporated into the single-phase layered materials depends on the overall metal composition, especially the Co concentration. Low-IRC behavior is correlated to the fraction of metal-site vacancies in the layered phase in both the single-phase and the two-phase materials. Li-7 nuclear magnetic resonance (NMR) studies of low-IRC materials revealed the relative population of Li between the Li and TM layer. Formula unit calculation based on Li-7 NMR results suggests that metal-site vacancies preferably occupy the sites in the Li layer, which could provide room for the intercalation of extra Li into the structure, hence reducing the irreversible capacity.
Structure and Dynamics in Functionalized Graphene Oxides through Solid-State NMR: Graphene oxide (GO), a derivative of the supermaterial graphene, has intrinsic proton conductivity, which is similar to Nafion, the most popular proton exchange membrane material currently used in fuel cells. Research into acid-functionalized GOs and determining the role of acidic groups in increasing proton conductivity will help to improve polymer electrolyte membrane performance in fuel cell systems. Multinudear solid-state NMR (ssNMR) spectroscopy was used to analyze the structure and dynamics of GO and a number of sulfonic acid derivatives of GO, both novel and previously reported. C-13 CP-MAS spectra showed the disappearance of surface-based oxygen groups upon GO functionalization and can be used to identify linker group carbon sites in previously synthesized and novel functionalized GO samples with high specificity. Dehydration of these samples allows the collection of spectra with resolved acidic proton and water peaks. The effect of dehydration on the proton spectrum is partially reversible through rehydration. Deuteration of the acidic groups in high temperature and acidic conditions was virtually unsuccessful, indicating that only the surface and not the intercalated functional groups play a role in the enhanced proton conductivity of ionomer/functionalized GO composites. Increased surface area and increased delamination of functionalized GO are suggested to be important to improved proton exchange membrane fuel cell performance. This synthesis and method of analysis prove the utility of ssNMR in the study of structure and dynamics in industrially relevant amorphous carbon materials despite the obvious difficulties caused by naturally broad signals and low sensitivity.
Lithium Polyacrylate (LiPAA) as an Advanced Binder and a Passivating Agent for High-Voltage Li-Ion Batteries: Intensive studies of an advanced energy material are reported and lithium polyacrylate (LiPAA) is proven to be a surprisingly unique, multifunctional binder for high-voltage Li-ion batteries. The absence of effective passivation at the interface of high-voltage cathodes in Li-ion batteries may negatively affect their electrochemical performance, due to detrimental phenomena such as electrolyte solution oxidation and dissolution of transition metal cations. A strategy is introduced to build a stable cathode-electrolyte solution interphase for LiNi0.5Mn1.5O4 (LNMO) spinel high-voltage cathodes during the electrode fabrication process by simply using LiPAA as the cathode binder. LiPAA is a superb binder due to unique adhesion, cohesion, and wetting properties. It forms a uniform thin passivating film on LNMO and conducting carbon particles in composite cathodes and also compensates Li-ion loss in full Li-ion batteries by acting as an extra Li source. It is shown that these positive roles of LiPAA lead to a significant improvement in the electrochemical performance (e.g., cycle life, cell impedance, and rate capability) of LNMO/graphite battery prototypes, compared with that obtained using traditional polyvinylidene fluoride (PVdF) binder for LNMO cathodes. In addition, replacing PVdF with LiPAA binder for LNMO cathodes offers better adhesion, lower cost, and clear environmental advantages.
Evaluation of the Stability of Trimethyl Phosphate as a Li-O-2 Battery Electrolyte via Multinuclear Solid-State NMR: Solid-state O-17 NMR was used to compare the stability of two potential Li-O-2 electrolytes tetraethylene glycol dimethyl ether (TEGDME) and trimethyl phosphate (TMP). The TEGDME electrolyte demonstrated superior stability to the TMP electrolyte. Li2O2 and evidence of electrolyte breakdown was observed in the TEGDME cell, whereas only electrolyte breakdown products were discovered within the TMP cell. Potential decomposition pathways of TMP are proposed here that account for the formation of the discharge species observed in the O-17, Li-7, H-1, and P-31 solid-state NMR of the cycled cathodes.
Accurate Characterization of Ion Transport Properties in Binary Symmetric Electrolytes Using In Situ NMR Imaging and Inverse Modeling: We used NMR imaging (MRI) combined with data analysis based on inverse modeling of the mass transport problem to determine ionic diffusion coefficients and transference numbers in electrolyte solutions of interest for Li-ion batteries. Sensitivity analyses have shown that accurate estimates of these parameters (as a function of concentration) are critical to the reliability of the predictions provided by models of porous electrodes. The inverse modeling (IM) solution was generated with an extension of the Planck-Nernst model for the transport of ionic species in electrolyte solutions. Concentration-dependent diffusion coefficients and transference numbers were derived using concentration profiles obtained from in situ F-19 MRI measurements. Material properties were reconstructed under minimal assumptions using methods of variational optimization to minimize the least-squares deviation between experimental and simulated concentration values with uncertainty of the reconstructions quantified using a Monte Carlo analysis. The diffusion coefficients obtained by pulsed field gradient NMR (PFG-NMR) fall within the 95\% confidence bounds for the diffusion coefficient values obtained by the MRI+IM method. The MRI +IM method also yields the concentration dependence of the Li+ transference number in agreement with trends obtained by electrochemical methods for similar systems and with predictions of theoretical models for concentrated electrolyte solutions, in marked contrast to the salt concentration dependence of transport numbers determined from PFG-NMR data.
Identification of electrochemical reaction products in lithium-oxygen cells with Li-7 nutation spectroscopy: In the Li-O-2 battery system, it is has been shown to be challenging to differentiate the discharge products or determine the electrolyte stability with direct Li-7 NMR. Defined Li-7 quadrupole lineshapes are not observed for cycled cathodes. Here, Li-7 nutation NMR is demonstrated to be an effective method for the identification of Li2O2 in cycled cathodes. The Li-7 quadrupole interaction of Li2O2 (35 kHz) and Li2CO3 (120 kHz) are of similar magnitude to typically radiofrequency fields (ranging from 40 to 60 kHz). The Li-7 nutation frequency will therefore be influenced by both interactions. The discharge products of the cycled cathodes were determined by comparing the Li-7 nutation frequencies of the cycled cathodes to the Li-7 nutation frequency of the pristine materials when the applied radiofrequency field was 30 kHz. Li2CO3 was determined to be the main discharge product in the propylene carbonate/dimethyl carbonate and trimethyl phosphate electrolyte systems, since the Li-7 nutation frequencies of the cathodes corresponded to the Li-7 nutation frequency of pristine Li2CO3. The Li-7 nutation frequency of the tetraethylene glycol dimethyl ether cathode was between the Li-7 nutation frequencies of both pristine Li2O2 and pristine Li2CO3, indicating that both Li2O2 and Li2CO3 were discharge products influencing the observed nutation frequency. From Li-7 nutation NMR the novel trimethyl phosphate electrolyte was determined to be an unsuitable Li-O-2 electrolyte, as the fast Li-7 nutation frequency indicated that Li2O2 was not a primary discharge species. With O-17 NMR, Li2CO3 was confirmed to be a main discharge product formed with the trimethyl phosphate electrolyte.
Correlation of Electrochemical Performance with Lithium Environments and Cation Dynamics in Li-2(Mn1-yFey)P2O7 using Li-6 Solid-State NMR: Li-6 solid-state nuclear magnetic resonance (ssNMR) is used here to evaluate a series of Li2Mn1-yFeyP2O7 cathode materials in an effort to quantify ion exchange rates and diffusion pathways. Magic angle spinning (MAS) NMR of the series of mixed metal pyrophosphates reveals a trade-off between electrochemical performance and well-resolved NMR spectra resulting from the change in electronic structure of the transition metal redox center. In addition, 1D Li-6 selective inversion NMR is employed to characterize Li ion dynamics in the fully Mn substituted member of the pyrophosphate series, where three of the four unique Li resonances are well resolved and labeled AB, C, and D, with AB corresponding to Li ions within one tunnel, and C and D Li ions residing in another. Despite limited inversion efficiency it is found that the utility of this experiment is not compromised so long as the initial magnetization conditions are well-defined. Initial fitting procedures involved the inclusion of all possible exchange pairs, a process which gave rise to consistently negative rate constants for C-AB or D-AB exchange, suggesting negligible exchange between these Li ions. Upon limiting the exchange model to ion exchange processes between the pairs of high and low frequency sites, rate constants of 45 +/- 25 and 100 +/- 30 Hz were obtained for C-D exchange at room temperature and 350 K respectively. Ion exchange pathways that are revealed by the exchange experiments imply limited mobility across distinct two-dimensional tunnels and slow exchange for within-tunnel ions. These exchange results provide corroboration for the geometrically determined site assignment in the 1-D spectrum, as well as support the notion of limited ion mobility in the Mn-phase resulting in poor electrochemical capability.
Direct Measurement of Surface Termination Groups and Their Connectivity in the 2D MXene V2CTx Using NMR Spectroscopy: The MXenes are a class of 2D materials composed of transition-metal sheets alternating with carbide/nitride sheets, stacked just a few atoms thick. MXenes discovered thus far also have a surface termination layer that is likely a mixture of hydroxides and fluorides. While reasonable structural models based on X-ray diffraction and transmission electron microscopy data exist, the exact nature and distribution of the surface termination species are not well understood. Here, H-1, F-19, and C-13 solid-state NMR spectroscopies are used to investigate the model MXene V2CTx, where T signifies the surface termination groups. H-1 NMR experiments provide direct proof of hydroxide moieties in the surface layer by measuring interactions with the MXene surface. Furthermore, H-1 NMR spectroscopy shows a significant amount of water hydrogen bonded to the surface hydroxide layer. F-19 NMR experiments show fluoride moieties bonded to the MXene surface, with extremely unusual F-19 spectra caused by strong interactions with the metallic/semiconducting MXene. C-13 NMR observes the sample from the center of the MXene layer and shows that the C-13 chemical shift is extremely sensitive to the MAX -> MXene transformation. Nuclear-spin magnetization transferred from H-1 nuclei in the hydroxide surface termination layer to C-13 nuclei in the center of the MXene sheet yields further evidence of this connectivity. The multinuclear NMR experiments provide direct experimental verification of the structural models and depict the MXene V2CTx as infinite sheets of small-bandgap V2C sheets terminated by a mixed hydroxide/fluoride layer embedded in a matrix of strongly hydrogen-bonded water molecules.
Electrochemical Changes in Lithium-Battery Electrodes Studied Using Li-7 NMR and Enhanced C-13 NMR of Graphene and Graphitic Carbons: An anode composed of tin-core, graphitic-carbon-shell nanopartides distributed on graphene nanosheets, Sn@C-GNs, is studied during the lithiation process. Li-7 NMR provides an accurate measure of the stepwise reduction of metallic Sn to lithium tin alloys and reduction of the graphitic carbon. The metallic nanopartide cores are observed to form ordered, crystalline phases at each step of the lithiation process. The Li-7 2D experiments presented provide insight into the proximity of the various phases, reflecting the mechanism of the electrochemical reaction. In particular, a sequential model of nanopartide lithiation, rather than a simultaneous process, is suggested. Movement of lithium ions between two elements of the nanostructurecl Sn@C-GNs material, the metallic core and carbon shell, is also Observed. Conventional C-13 solid-state NMR, SSNMR, experiments on <5 mg of active material from electrochemical cells were found to be impossible, but signal enhancements (up to 18-fold) via the use of extended echo trains in conjunction with magic-angle spinning enabled NMR characterization of the carbon. We demonstrate that the C-13 data is extremely sensitive to the added electron density when the graphitic carbon is reduced. We also investigate ex situ carbon electrodes from cycled Li-O-2 cells, where we find no evidence of charge sharing between the electrochemically active species and the graphitic carbon in the C-13 NMR spectroscopy.
Site Occupation of Ga and Al in Stabilized Cubic Li7-3(x plus y)GaxAlyLa3Zr2O12 Garnets As Deduced from (27)AI and Ga-71 MAS NMR at Ultrahigh Magnetic Fields: Li-containing garnets, which are stabilized in their cubic modification by doping with Al or Ga, show very high Li-ion conductivities. This property qualifies them to be used as solid electrolytes in advanced all-solid-state batteries. The relation between local structures and dynamic properties, however, is still not fully understood. Here, cubic mixed-doped Li7-3(x+y)GaxAlyLa3Zr2O12 garnet solid solutions with different portions of Al and Ga were synthesized. It turned out that the solubility of Ga is higher than that of Al; the evaluation of 42 different doping compositions indicated an increase of the lattice parameter a(o) with increasing Ga content. Ga-71 MAS NMR spectra recorded at 21.1 T revealed two Ga-71 NMR resonances, corresponding to Ga occupying both the 24d (243 ppm) and 96h sites (193 ppm). This behavior, which has been observed for the first time in this study, is very similar to that of Al. The Ga-71 NMR line at 193 ppm observed here remained invisible in previous NMR studies that were carried out at lower magnetic fields. The invisibility at lower field is because of large second-order quadrupolar broadening that has a lower effect on the Ga-71 NMR spectra at higher magnetic field. Most importantly, the similarity in site preference of Al and Ga found here inevitably raises a question about the significance of a blocking effect on long-range Li-ion transport. It weakens the assumption that the site preference of dopants is responsible for the higher Li diffusivity of Ga-doped samples compared to the Al-doped analogues. Concerning Li-ion dynamics, our 7Li NMR line shape measurements indicate that the change in lattice constant ao with increasing doping level seems to have a larger influence on Li-ion dynamics than the Al.:Ga ratio.
H-1 Solid-State NMR Study of Nanothin Nafion Films: The unique behaviors of Nafion nanothin films with thicknesses of 10 nm (ultrathin) and 160 nm (thin) were evaluated using variable-temperature and variable-humidity solid-state H-1 NMR spectroscopy. These unprecedented measurements of nanothin films stacked within an NMR rotor represent a remarkable experimental achievement and demonstrate that H-1 NMR spectroscopy of such minute amounts of ionomer might be possible within active catalyst layers in polymer electrolyte fuel-cell electrodes. This study was motivated by the observation, in a separate work, of thickness-dependent and highly suppressed conductivity in nanothin films of Nafion (4-300 nm) compared to counterpart free-standing Nafion membranes. Trends in the line width and, more precisely, the T2 relaxation, as probed using a Hahn echo, showed that the local mobility within the hydrogen-bonded domain is equivalent for 10 and 160 nm films and is governed by the fast exchange limit in terms of NMR time scales. Subtle differences in the chemical shift trends provide insight into the domain structures, where the 10 nm films show no changes whereas the thicker 160 nm films exhibit chemical shift trends that indicate a rearranging hydrogen-bonded network. Thus, it is inferred that domain structure formation is influenced by film thickness and that the interaction with the substrate becomes limiting as the film becomes thinner.
Manganese sequestration and improved high-temperature cycling of Li-ion batteries by polymeric aza-15-crown-5: Mn cation trapping by polymeric aza-15-crown-5 ethers is an effective means for mitigating the consequences of Mn dissolution in Li-ion batteries. Mn cations trapping was investigated in lithium manganese oxide (LMO) spinel-graphite (GR) cells containing 1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) 1:2 v/v. A commercial polyolefin separator membrane coated with poly[divinylbenzene(vinylbenzyl-aza-15-crown-5)-vinylbenzylchloride)] effected a 39\% reduction in capacity loss rate during cycling at 50 degrees C with 100\% depth of discharge (DOD) at C/5 rate. Simultaneously, a 50-60\% reduction in the Mn deposited at the negative electrode, and a 6x to 10x increase in the Mn on the coated separator were observed for cells with coated separators, over baseline cells with plain separators. X-ray absorption near-edge spectroscopy (XANES) yielded average oxidation states near +3 for Mn cations in graphite electrodes and separators from cycled cells, suggesting that Mn metal or in oxidation state +2 can only be minor fractions of the Mn existing outside the positive electrode. We discuss the implications of these results for choosing an optimal chelating agent. We also show that the cation chelating polymer reported here is compatible with existing manufacturing processes for Li-ion battery separators. (C) 2014 Elsevier B.V. All rights reserved.
Proton dynamics in sulfonated ionic salt composites: Alternative membrane materials for proton exchange membrane fuel cells: Hydrated Nafion, the most prevalent proton exchange membrane utilizes a vehicular mechanism for proton conduction. However, there is an increasing need for such membranes to perform under anhydrous conditions, at high temperatures, which would employ a structural transport mechanism for proton conductivity. Here, several solid-acids are characterized, both as pristine salts, and as polymer composites. Materials of interest include benzimidazolium methanesulfonate (BMSA), imidazolium methanesulfonate (IMSA), and imidazolium trifluoromethanesulfate (IFMS). The proton dynamics of these solid acids are characterized as pure salts, and as composites, embedded into porous Teflon, by solid state NMR. It was determined that spin lattice (T-1) relaxation of the composites are systematically lower than that of the pure salt, indicating that local dynamics are enhanced in the composites. Spin spin relaxation (T*(2)) was measured as a function of temperature to determine the activation energy for local mobility for each salt and composite. The activation energy for local proton mobility in each salt decreased after being inserted into porous Teflon. Finally, the long-range ion transport was characterized using impedance spectroscopy. The IFMS Teflon composite possessed the lowest activation energy for local proton mobility, the highest thermal stability, and the most favorable proton conductivity, among the investigated materials. (C) 2014 Elsevier B.V. All rights reserved.
Environmental In Situ X-ray Absorption Spectroscopy Evaluation of Electrode Materials for Rechargeable Lithium-Oxygen Batteries: Lithium-oxygen batteries have attracted much recent attention due their high theoretical capacities, which exceeds that of Li-ion batteries. Among all the metal oxides that have been investigated in oxygen cathodes, alpha-MnO2 materials have shown unique electrochemical properties in rechargeable lithium oxygen batteries. Although extensive research has been performed to investigate the structure of alpha-MnO2 upon lithium intercalation, its behavior upon reacting with lithium under an oxygen environment remains to be fully explored. Here, we performed a systematic study on the behavior of two forms of alpha-MnO2 nanowires (i.e., potassium and ammonia versions) together with bulk alpha-MnO2 in oxygen cathodes through environmental in situ X-ray absorption spectroscopy. The results show that the alpha-MnO2 materials undergo lithium insertion/removal and lithium peroxide formation/decomposition simultaneously. The former causes a self-switching of the oxidation state of Mn during cycling. Additionally, we found that potassium-containing alpha-MnO2 nanowires exhibit a suppression of Mn reduction until late in cell discharge under oxygen, retaining a higher degree of Mn4+. character for enhanced oxygen reduction activity than other, similar alpha-MnO2 materials. During cell recharge along with oxygen evolution, the materials were found to return to their initial states at low overpotential.
Imidazolium Trifluoromethanesulfonate sPEEK Composites for Anhydrous High Temperature Proton Exchange Membrane Fuel Cells: Imidazolium trifluoromethanesulfonate (IFMS) was inserted into a sPEEK host, and a systematic evaluation of the pristine sPEEK, and reference composites was performed. With electrochemical impedance spectroscopy (EIS) data from the series of polymer, solid-acid, and composites, it was shown that both the anion and cation are involved in proton conduction as well as the sulfonate group on the polymer. EIS also depicted the maximum salt content for highest proton conductivity, collected in anhydrous conditions at 120 degrees C, of 1.1x10(-)3 S/cm with the least amount of salt, which was 37\%. These dynamics were observed in H-1 solid state NMR.
Manganese Sequestration and Li-Ion Batteries Durability Enhancement by Polymeric 18-Crown-6 Ethers: We propose trapping of Mn cations by polymeric crown ethers as a mitigation measure for the consequences of Mn dissolution in Li-ion batteries (LIBs). Mn cations trapping by poly(vinylbenzo-18-crown-6) and poly(undecylenyloxymethyl-18-crown-6) was investigated for 1M LiPF6 solutions in binary carbonates containing Mn(11) salts and in lithium manganese oxide (LMO) spinel - graphite (GR) cells. Trapping site occupancies by Mn+2 exceeding 90\% were measured in bench top experiments. Polyethylene separators coated with poly(vinylbenzo-18-crown-6) trapped Mn cations in LMO - OR cells and decreased capacity fade during 100 cycles at high temperature (60 degrees C) and C/4 rate, retaining 26\% more capacity than the baseline cells. We also address the important distinction between using free (molecular) vs. tethered (polymeric) macrocycles, and its consequences for LIB performance. (C) 2014 The Electrochemical Society. All rights reserved.
The use of Li-6\Li-7\-REDOR NMR spectroscopy to compare the ionic conductivities of solid-state lithium ion electrolytes: Garnet-like solid-state electrolyte materials for lithium ion batteries are promising replacements for the currently-used liquid electrolytes. This work compares the temperature dependent Li+ ion hopping rate in Li6BaLa2M2O12 (M = Ta, Nb) using solid-state Li-6\Li-7\-REDOR NMR. The slope of the Li-6\Li-7\-REDOR curve is highly temperature dependent in these two phases, and a comparison of the changes of the REDOR slopes as a function of temperature has been used to evaluate the Li+ ion dynamics. Our results indicate that the Nb phase has a higher overall ionic conductivity in the range of 247 K to 350 K, as well as a higher activation energy for lithium ion hopping than the Ta counterpart. For appropriate relative timescales of the dipolar couplings and ion transport processes, this is shown to be a facile method to compare ion dynamics among similar structures.
An Improved Understanding of Li+ Hopping Pathways and Rates in Li3Fe2(PO4)(3) Using Selective Inversion Li-6 NMR Spectroscopy: Li-6 selective inversion NMR experiments are used to reveal Li ion exchange rates and energy barriers for Li ion hopping in monoclinic Li3Fe2(PO4)(3). The three crystallographically unique Li sites in this material are well resolved by magic-angle spinning, thus allowing for the examination of all three exchange processes. We have revisited this material using selective inversion to probe dynamics, and energy barriers over the temperature range 268-397 K are found to be 0.37 +/- 0.07, 0.53 +/- 0.02, and 0.52 +/- 0.03 eV for the three unique exchange pairs. The results presented here are consistent with the known Li3Fe2(PO4)(3) crystal structure. The selective inversion experiment is more robust than 2D EXSY for the determination of energy barriers by NMR; this can be attributed to the efficiency of the one-dimensional technique, and an exchange model that accounts for multisite exchange and fast spin-lattice relaxation. Moreover, bond valence sum density maps provide a meaningful depiction of lithium ion diffusion pathways in this material that complement the NMR results.
Slice-Selective NMR Diffusion Measurements: A Robust and Reliable Tool for In Situ Characterization of Ion-Transport Properties in Lithium-Ion Battery Electrolytes: The main impediments to the widespread acceptance of electric drive vehicles are the cost, energy-storage capacity, and durability of portable electrical energy sources and, in particular, batteries. In situ experimental techniques that can accurately detect and monitor performance degradation mechanisms on the nanoscale, including the identities of short-lived chemical species and changes in materials properties as a function of cycling rate, temperature, or time, are not widely used. Herein we demonstrate the combination of in situ ID imaging and slice-selective NMR diffusion measurements as a tool for the spatially and temporally resolved determination of lithium diffusivities in a conventional liquid electrolyte (1.0 M lithium bis(trifluoromethanesulfonyl)imide solution in propylene carbonate) under application of a constant electrical current. All experiments were carried out using standard NMR equipment, so the proposed technique can be easily implemented in any modern R&D facility.
Impact of Lithium Bis(oxalate)borate Electrolyte Additive on the Performance of High-Voltage Spinel/Graphite Li-Ion Batteries: The impact of lithium bis(oxalate)borate (LiBOB) electrolyte additive on the performance of full lithium-ion cells pairing the high-voltage spinel cathode with the graphite anode was systematically investigated. Adding 1 wt \% LiBOB to the electrolyte significantly improved the cycle life and Coulombic efficiency of the full-cells at 30 and 45 degrees C. As the LiBOB was preferentially oxidized and reduced compared with LiBOB-free electrolyte during cycling, their relative contributions to the improved capacity retention in full-cells was gauged by pairing fresh and LiBOB-treated electrodes with various combinations. The results indicated that a solid-electrolyte interphase (SEI) film on graphite produced by the reduction of the LiBOB additive is more robust and stable against Mn dissolution problem during cycling at 45 degrees C compared with the SEI formed by the reduction of the base (LiBOB-free) electrolyte. In addition, a 3 wt \% LiBOB-added electrolyte showed reduced Mn dissolution compared with the base electrolyte after storing the fully charged Li1-xNi0.42Fe0.08Mn1.5O4 (LNFMO) electrodes at 60 degrees C for one month. It is believed that LiBOB aids in stabilizing the electrolyte by trapping the PF5, i.e., sequestering the radical which tends to oxidize EC and DEC electrolyte solvents. Thus, oxidation is suppressed on the carbon black particles in the positive electrode, as evidenced by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) analyses. As a result, HF generation is suppressed, which in turn results in less Mn dissolution from the spinel cathode.
Dynamics of Ag+ Ions in RbAg4I5 Probed Indirectly via Rb-87 Solid-State NMR: Solid-state Rb-87 NMR has been used to determine the ionic hopping rate of Ag+ ions in a powdered sample of alpha-RbAg4I5 as a function of temperature from 20 to 250 degrees C. In this phase, Rb is a stationary framework atom, which does not take part in ionic conduction in this material. At the same time Rb-87 has a large quadrupole moment, making Rb-87 NMR capable of detecting mobile species in close proximity. Simulation of the static Rb-87 NMR powder pattern under the influence of Ag+ motion was performed using the EXPRESS software. Lineshape simulations were used to extract an ionic hopping rate for each temperature, and changes in the line shape were correlated to changes in ion mobility, which increases with temperature. Ag+ hopping rates were found to be in the range of 7.0 +/- 0.5 kHz to 30 +/- 2 kHz within the temperature window of 20 to 100 degrees C, resulting in an activation energy of 17 +/- 3 kJ/mol (0.18 +/- 0.03 eV), for silver ion hopping. This result is in agreement with previous studies, suggesting that this indirect method of detecting ion mobility can be extended to other ion conducting materials.
Differentiating Lithium Ion Hopping Rates in Vanadium Phosphate versus Vanadium Fluorophosphate Structures Using 1D Li-6 Selective Inversion NMR: The electrochemical performance of lithium ion batteries is strongly correlated with the ion dynamics within the electrode structures. This study characterizes Li ion hopping rates and energy barriers in the layered phase, Li5V(PO4)(2)F-2, using Li-6 selective inversion (SI) NMR measurements. Li5V(PO4)(2)F-2 has six crystallographically distinct lithium sites giving the possibility of fifteen exchange partners between nonequivalent lithium environments. Here, Li-6 1D SI measurements over a variable temperature range were used to quantify the time scales and energy barriers of ion mobility for several ion pairs observed to participate in ion hopping The rates determined in this material are similar in range to the previously determined rates found in tavorite Li2VPO4F yet considerably slower than results from both alpha-Li3V2(PO4)(3) and alpha-Li3Fe2(PO4)(3). A detailed analysis of the structural features that enhance or inhibit fast ion mobility is discussed. This includes a consideration of the bond valence density maps of the diffusion pathway. Comparison of the ion mobilities in the phosphates and fluorophosphates shows how the gains in redox potential come at the expense of fast ion mobility, meaning that any improvements to the energy output of the lithium ion battery through higher voltage may be compromised due to slow charge/discharge rates.
Dynamics of benzimidazole ethylphosphonate: a solid-state NMR study of anhydrous composite proton-conducting electrolytes: Imidazole phosphate and phosphonate solid acids model the hydrogen-bonding networks and dynamics of the anhydrous electrolyte candidate for proton exchange membrane fuel cells. Solid-state NMR reveals that phosphate and phosphonate anion dynamics dominate the rate of long-range proton transport, and that the presence of a membrane host facilitates proton mobility, as evidenced by a decreased correlation time of the composites (80 +/- 15 ms) relative to the pristine salt (101 +/- 5 ms). Benzimidazole ethylphosphonate (Bi-ePA) is chosen as a model salt to investigate the membrane system. The hydrogen-bonding structure of Bi-ePA is established using X-ray diffraction coupled with solid-state H-1-H-1 DQC NMR. The anion dynamics has been determined using solid-state P-31 CODEX NMR. By comparing the dynamics of ethylphosphonate groups in pristine salt and membrane-salt composites, it is clear that the rotation process involves three-site exchange. Through data interpretation, a stretched exponential function is introduced with the stretching exponent, beta, ranging 0 < beta <= 1. The P-31 CODEX data for pristine salt are fitted with single exponential decay where beta = 1; however, the data for the membrane-salt composites are fitted with stretched exponential functions, where b has a constant value of 0.5. This beta value suggests a non-Gaussian distribution of the dynamic systems in the composite sample, which is introduced by the membrane host.
Special Issue: Solid-State NMR in Materials for Energy Storage and Conversion
Studies of lithium ion dynamics in paramagnetic cathode materials using Li-6 1D selective inversion methods: The effectiveness of two different selective inversion methods is investigated to determine timescales of Li ion mobility in paramagnetic Li intercalation materials. The first method is 1D exchange spectroscopy, which employs a 90 degrees-tau(1)-90 degrees sequence for selective inversion of a Li resonance undergoing site exchange. The experiment is most easily applied when the first delay period, tau(1), is set to the frequency difference between two resonances undergoing ion exchange. This enables the determination of ion hopping timescales for single exchange pair systems only. To measure ion dynamics in systems having more than one exchange process, a second selective inversion method was tested on two paramagnetic Li intercalation materials. This second technique, replaces the 90 degrees-tau(1)-90 degrees portion of 1D EXSY with a long, selective shaped pulse (SP). Two paramagnetic solid-state materials, which are both cathode materials for lithion ion batteries, were chosen as model compounds to test the effectiveness of both the selective inversion methods. The first compound, Li2VPO4F, was chosen as it hosts two Li sites with 1-exchange process. The second model compound is a 3-site, 3-exchange process system, Li2VOPO4. For the 2-site material, Li2VPO4F, the timescales of the single A-B exchange process were found to be within error of one another regardless of the inversion method. For the 3 Li-site material Li2VOPO4, the three exchange processes, AB, BC, and AC, were found to be on the millisecond timescale as revealed using the SP method. These timescales were determined over a variable temperature range where activation energies extended from 0.6 +/- 0.1 eV up to 0.9 +/- 0.2 eV. (C) 2012 Elsevier Inc. All rights reserved.
Direct Detection of Discharge Products in Lithium-Oxygen Batteries by Solid-State NMR Spectroscopy
Structure and Electrochemistry of Two-Electron Redox Couples in Lithium Metal Fluorophosphates Based on the Tavorite Structure: An electrochemical and structural study of the two-electron redox couple comprising the tavorite-type series of fluorophosphates Li(1 +/- x)VPO(4)F (x = 0, 1) shows that both intercalation of LiVPO(4)F with Li (to give Li(2)VPO(4)P) and deintercalation (to give VPO(4)F) proceed by a two-phase mechanism. Structural models for each of the three phases were determined by Rietveld refinements of combined neutron and X-ray diffraction data of the isolated pure phase materials. LiVPO4F crystallizes in the triclinic space group P (1) over bar and is isostructural to many known fluorophosphates whereas both Li(2)VPO(4)F and VPO(4)F crystallize in the monoclinic space group C2/c, although they have very closely related structures to the parent. Solid-state (6,7)Li NMR studies of Li(2)VPO(4)F reveal the two lithium sites are clearly distinguishable, with more than 100 ppm separation between the resonances. 2D exchange NMR is used to demonstrate the time scale of ion dynamics between the two sites.
Li-6 1D EXSY NMR Spectroscopy: A New Tool for Studying Lithium Dynamics in Paramagnetic Materials Applied to Monoclinic Li2VPO4F: Li-6 selective inversion NMR studies are used to probe details of Li mobility in Li2VPO4F. Two crystallographically unique Li sites were resolved under magic-angle spinning (25-40 kHz) with paramagnetic shifts arising at 46 and -47 ppm (330 K). The rate of exchange between these sites was evaluated using selective inversion (or one-dimensional exchange (1D EXSY)) NMR. This methodology relies on relaxation-based experiments that provide a means for mobility time scales to be determined for materials in which Li+ ions exchange slowly relative to their T-1 spin-lattice relaxation. This situation is particularly relevant to cathode materials for lithium ion batteries, where the unpaired electrons of the transition-metal centers provide a dominant mechanism for rapid relaxation. In Li2VPO4F, Li1-Li2 exchange pair jump rates extend from 24 (+/- 1) to 55 (+/- 4) Hz over a temperature range of 330-350 K The activation energy for this ion exchange process was measured to be 0.44 (+/- 0.06) eV.
Structural analysis of lanthanum-containing battery materials using La-139 solid-state NMR: La-139 solid-state NMR spectra, acquired at 21.1 and 11.7 T, have been used to evaluate the structural properties of the lithium ion battery materials, La32Li16Fe6.4O67 and Li3xLa2/3-xTiO3. In particular, atomic-level disorder in the second coordination sphere environment of lanthanum in these materials has been indicated by the observation of a distribution in the asymmetry parameters and the quadrupolar coupling constants derived from experimental NMR spectra, and supported by theoretical calculations. For comparison, La-139 NMR has been obtained for the two model compounds La2O3 and LaNbO4, in which there is no atomic-level disorder. Quadrupolar coupling constants in the range of 17 to 59 MHz have been measured, and these values are supported by previous work as well as theoretical predictions performed in CASTEP. It has been shown that La-139 NMR is a useful tool for the structural analysis of lithium ion battery materials, and when combined with Li-7 MAS NMR and powder X-ray diffraction, can be used to determine the structure of complex solid-state electrolyte and electrode materials.
Exact calculation of the response of a quadrupolar nucleus to radio frequency irradiation: The effect of a pulse in NMR is usually considered as a rotation of the frame of reference of the spin system. For spins-1/2, this concept is an important and very useful tool. The assumption behind this concept is that while the radio frequency irradiation is on, this term dominates all other interactions. Although this is usually true for spins-1/2, typical interactions for a quadrupolar nucleus can be very large and the assumption is no longer valid. The full solution is complex, but two extreme cases are already solved. If the quadrupole interaction is very small, then the assumption is valid and the pulse does act like a rotation of the frame of reference. At the other extreme, if the interaction is large and the spin, I, is half-integral, then the central transition remains relatively narrow and can be treated as a fictitious spin-1/2. The pulse then acts as a rotation, but with a scaling factor of I + 1/2. This paper treats the general case, where no approximations are made. The effects can be observed in a nutation experiment, in which the observed signal is plotted as a function of pulse width, in a simple one-pulse experiment. If the pulse acts as a rotation, then the nutation plot will be a sine wave, but otherwise it will be a sum of sinusoids. This is true even for a single-crystal sample with a single quadrupolar coupling. If the sample is a powder, then the nutation plot will be a sum of many sinusoids, since the quadrupole coupling will vary with the powder average. This paper sketches out the theory of these effects based on a full and exact description of a quadrupolar system and illustrates it with some nutation spectra of Na-23 in a powdered sample of sodium nitrate.
Structural Complexity and Electrical Properties of the Garnet-Type Structure LaLi0.5Fe0.2O2.09 Studied by Li-7 and La-139 Solid State NMR Spectroscopy and Impedance Spectroscopy: Garnet-like structures containing lithium are of interest for applications in lithium ion batteries because of their inherent lithium ion conductivity and stability against chemical reaction with Li. Here, a series of materials, with parent composition LaLi0.5Fe0.2O2.09, are synthesized using solid-state chemistry, and characterized, in terms of their structure, using a combination of powder X-ray diffraction (PXRD), Li-7, and La-139 solid-state NMR, which reveal disorder on the Li and Fe sites in the lattice. The Li-7 spectra comprise a set of peaks that are distinguished based on their T-1 relaxation properties, as a diamagnetic set and a paramagnetic set of peaks. The La-139 spectra include two La environments, one well-defined, with a C-Q of 56 MHz +/- 1 MHz and asymmetry parameter, eta of 0.05 +/- 0.05, and a second, which experiences a range of local environments, because of the Li/Fe substitution, and has a C-Q of 29 MHz +/- 2 MHz, and eta of 0.6 +/- 0.1. The dynamics within the materials were characterized using impedance spectroscopy, and trends were correlated with the lithium content and structural features. The best conductivity was determined for the parent material, LaLi0.5Fe0.2O2.09, after sintering at 850 degrees C. The complex Li-7 and La-139 NMR spectra, interpreted together with (PXRD) data, indicate that the increasing concentration of lithium in the material populates an iron site with excess lithium, in a range of possible local environments, which appears to decrease the total ionic and electronic conductivity.
Elucidating the Time Scale and Geometry of Phosphate and Phosphonate Rotation in Solid Acid Electrolytes Using Multinuclear NMR: The dynamics of the anions in a series of solid acid electrolyte materials have been determined using solid-state (31)P CODEX NMR. The time scale and activation energy of the phosphate reorientation were quantified; however, the characterization proved to be nontrivial due to interactions with the quadrupolar (133)Cs nuclei. The series of compounds studied included other known proton conductors, where the alkali cation was replaced systematically with Rb(+), Tl(+), or K(+) The nuclear properties of the cations, together with the relative proton conductivities of these electrolyte analogues, allowed for a complete analysis of the interactions (both structure reorientations and spin dynamics) giving rise to the observed CODEX buildup curves. The high-temperature super-protonic transition, and the narrow range of thermal stability, limits the usefulness of cesium dihydrogen phosphate (CDP) in fuel cell applications. With the goal of achieving a broader range of thermal stability and enhanced proton dynamics, the phosphate anion was substituted for methyl phosphonate. This altered the type of motion available to the anion and significantly reduced the energy barrier for anion rotation in this electrolyte.
Influence of particle size on solid solution formation and phase interfaces in Li0.5FePO4 revealed by P-31 and Li-7 solid state NMR spectroscopy: Here we report the observation of electron delocalization in nano-dimension xLiFePO(4):(1 - x)FePO4 (x = 0.5) using high temperature, static, P-31 solid state NMR. The P-31 paramagnetic shift in this material shows extreme sensitivity to the oxidation state of the Fe center. At room temperature two distinct P-31 resonances arising from FePO4 and LiFePO4 are observed at 5800 ppm and 3800 ppm, respectively. At temperatures near 400 degrees C these resonances coalesce into a single narrowed peak centered around 3200 ppm caused by the averaging of the electronic environments at the phosphate centers, resulting from the delocalization of the electrons among the iron centers. Li-7 MAS NMR spectra of nanometre sized xLiFePO(4):(1 x)FePO4 (x - 0.5) particles at ambient temperature reveal evidence of Li residing at the phase interface between the LiFePO4 and FePO4 domains. Moreover, a new broad resonance is resolved at 65 ppm, and is attributed to Li adjacent to the anti-site Fe defect. This information is considered in light of the Li-7 MAS spectrum of LiMnPO4, which despite being iso-structural with LiFePO4 yields a remarkably different Li-7 MAS spectrum due to the different electronic states of the paramagnetic centers. For LiMnPO4 the higher Li-7 MAS paramagnetic shift (65 ppm) and narrowed isotropic resonance (FWHM approximate to 500 Hz) is attributed to an additional unpaired electron in the t(2g) orbital as compared to LiFePO4 which has delta(iso) = -11 ppm and a FWHM = 9500 Hz. Only the delithiated phase FePO4 is iso-electronic and iso-structural with LiMnPO4. This similarity is readily observed in the Li-7 MAS spectrum of xLiFePO(4):(1 - x)FePO4 (x = 0.5) where Li sitting near Fe in the 3+ oxidation state takes on spectral features reminiscent of LiMnPO4. Overall, these spectral features allow for better understanding of the chemical and electrochemical (de)lithiation mechanisms of LiFePO4 and the Li-environments generated upon cycling.
Study of Lithium Dynamics in Monoclinic Li3Fe2(PO4)(3) using Li-6 VT and 2D Exchange MAS NMR Spectroscopy: Details of Li-mobility in Li3Fe2(PO4)(3) are elucidated using solid-state Li-6 NMR. Three crystallographically unique Li sites were resolved under magic angle spinning (25 kHz) with paramagnetic shifts arising at 45 ppm, 102 ppm, and 216 ppm. These resonances were assigned to the crystallographic positions based on the degree of the Fermi-contact interaction with the paramagnetic iron center. Li-6 2D exchange NMR experiments were performed under variable temperature conditions in order to determine the activation energies for hopping between lithium sites. Activation energies ranged from 0.59 (+/- 0.05) eV to 0.81 (+/- 0.04) eV, where shorter Li internuclear distances and larger Li-O bottlenecks yielded lower activation energies. These results were compared to it previous study on the isostructural Li3V2(PO4)(3), which showed similar trends of increased internuclear distance (and constricted bottlenecks) yielding larger energy barriers for Li-Li exchange. Overall, the average activation energy for lithium ion hopping in the iron-based structure is lower than the vanadium analogue, which is attributed to the more open framework of the former.
Synthesis of Li4V(PO4)(2)F-2 and Li-6,Li-7 NMR studies of its lithium ion dynamics: A single phase, well-crystallized Li4V(PO4)(2)F-2/carbon nanocomposite has been prepared by an optimized solid-state route via oxidation of Li5V(PO4)(2)F-2. The Li4V(PO4)(2)F-2 composition exhibits lattice parameters close to those of the oxidized parent (a = 6.898 angstrom, b = 10.673 angstrom, c = 9.977 angstrom; beta - 87.84 degrees; V - 734.0 angstrom(3)). 1D Li-6 solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) studies identified which of the six lithium ions are removed from the lattice of the parent Li5V(PO4)(2)F-2. The results are in perfect accord with previous NMR-based predictions of which sites would be the most mobile and thereby most easily extracted upon cycling. Variable-temperature NMR studies and 2D exchange spectroscopy (EXSY) are used to probe the Li ion dynamics in Li4V(PO4)(2)F-2. Importantly, our studies show that upon delithiation, the ion mobility was found to increase significantly vis a vis the parent Li5V(PO4)(2)F-2. We ascribe this to the creation of lithium vacancies within the structure that open up pathways for ion transport.
Investigations of Proton Conduction in the Monoclinic Phase of RbH2PO4 Using Multinuclear Solid-State NMR: To understand the nature of the high-temperature phase transition and the origin of proton conductivity in rubidium dihydrogen phosphate RbH2PO4, (RDP), we utilized variable-ternperature H-1, P-31, and Rb-87 MAS NMR at 11.7 and 21.1 T. The variable-temperature H-1 MAS NMR spectra do not show water peaks in the temperature range of 300-400 K, which argues against a partial polymerization and/or decomposition. The H-1 DQ MAS NMR spectra acquired at high field (21.1 T) reveal three different proton sites, and dipolar recoupling experiments give estimates of distances. This evidence supports the formation of a superstructure (doubling of the unit cell along the a-axis) in the high-temperature monoclinic phase. The variable-temperature proton spin-lattice relaxation (T-1) reveals significant proton dynamics above 340 K, coinciding with the temperature at which an increase of bulk proton conductivity is reported in monoclinic RDP, but well below the macroscopic phase transition. These precursor dynamics foreshadow the onset of superprotonic conductivity. From variable-temperature P-31 and Rb-87 MAS NMR spectra, it is deduced that significant proton dynamics above 340 K are due to the proton transfer between hydrogen bonds accompanied by reorientation of PO4 tetrahedra.
Measurement and calculation of C-13 and N-15 NMR chemical-shift tensors of a push-pull ethylene: Methyl 3-dimethylamino-2-cyanocrotonate (MDACC) has a remarkably weak carbon-carbon double bond. It has strong electron-withdrawing groups on one end and electron-donating groups on the other: a so-called push-pull ethylene. To investigate this unusual electronic structure, we have determined the crystal structure and measured both the C-13 and N-15 NMR chemical-shift tensors. These measurements are supplemented by shielding-tensor calculations done with density functional methods. The large difference (approximately 100 ppm) between isotropic chemical shifts of the two alkenyl carbons reflects a large charge release from the electron-donating side of C=C double bond to the electron-withdrawing groups. Comparison of the calculated orientations of the principal components of the alkenyl carbons obtained from ab initio calculations shows that the primary changes in charge density occur in the molecular plane. On the other hand, smaller charge density changes above and below the plane of the C=C double bond establish the conjugation of donor and acceptor groups with pi* and pi molecular orbitals of the central double bond, respectively, which lowers the barrier to rotation about this bond.
Ab initio structure determination of SrBi2OB4O9 by powder X-ray/neutron diffraction and NMR spectroscopy: SrBi2OB4O9 is a novel centrosymmetric borate oxide forming in the SrO-Bi2O3-B2O3 system. Its crystal structure has been determined ab initio from high-resolution X-ray and neutron powder diffraction data with the help of B-11 MAS-NMR data. SrBi2OB4O9 crystallizes with a triclillic unit-cell with a=6.9657(1) angstrom, b=9.7976(1) angstrom, c=6.8148(1) angstrom
Layered lithium vanadium fluorophosphate, Li5V(PO4)(2)F-2: A 4 V class positive electrode material for lithium-ion batteries: A single-phase, well-crystallized Li5V(PO4)(2)F-2/carbon nanocomposite has been prepared by an optimized solid-state route, and its electrochemical behavior was examined as a positive electrode active material in lithium-ion batteries. The microcrystalline powder was synthesized by the reaction of amorphous VPO4/C with Li3PO4 and LiF. Synthesis of the VPO4/C precursor utilized carbon to reduce V2O5 and simultaneously create a nanoparticle reactive material, with the amount being tailored to result in a small excess at the end of the reaction to increase the bulk electronic conductivity of the final Li5V(PO4)(2)F-2 composite. Quench techniques were used to isolate the metastable two-dimensional Li5V(PO4)(2)F-2 as a pure single phase. Li-6 MAS NMR studies identified the crystallographic lithium sites and provided information on lithium-ion mobility and exchange in the two-dimensional layered structure. Lithium cells of the Li5V(PO4)(2)F-2/C composite exhibited a reversible plateau at 4.15 V followed by a second plateau at 4.65 V, which correspond to the V3+/V4+ and V4+/V5+ redox couples, respectively, and a charge capacity of 165 mA h g(-1), which is close to theoretical (170 mA h g(-1)). These operating voltages are very similar to those of the V3+/V4+ couple in Li3V2(PO4)(3) or LiVPO4F and the V4+/V5+ couple in Li3V2(PO4)(3).
Influences of casting solvents on proton dynamics within sulfonated polyether ether ketones (S-PEEKs) studied using high-resolution solid-state NMR: Proton mobilities of S-PEEK membranes with variable degrees of sulfonation (DS), and of S-PEEK membranes cast from different solvents were studied using high-resolution solid-state H-1 MAS NMR. In the hydrated S-PEEKs, a single resonance for water-associated sulfonic acid protons was observed for S-PEEK with DS = 58 and 95\%, which indicates that the acid protons in either of these S-PEEK samples have similar environments, suggesting a similar distribution of water at each sulfonic acid site. However, there are multiple resonances for water-associated sulfonic acid protons in S-PEEK with DS = 72 and 86\%. This suggests distinct acid proton environments resulting from an uneven distribution of water to sulfonic acid protons in these S-PEEK membranes, explaining the fact that these two fully hydrated S-PEEKs show similar conductivities although they have large differences in water content and DS. For S-PEEK membranes cast from different solvents, stronger polymer chain interactions were observed in S-PEEK from N,N-dimethylformamide (DMF) than in those from N,N-dimethylacetamide (DMAc). This could explain why the former exhibits lower proton conductivity although it has the same water content and DS as the latter. S-PEEKs cast from DMAc and dimethylsulfoxide (DMSO) have large differences in water content, but show very similar proton conductivity at high relative humidity. In all cases, solid-state H-1 NMR is shown to be a very sensitive probe of local environments as a function of the casting solvent, degree of sulfonation, and relative humidity, as such spectra provide detailed information about the distribution of acid protons in the membranes. (c) 2008 Elsevier B.V. All rights reserved.
POLY 287-Proton dynamics in polymer electrolytic membranes by solid state NMR
Investigations of the phase transition and proton dynamics in rubidium methane phosphonate studied by solid-state NMR: In search of new solid acid proton conductors, we prepared the solid acid rubidium methane phosphonate (RMP). These crystals have a monoclinic structure (P2/c; a = 9.3452, b = 9.3142, and c = 7.5021 angstrom; beta = 101.12). The salt incorporates a hydrated lamellar structure. The H-1 MAS NMR reveals two different types of acidic protons as well as the water protons in the lamella. The H-1 VT MAS NMR of RMP center dot 2H(2)O single crystal shows a structural phase transition around 320 K, and the high-temperature phase exhibits significant proton dynamics. The proton proximities are established by solid state H-1 DQF NMR. The dehydration of RMP crystal leads to structural collapse, and the resultant RMP powder is extremely hygroscopic. The proton environment and dynamics are examined using H-1 DQF NNIR, which reveals that the dehydrated RMP powder has rigid lattice, in contrast with the hydrated form. Further the H-1 VT MAS NMR shows that dehydrated RMP powder has no phase transition, and no significant proton dynamics are observed in the temperature range of 250-350 K. The new hydrated crystal, RMP center dot 2H(2)O, shows high proton mobility at relatively low temperature (similar to 330 K) and a proton transport mechanism that uniquely relies on crystalline water.
Li-6\P-31\ rotational-echo, double-resonance studies of lithium ion site dynamics in Li3V2(PO4)(3): Low-temperature Li-6\P-31\ rotational-echo, double-resonance (REDOR) measurements were used to study lithium mobility within each of the three Li sites in monoclinic Li3V2(PO4)(3). Each of the Li ions was found to experience a different reduced dipolar coupling with the nearest P-31 nucleus. Under fast magic-angle spinning (MAS) conditions (40 kHz), the three crystallographic P-31 sites in Li3V2(PO4)(3) were resolved. On the basis of the known Li-P internuclear distances, the P-31 resonances were assigned by determining the degree of signal attenuation at each phosphorous site. The attenuation of the Li-P dipolar coupling, compared to simulations of the static case, was associated with rattling of the Li ions within the lattice. The relative mobility at each Li site is correlated with structural properties including the shortest Li-O contact within the void and bond valence calculations.
The challenge of paramagnetism in two-dimensional Li-6,Li-7 exchange NMR: Li-6,Li-7 fast magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy is used to study LiMn2O4 and Li3V2(PO4)(3). The presence of paramagnetic transition metal centers in these materials has a profound effect on the resulting NMR spectra. Lithium ion mobility has been studied by two-dimensional (2-D) exchange spectroscopy (EXSY) in Li3V2(PO4)(3) but an absence of lithium ion exchange was observed for LiMn2O4. Several differences between the two materials are explored to explain these results. LiMn2O4 experiences a greater donation of electron spin density to the Li nucleus via the Fermi-contact interaction when compared with Li3V2(PO4)(3). This contributes to a greater hyperfine chemical shift and a larger dependence of chemical shift on temperature. The delocalized electrons in LiMn2O4 cause temperature-independent T (1) relaxation rates and shorter relative T (2) values. The relative rates of ionic conductivity and spin-lattice or spin-spin relaxation in LiMn2O4 and Li3V2(PO4)(3) are contrasted to illustrate the constraints on the use of 2-D EXSY to characterize ion dynamics in paramagnetic materials.
Probing proton mobility in polyvinazene and its sulfonated derivatives using H-1 solid-state NMR: The proton dynamics of PV and its sulfonated derivatives have been studied using high-resolution solid state H-1 MAS NMR. Variable temperature experiments were used to determine the activation energy for transportation of hydrogen bonded protons, found to be 22 +/- 1 kJ center dot mol(-1) for PV and 13 +/- 1 kJ - mol(-1) for PV-B25. The proton exchange between sulfonic acid group and vinazene ring observed from both variable temperature experiments and H-1 EXSY NMR experiments provides a good explanation for this difference. A rotorsynchronized homonuclear double quantum filter sequence was used to distinguish protons of differing mobilities. A model is proposed to understand the distinct proton mobilities in these materials.
Proton dynamics of nafion and Nafion/SiO2 composites by solid state NMR and pulse field gradient NMR: Proton mobilities in Nafion and Nafion/SiO2 composites have been studied using high-resolution solid-state MAS NMR. High-resolution solid-state H-1 NMR show that low concentrations of TEOS or short permeation times are necessary to allow complete hydrolysis of TEOS in Nafion. Incomplete hydrolysis of TEOS leaves residual ethyl groups on the surface of silica, which not only reduces the amount of water adsorbed by silica but also blocks the pathway of proton transport in the Nafion/SiO2 composites. The diffusion coefficients established using PFG NMR show that the best Nafion/SiO2 composite can be obtained from synthesis with a low concentration of TEOS in a methanol solution. This composite gives a higher diffusion coefficient than pure Nafion under dry conditions, although no differentiation in performance is observed when the membranes are hydrated. Si-29 NMR shows that this composite has a high ratio of Q(3)/Q(4) sites, consistent with a small particle size and many surface hydroxyl groups. Together, these data demonstrate the role of high-surface-area SiO2 particles in trapping water and building a pathway for structural (Grotthuss mechanism) proton diffusion. Good proton transport under low relative humidity is the holy grail of the PEM-FC community, and this molecular level study shows how conditions can be iteratively optimized to target desirable structure-property relationships.
Unraveling the complex hydrogen bonding of a dual-functionality proton conductor using ultrafast magic angle spinning NMR: Hydrogen bonding plays a critical role in proton-conducting polymers, as it provides the network necessary for structural (Grotthus mechanism) diffusion. This network must be both pervasive and dynamic in order for long-range proton transport to be achieved. The structural motifs must be understood, even in amorphous materials, and moreover, the lattice energies in the structure must be low enough to allow rearrangement and mobility. To this end, a novel proton-conducting candidate, 1,10-(1-H-imidazol-5yl) decanephosphonic acid and its HBr doped counterpart are considered from the molecular level as potential proton-conducting membranes. The use of high-resolution solid-state H-1 NMR to elucidate structure and dynamics of such systems is highlighted in this material. We compare our molecular-level results to macroscopic probes of proton transport in related polymers, achieved using impedance spectroscopy.
PMSE 552-Superabsorbent chitosan-PEG gel
Li-6,Li-7 NMR study of ion mobility on the molecular scale in lithated imidazole complexes: Two model compounds, lithium imidazoliurn (LiIm) and lithium 2-undecylimidazolium (und-LiIm), were synthesized. These materials are chosen as models of potential lithium ion conductors for use as electrolytes in lithium batteries. Solid-state NMR was used to provide information on the microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Lithium imidazolium was mixed with lithium methylsulfonate, generating a doped complex in which a doubly lithiated imidazole ring was inferred based on the Li-7 NMR chemical shifts. Our research includes Li-6,Li-7 variable temperature MAS NMR experiments at intermediate spinning speeds, relaxation studies to determine spin-lattice relaxation times (T-1) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. The possibility of 2-site ring reorientation for the doubly lithiated imidazole ring was supported by exchange spectroscopy. Comparisons of spin-lattice relaxation times and corresponding activation energies of the lithium imidazolium and the doped complex point to a higher degree of mobility in the latter. Lithium 2-undecylimidazolium was prepared and exhibited a lower melting point than the parent lithium imidazolium, as expected. This small 7 molecule was chosen as representative of a side-chain functionalized polyethylene-based material. Li-7 MAS spectra show mainly the presence of the doubly lithiated imidazole ring in pure und-LiIm, and in the LiCH3SO3-und-LiIm mixture. The data clearly indicate local mobility of the lithium ions in the materials. (c) 2006 Elsevier B.V. All rights reserved.
Solid-state NMR study of two classic proton conducting polymers: Nafion and sulfonated poly(ether ether ketone)s: Proton mobilities in Nafion and sulfonated poly(ether ether ketone) (S-PEEK) have been studied using high-resolution solid-state H-1 NMR under fast magic angle spinning (MAS). These studies demonstrated proton exchange between sulfonic acid groups and water within both Nafion and S-PEEK. Variable temperature experiments were used to determine the activation energy for proton transport in pure Nafion, found to be 11.0 kJ/mol, which is lower than those determined for S-PEEKs with different degrees of sulfonation. Increasing proton exchange rates with increasing temperature indicate the expected dependence of proton mobility on temperature. A rotor-synchronized homonuclear double quantum filter sequence (BaBa) was used to disclose the nature of the H-bonding interactions in the two polymers, from which a model of the proton interactions in the polymers is developed.
Li-7 NMR and two-dimensional exchange study of lithium dynamics in monoclinic Li3V2(PO4)(3): High-resolution solid-state Li-7 NMR was used to characterize the structure and dynamics of lithium ion transport in monoclinic Li3V2(PO4)(3). Under fast magic-angle spinning (MAS) conditions (25 kHz), three resonances are clearly resolved and assi ned to the three unique crystallographic sites. This assignment is based on the Fermi-contact delocalization interaction between the unpaired d-electrons at the vanadium centers and the lithium ions. One-dimensional variable-temperature NMR and two-dimensional exchange spectroscopy (EXSY) are used to probe Li mobility between the three sites. Very fast exchange, on the microsecond time scale, was observed for the Li hopping processes. Activation energies are determined and correlated to structural properties including interatomic Li distances and Li-O bottleneck sizes.
Li-6 NMR studies of cation disorder and transition metal ordering in Li[Ni1/3Mn1/3Co1/3]O-2 using ultrafast magic angle spinning: Studies of Li[Ni1/3Mn1/3Co1/3]]O-2 prepared under six different conditions are compared using high-resolution solid-state Li-6 NMR. Differing degrees of cation disorder are established via intearation of the NMR resonances, and this quantification of cation disorder is compared with Rietveld refinements of powder X-ray and neutron diffraction studies. Chemical shift trends to high frequency with decreasing degrees of disorder are established among this family of samples and explained according to the orbital overlap experienced by Li nuclei in the two environments: within the lithium layers and exchanged with nickel into the transition metal layers. Finally, an interesting case of local transition metal charge ordering is observed. Three unique environments are described, which can be accounted for based on electroneutrality arguments, and the known clustering of Ni2+ and Mn4+. This effect has not been detected in these materials by other methods including neutron and X-ray diffraction. Thus, the local ordering, which is observed in the dominant NMR resonance of Li in its own layers is thought to be pervasive (affecting the majority of the NMR nuclei), but very local, so as to be seen only by techniques such as NMR which probe immediate neighborhoods.
Study of imidazole-based proton-conducting composite materials using solid-state NMR: The application of solid-state NMR methods to characterize the structure and dynamics of imidazole-based proton-conducting polymeric materials provides insight into the mechanism (Grotthus vs vehicle) of proton-mobility. The presented materials are built on a siloxane backbone, and are of interest as potential new proton-conducting membranes for fuel cells able to function at temperatures above 130 degrees C. This is expected to improve the CO tolerance of the catalyst in the fuel cell, as compared to water-based systems. High-resolution solid-state H-1 NMR is achieved under fast magic-angle spinning (MAS) conditions (30 kHz), and provides resolution of resonances in the hydrogen-bonding region. Homonuclear double quantum filtered (DQF) NMR spectra, acquired using the back-to-back sequence, provided identification of mobile protons. It was found that proton conductivity, observed macroscopically using impedance spectroscopy, is con-elated with local proton mobility, observed via H-1 NMR line width trends observed for the hydrogen-bonded protons. H-1 MAS and DQF NMR experiments show no crystal packing of these materials in contrast to model oligo-ethyleneoxide-tethered imidazole materials (Imi-nEO) Studied previously. Comparisons of macroscopic and microscopic measures of proton mobility are also presented in the activation energies of pure and acid-doped siloxane oligomers and polymers functionalized with imidazole. The acid-doped materials show enhanced proton mobility, and hence higher conductivity, relative to the pure material.
Probing hydrogen bonding and proton mobility in dicyanoimidazole monomers and polymers: Hydrogen bonding and proton mobility are important features in many polymers. In this work, hydrogen bonding is studied in both monomers and polymers of dicyanoimidazoles using infrared and solid-state NMR spectroscopy and polymer viscosity studies. Hydrogen bonding accounts for an unusual complexity in the nitrogen -hydrogen stretching region of the infrared spectra. The influence of hydrogen bonding on properties was observed in several dicyanoimidazole polymers through polymer viscosity studies and estimation of Mark-Houwink parameters. The Mark-Houwink a value decreases, representing a less stiff chain, in poly(l-methyl-2-vinyl-4,5-dicyanoimidazole) compared to poly(2-vinyl-4,5-dicyanoimidazole) because hydrogen bonding is eliminated. By dissolving poly(2-vinyl-4,5-dicyanoimidazole) in NH3(aq), a polymer electrolyte results. Although hydrogen bonding is eliminated, electronic repulsions contribute to an increase in a or chain stiffness. Proton mobility in dicyanoimidazole polymers was studied with an innovative solid-state NMR technique using double-quantum (DQ) filtering and fast magic angle spinning (MAS = 30 kHz). Using this approach, several types of hydrogen bonds were identified and proton mobility in poly(2-vinyl-4,5-dicyanoimidazole) was detected.
Investigation of imidazole-based lithium conducting materials: This study aims to develop novel polyelectrolytes including lithiated imidazole heterocycles for use in lithium ion rechargeable batteries. Lithium ion local mobility in these materials is characterized by Li-6,Li-7 solid-state NMR. By comparing these results with macroscopic ionic conductivity, measured by impedance spectroscopy, we will be able to develop a picture of the ionic conductivity at the microscopic level. Multinuclear solid state NMR provides information on microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Our research includes Li-6,Li-7 variable MAS NMR studies at intermediate spinning speeds, relaxation investigations to determine spin-lattice relaxation times (T-1) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. A very long T-1 (135 s at ambient temperature) and an activation energy Ea = 17.2 kJ/mol suggests rigid molecule structure and the absence of the ring reorientation of the model compound, lithium imidazolium (LiIm). We compare this to the behavior of LiIm doped with lithium methanesulfonate, which we show to form a new ionic complex with lower T-1 and corresponding lower activation energy. With the goal of creating new polyelectrolytes, we have synthesized electrolytes incorporating lithiated imidazole rings, where lithium transport may be independent of polymer-backbone flexibility, and thus polymers with high T-g may be viable. Such materials are highly desirable for secondary lithium polymer battery applications.
Solid state NMR spectroscopic investigations of model compounds for imidazole-based proton conductors: The temperature dependence and the exact geometry of slow molecular reorientations in imidazolium methyl sulfonate are investigated using modern one-dimensional MAS exchange spectroscopy. Earlier high-temperature studies have evidenced a fast 180degrees flip motion of the imidazole ring, which is shown here to slow on cooling and is believed to be a prototypical molecular process involved in Grotthus-type proton transport in imidazole-based proton conductors intended for fuel cell applications. It is further shown that valuable information on the relative orientations of CH and NH dipolar coupling tensors with respect to the chemical shift anisotropy tensors of the respective heteronuclei can be obtained from the MAS exchange data as well as from static C-13 and N-15 line shapes, without the necessity of performing more involved single-crystal NMR experiments. The principal axes of the CSA tensors are found to not coincide with the CH or NH bond axes, in contrast to earlier assumptions involving similar compounds. Imidazole itself is shown to be more complex than might be expected, based on its simple structure. Implications on earlier studies of pure imidazole, where ring flips were claimed to be absent, are discussed.
Polymer-functionalized carbon nanotubes investigated by solid-state nuclear magnetic resonance and scanning tunneling microscopy: Carbon nanotubes are an intriguing new form of carbon, comprising molecular-scale cylinders of nanometer diameter and micrometer to centimeter lengths. They exhibit many extraordinary mechanical and electrical properties and have a wide variety of anticipated applications. However, to realize these potential applications, chemists need to develop means by which to manipulate these nanotubes in a predictable and controllable way. Novel sidewall-modified carbon nanotubes functionalized with polymers, such as poly(methyl methacrylate) (PMMA), have been prepared to gain control over the properties of nanocomposites on the molecular level. Characterization of these materials has been limited by their insolubility in organic solvents. Here the interaction between the carbon nanotube and the polymer has been studied through the use of solid-state nuclear magnetic resonance (NMR) and scanning tunneling microscopy (STM). Fast magic-angle spinning (30 kHz), to achieve high-resolution H-1 NMR, together with advanced pulse sequences such as H-1 double quantum NMR with the BABA (back-to-back) sequence, and heteronuclear H-1-C-13 sequences, are used to demonstrate the association of the initiator moieties and polymers with the surface of the nanotubes. The findings are supported by STM data of nanotubes before and after functionalization with the initiator groups.
Benzoxazine oligomers: Evidence for a helical structure from solid-state NMR spectroscopy and DFT-based dynamics and chemical shift calculations: A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and helical conformations of trimeric and tetrameric units. In fast MASH NMR spectra, the resonances of the protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by means of DFT-based geometry optimizations and H-1 chemical-shift calculations. Also supporting these proposed structures are homonuclear H-1-H-1 double-quantum NMR spectra, which identify the local proton-proton proximities in each material. Additionally, quantitative N-15-H-1 distance measurements obtained by analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the N...H...O and O...H...O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more confidence than the individual measurements or calculations alone, in particular, because the microscopic structure of many disordered systems cannot be elucidated by means of conventional methods due to lack of long-range order.
High-resolution solid-state NMR studies of imidazole-based proton conductors: Structure motifs and chemical exchange from H-1 NMR: High-resolution solid-state H-1 NMR under fast magic angle spinning is used for the first time to study proton conductivity. The materials of interest, ethylene oxide tethered imidazole heterocycles (Imi-nEO), are characterized by variable temperature experiments, as well as 2D homonuclear double quantum (DQ) NMR and 2D exchange spectroscopy. Quantum chemical calculations provide a full assignment and understanding of the H-1 chemical shifts, based on a single-crystal structure obtained for Imi-2EO. Three types of hydrogen-bonded N-H-1 resonances are observed by H-1 MAS NMR at 30 kHz. Double quantum NMR experiments identify those hydrogen-bonded protons that are mobile on the time scale of the experiment, and thereby, those which are able to participate in charge transport. Characterized by their spin-spin relaxation (T-2*) behavior, the local mobility of these protons as a function of temperature is compared to the conductivity of the materials. Homonuclear H-1 2D DQ MAS spectra provide evidence for locally ordered domains within all the Imi-nEO materials. Disordered (mobile) and ordered components in Imi-2EO dramatically differ in their H-1 spin-lattice relaxation times. 2D NOESY spectra show no evidence of chemical exchange processes between the ordered and disordered domains. These results indicate that the highly ordered regions of the materials do not (or only poorly) contribute to proton conductivity, which is rather taking place in the disordered regions. Molecules in the disordered domains are in a state of dynamic or fluctuating hydrogen-bonding, allowing for Grotthus mechanism proton transport, while molecules in the ordered domains do not experience exchange, and do not participate in long-range proton conductivity. At the interface between these regimes a small number of molecules undergo slow exchange. With increasing temperature, this exchange becomes fast on the NMR time scale, and the final chemical shift of 12.5 ppm in Imi-5EO implies the persistence of strongly and weakly hydrogen-bonded domains, which reorganize rapidly to support the proton transport process.
Investigation of an N center dot center dot center dot H hydrogen bond in a solid benzoxazine dimer by H-1-N-15 NMR correlation techniques under fast magic-angle spinning: The N . . .H distance within the unusual hydrogen-bonding arrangement adopted by a pair of methyl-substituted benzoxazine dimers (C6H3(OH)(2)CH2)(2)N(CH3) has been determined by solid-state NMR to be 194 +/- 5 pm. This indicates that the proton is shared between the nitrogen and oxygen atoms, with a preference for an O-H rather than an N-H bond character. It is to be noted that a previous X-ray single crystal study was unable to localize the position of this hydrogen-bonded proton. The advanced solid-state NMR methods employed utilize REDOR-type recoupling under fast magic-angle spinning to recouple the heteronuclear H-1-N-15 dipole-dipole interaction, such that rotor-encoded spinning-sideband patterns are obtained, the analysis of which yields the H-1-N-15 dipole-dipole coupling and hence the N . . .H distance. Different designs of recoupling pulse sequences are discussed, which allow the experiment to be adapted to the system under investigation in terms of the required N-15 or H-1 chemical shift resolution, conventional (N-15) or inverse (H-1) detection as well as the importance of the perturbing influences of further spins. The chosen recoupling scheme employs inverse, i.e. H-1, detection, because it provides a dramatic increase in signal sensitivity, resulting in savings in measurement time by a factor of at least 20, as well as H-1 chemical-shift resolution in the directly detected spectral dimension. This is the method of choice for cases such as this, where chemical shift resolution is not required in the N-15 dimension. In addition, the perturbing effect of further protons on the N . . .H coupling of interest is minimized, such that a relatively long N... H distance can be determined despite the presence of several other couplings of comparable strength. Copyright (C) 2001 John Wiley & Sons, Ltd.
The true crystal structure of Li17M4 (M=Ge, Sn, Pb)-revised from Li22M5: Single crystal studies of the compounds Li17Ge4, Li17Sn4 and Li17Pb4, previously thought to be of stoichiometry Li22M5 (M=Ge, Sn, Ph) are presented. Their space group is F (4) over bar 3m, rather than the F23 previously reported for Sri and Pb, with a = 18.756(2) Angstrom, 19.690(2) Angstrom and 19.842(1) Angstrom for the Ge, Sn and Ph compounds, respectively, with very good refinements, particularly for the tin structure (R(F greater than or equal to 4 sigma (F)) = 1.85\% for Ge, 1.91\% for Sn and 2.3\% for Pb). Comparison with the related structure of Li21Si5 reveals that as one progresses down Group IV, there is a variation in the occupation of the lithium tetrahedra about the tetrahedral Wickoff sites based on the space apportioned by the surrounding metal atoms. Partial or complete occupation of some tetrahedral sites increases from Si to Ph resulting in centred tetrahedral Li-5 groups replacing Li-4 units, and hence stoichiometries that can be effectively described as Li21Si5 ('Li16.8Si4'); Li(21 + 3/16)Ge5 ('Li17Ge4'); Li(21 + 5/16)Sn5 ('Li17Sn4'); and Li(21 + 1/4)Pb5 (or Li17Pb4). (C) 2001 Elsevier Science B.V. All rights reserved.
Electrochemical and multinuclear solid-state NMR studies of tin composite oxide glasses as anodes for Li ion batteries: Electrochemical and multi-nuclear solid-state NMR studies of various tin oxide and two tin composite oxide (TCO Sn1.0Al0.42B0.56P0.40O3.6 and Sn-rich TCO Sn1.5Al0.42B0.56P0.40O4.2) samples are described, which give a coherent picture of the different processes occurring within these systems. Li-6,Li-7 NMR results demonstrate that the agglomeration of Li-Sn domains is inhibited in TCO; in contrast, in SnO, the aggregation of particles is observed. This difference results in part from the facile back-reaction between Sn and O. The interfacial energy of the most highly divided particles (TCO) allows the ``back-reaction'' of lithium with oxygen to be reversible at a lower potential than predicted from simple thermodynamic considerations that exclude surface energy contributions. Thus, the proximity and availability of oxygen in the host matrix may indirectly enhance the reversibility and cyclability of the cell in these materials, by ``trapping the Sn particles''. Aggregation may also be limited in TCO owing to the participation of the matrix observed by Al-27, P-31, and B-11 NMR, where reversible changes in the coordination environment are observed during lithium uptake and removal. The size-limiting role of the matrix ions is key to the enhanced electrochemical properties of the TCO glass. The initial rearrangement of the glass network is kinetically limited, as demonstrated by galvanostatic intermittent titration technique (GITT) experiments. The combined results of this study demonstrate the unique nature of the reaction between lithium and TCO.
EXAFS study of the tin coordination change upon lithiation of SnO and tin oxide glasses: The behavior of TCO and SnO on electrochemical Li uptake and removal has been studied using ex-situ EXAFS spectroscopy. These results indicate that in both cases Sn clusters are formed on initial discharge, in which, or at the surface of them if they are sufficiently small, a significant number of oxygen atoms are coordinated. The lower Sn coordination number for TCO as compared to SnO indicates that the particles are much smaller. The RDF obtained on full discharge (to 10mV) in SnO, followed by removal of about 1/3 of the Li on charge shows that the oxygen contribution is absent at deep discharge but is readily recovered on charge. The high degree of disorder in the TCO precludes definitive EXAFS analysis, but suggests that some oxygen may be maintained in the Sn coordination shell even at full discharge.
On the nature of Li insertion in tin composite oxide glasses: Tin composite oxide (TCO) glass, Sn1.0Al0.4B0.56P0.4O3.6, is a high capacity lithium storage medium that may perform as the next generation of anodes in Li-ion batteries. By using X-ray absorption spectroscopy in conjunction with lithium-7 nuclear magnetic resonance, we have obtained a picture of the Li insertion and deinsertion process in TCO compared to that in SnO, and show that these processes are more complex than was originally thought. Our results demonstrate the significance of oxygen participation in the reversible reactions at nanophase Li-Sn or Li-Sn-O centers, and suggest that the presence of a quasi-flexible Al coordination environment in the glass may enhance the cycling properties. (C) 1999 The Electrochemical Society. S1099-0062(99)02-022-2. All rights reserved.
Understanding the nature of low-potential Li uptake into high volumetric capacity molybdenum oxides: A simple method to prepare (Na0.25MoO3) provides a material that displays very good properties as negative electrodes in lithium-ion batteries. Low- potential Li insertion below 300 mV is associated with dramatic transformation of the initial structure, leading to an amorphous material which exhibits excellent cyclability with a high reversible specific capacity of 940 mA/g in the voltage window of 3.0-0.005 V (volumetric capacity of 4000 mAh/cm(3)). For the first time, two complementary tools, Li-7 nuclear magnetic resonance and X-ray absorption spectroscopy, demonstrate that on Li insertion at low potential, a complex amorphous composite is formed consisting of a LiMo-suboxide in intimate contact with lithium oxide. (C) 1998 The Electrochemical Society. S1099-0062(98)02-049-5. All rights reserved.
Solid-state H-2 NMR determination of poly(aniline) conformation within a MoO3 nanocomposite: The conformation of poly(aniline) (PANI) within a MoO3, nanocomposite is investigated by solid-state H-2 MMR (see also cover). This technique is demonstrated to be highly suited to providing information about the structure of polymers intercalated within transition metal oxide hosts that cannot be obtained by other methods. Through a series of 1D and 2D experiments it is shown that PANI is organized within the interlayer regions of the inorganic host and that order within the composite is not determined solely by the oxide layers.
Solid-state H-2 NMR study of methyl-d(3)-cobalamin: A solid-state H-2 NMR study of methyl-d(3)-cobalamin has been performed as a function of temperature to provide information concerning the character and energetics of the motion performed by this unique bioorganometallic methyl group. Analysis of the H-2 NMR line shape indicates that the methyl group undergoes rapid three-fold rotation, and that the Co-C-H-2 angle lies between 105.9 and 109.5 degrees. Determination of the spin-lattice relaxation times T-1 shows that the relaxation is anisotropic, consistent with a ``jumping'' motion of the methyl group rather than rotational diffusion. This also provides the activation energy to methyl jumps as 8.3 +/- 1.3 kJ/mol. It is proposed that this energetic barrier may be a useful probe of changes in the electronic character of the Co-C bond that accompany the biological role of this molecule in such enzymes as methionine synthase.
Poly(pyrrole) and poly(thiophene)/vanadium oxide interleaved nanocomposites: positive electrodes for lithium batteries: Lithium insertion has been examined in a series of conductive polymer-V2O5 nanocomposites that have a structure comprised of layers of polymer chains interleaved with inorganic oxide lamellae. Poly(pyrrole), [PPY]; poly(aniline) [PANI]; poly(thiophene), [PTH] and its derivatives constituted the polymer component; PTH was prepared from the monomers bithiophene, terthiophene, 3-methylthiophene and 2,5-dimethylthiophene. Compositions of the corresponding nanocomposites were [PANI](0.4)V2O5, [PPY](x)V2O5 (x approximate to 0.4, 0.9), and [PTH],V2O5 (x approximate to 0.3-0.8). We find that for modified [PANI](0.4)V2O5, polymer incorporation results in better reversibility, and increased Li capacity in the nanocomposite compared to the xerogel. For PPY and PTH nanocomposites, the electrochemical response is highly dependent on the preparation method, nature of the polymer, and its location. Reversible Li insertion was maximized in the case of PTH when it was prepared from 3-methyl or terthiophene as the monomers, suggesting that chain conjugation length and polymer order area important factors. In some of these materials, the Li insertion capacity can be increased by 40\% by subjecting the electrode to an initial charge step. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.
X-ray Absorption and Solid-State NMR Spectroscopy of Fluorinated Proton Conducting Polymers: F-19 and C-13 solid-state nuclear magnetic resonance (SSNMR) spectra and near edge X-ray absorption fine structure (NEXAFS) spectra at the S 2p, C Is, O Is, and F Is edges of three different types of perfluorosulfonic acid (PFSA) proton conducting polymers are reported. The NEXAFS spectra were recorded in a scanning transmission X-ray microscope (STXM). They are reported on quantitative intensity scales, suitable for use as reference standards for analytical spectromicroscopy studies. The NEXAFS spectral features are assigned using comparisons to the corresponding spectra of small molecule and polymer analogues, including C is and F is spectra of polytetrafluoroethylene (PTFE) recorded using STXM. The F-19 and C-13 SSNMR spectra are reported and analyzed. Within each type of spectroscopy, the spectra of the three types of PFSA are very similar. However, small but statistically significant differences were identified which give insights into how minor molecular structure differences are reflected in the SSNMR and NEXAFS spectra. The relative chemical sensitivity of SSNMR and NEXAFS for studies of PFSA materials is compared. Solution-state NMR diffusion profile analyses of two PFSA ionomer dispersions are also reported.
The proton dynamics of imidazole methylphosphonate: an example of cooperative ionic conductivity: Imidazole methylphosphonate models the hydrogen bonding and dynamics of a potential anhydrous polymer electrolyte. Understanding the behavior of anhydrous electrolytes is crucial to creating polymer materials with better performance in a fuel cell environment. This model salt exhibits ionic conductivity in the solid-state and the method of ion conduction in the solid-state differs depending on the choice of anion and cation pairs. Previous investigation of a sulfonate analogue suggests fast ring dynamics, which contributes to the ionic conductivity. However, (13)C CODEX NMR shows that rotation of the imidazole ring is somewhat slower in the methlyphosphonate compound, with a timescale for the two-site ring flip of 31 +/- 9 ms at ambient temperature. (31)P CODEX and variable temperature (1)H MAS NMR spectra confirm that ionic conductivity is facilitated by dynamics at the bifurcated hydrogen bonds between anions, with a timescale of 57 +/- 4 ms at ambient temperature for rotation of the phosphonate about the C(3v) axis. Increasing temperature introduces thermal motion and promotes the rotation of the imidazole ring together with the rotation of the phosphonate group. This leads to a cooperative mechanism of ion conduction between imidazole and the methylphosphonate at higher temperatures, which was unseen in a previous study of the benzimidazole methylphosphonate analogue.
Solid-state NMR studies of hydrogen bonding networks and proton transport pathways based on anion and cation dynamics: Proton dynamics in polymer electrolyte membranes are multifaceted processes, and the relative contributions of various mechanisms can be difficult to distinguish. judicious choices of model systems can aid in understanding the critical steps. In this study, we characterize anion dynamics in a series of benzimidazole-alkyl phosphonate salts, and compare those dynamics to a membrane prototype, built on a decane backbone. The series of salts are characterized, using high resolution H-1 solid-state magic angle spinning (MAS) NMR, DQ MAS NMR, and P-31 centreband-only detection of exchange (CODEX) NMR spectroscopy, to determine the influence of the nature of the alkyl group on the rates and geometries of anion dynamics, and overall proton exchange processes. The alkyl group is shown to slow the correlation times for anion reorientation, when compared at ambient temperature. However, it is also apparent that the lowered lattice energy of the salt lowers the activation energy and allows good dynamics at intermediate temperatures in both the benzimidazolium ethylphosphonate and in the HBr adduct of 1,10-(1-H-imidazol-5-yl)decanephosphonic acid (Imi-d-Pa). Copyright (c) 2007 John Wiley & Sons, Ltd.
Unraveling the Rapid Performance Decay of Layered High-Energy Cathodes: From Nanoscale Degradation to Drastic Bulk Evolution: Lithium-rich layered oxides are promising cathode candidates because of their exceptional high capacity. The commercial application of these high-energy cathodes, however, is thwarted by the undesired rapid performance decay during cycling. Surface degradation has been widely considered to correlate with the performance decay of the cathodes, whereas, in this work, we demonstrate that the degradation of Li-rich high-energy Li1.2Ni0.13Mn0.54Co0.13O2 (HENMC) cathode material not only takes place at surfaces but also proceeds from its internal structure. In addition to demonstrating the surface reconstruction and the formation of a cathode-electrolyte interphase (CEI) layer of cycled HENMC cathode, this study uncovers the irreversible bulk phase transition from a Li-excess monoclinic (C2/m) solid solution into a conventional ``layered'' (R (3) over barm) phase, accompanied by complete loss of Li+ from the TM layers during cycling. Furthermore, the internal grains of HENMC bear lattice distortions, leading to the formation of ``nano-defect'' domains, which could limit the Li+ diffusion inside the grains. More prominently, the layered-to-spinel transition in the form of large spinel grains (Fd (3) over barm), hundreds of nanometers across, is discovered, and their detailed atomic arrangement is studied. The findings suggest that, instead of attributing the overall capacity fade to the surface degradation, these drastic bulk evolutions would be the main degradation mechanisms at the source of the rapid failure of Li-rich cathodes.
Visualization of Steady-State Ionic Concentration Profiles Formed in Electrolytes during Li-Ion Battery Operation and Determination of Mass-Transport Properties by in Situ Magnetic Resonance Imaging: Accurate modeling of Li-ion batteries performance, particularly during the transient conditions experienced in automotive applications, requires knowledge of electrolyte transport properties (ionic conductivity kappa, salt diffusivity 0 Salt diffusivity D, and lithium ion transference number t(+)) over a wide range of salt concentrations and temperatures. While specific conductivity data can be easily obtained with modern computerized instrumentation, this is not the case for D and t(+). A combination of NMR and MRI techniques was used to solve the problem. The main advantage of such an approach over classical electrochemical methods is its ability to provide spatially resolved details regarding the chemical and dynamic features of charged species in solution, hence the ability to present a more accurate characterization of processes in an electrolyte under operational conditions. We demonstrate herein data on ion transport properties (D and t(+)) of concentrated LiPF6 solutions in a binary ethylene carbonate (EC) dimethyl carbonate (DMC) 1:1 v/v solvent mixture, obtained by the proposed technique. The buildup of steady-state (time-invariant) ion concentration profiles during galvanostatic experiments with graphite lithium metal cells containing the electrolyte was monitored by pure phase-encoding single point imaging MRI. We then derived the salt diffusivity and Li+ transference number over the salt concentration range 0.78-1.27 M from a pseudo-3D combined PFG-NMR and MRI technique. The results obtained with our novel methodology agree with those obtained by electrochemical methods, but in contrast to them, the concentration dependences of salt diffusivity and Li+ transference number were obtained simultaneously within the single in situ experiment.
Multi-Temperature in Situ Magnetic Resonance Imaging of Polarization and Salt Precipitation in Lithium-Ion Battery Electrolytes: Accurate electrochemical modeling of lithium-ion batteries is an important direction in the development of battery management systems in automotive applications, for both real-time performance control and long-term state-of-health monitoring. Measurements of electrolyte-domain transport parameters, under a wide regime of temperature and current conditions, are a crucial aspect of the parametrization and validation of these models. This study constitutes the first exploration of the temperature dependence for the steady-state electrolyte concentration gradient under applied current with spatial resolution via the in situ magnetic resonance imaging (MRI) technique. The use of complementary pure phase-encoding MRI methods was found to provide quantitatively accurate measurements of the concentration gradient, in strong agreement with predictions based on ex situ NMR and electrochemical techniques. Temperature is demonstrated to have a marked influence on the steady-state concentration gradient as well as the rate of its buildup. This finding underlines the importance of utilizing spatially varying electrolyte transport parameters in modeling approaches. Additionally, a surprising outcome of this investigation was that a conventional 1.00 M LiPF6 electrolyte in an equal-parts ethylene carbonate/diethylene carbonate solvent mixture generated salt precipitation under polarization at 10 degrees C. The loss of salt under strong polarization and at low temperature is a previously unaddressed potential source of long-term capacity fade in lithium-ion batteries, and on the basis of this result, warrants further investigation.
Complete description of the interactions of a quadrupolar nucleus with a radiofrequency field. Implications for data fitting: We present a theory, with experimental tests, that treats exactly the effect of radiofrequency (RF) fields on quadrupolar nuclei, yet retains the symbolic expressions as much as possible. This provides a mathematical model of these interactions that can be easily connected to state-of-the-art optimization methods, so that chemically-important parameters can be extracted from fits to experimental data. Nuclei with spins >1/2 typically experience a Zeeman interaction with the (possibly anisotropic) local static field, a quadrupole interaction and are manipulated with RF fields. Since RF fields are limited by hardware, they seldom dominate the other interactions of these nuclei and so the spectra show unusual dependence on the pulse width used. The theory is tested with Na-23 NMR nutation spectra of a single crystal of sodium nitrate, in which the RF is comparable with the quadrupole coupling and is not necessarily on resonance with any of the transitions. Both the intensity and phase of all three transitions are followed as a function of flip angle. This provides a more rigorous trial than a powder sample where many of the details are averaged out. The formalism is based on a symbolic approach which encompasses all the published results, yet is easily implemented numerically, since no explicit spin operators or their commutators are needed. The classic perturbation results are also easily derived. There are no restrictions or assumptions on the spin of the nucleus or the relative sizes of the interactions, so the results are completely general, going beyond the standard first-order treatments in the literature. (C) 2013 Elsevier Inc. All rights reserved.
Monitoring the Electrochemical Processes in the Lithium-Air Battery by Solid State NMR Spectroscopy: A multi-nuclear solid-state NMR approach is employed to investigate the lithium-air battery, to monitor the evolution of the electrochemical products formed during cycling, and to gain insight into processes affecting capacity fading. While lithium peroxide is identified by O-17 solid state NMR (ssNMR) as the predominant product in the first discharge in 1,2-dimethoxyethane (DME) based electrolytes, it reacts with the carbon cathode surface to form carbonate during the charging process. C-13 ssNMR provides evidence for carbonate formation on the surface of the carbon cathode, the carbonate being removed at high charging voltages in the first cycle, but accumulating in later cycles. Small amounts of lithium hydroxide and formate are also detected in discharged cathodes and while the hydroxide formation is reversible, the formate persists and accumulates in the cathode upon further cycling. The results indicate that the rechargeability of the battery is limited by both the electrolyte and the carbon cathode stability. The utility of ssNMR spectroscopy in directly detecting product formation and decomposition within the battery is demonstrated, a necessary step in the assessment of new electrolytes, catalysts, and cathode materials for the development of a viable lithium-oxygen battery.
Spatially resolved surface valence gradient and structural transformation of lithium transition metal oxides in lithium-ion batteries: Layered lithium transition metal oxides are one of the most important types of cathode materials in lithium-ion batteries (LIBs) that possess high capacity and relatively low cost. Nevertheless, these layered cathode materials suffer structural changes during electrochemical cycling that could adversely affect the battery performance. Clear explanations of the cathode degradation process and its initiation, however, are still under debate and not yet fully understood. We herein systematically investigate the chemical evolution and structural transformation of the LiNixMnyCo1-x-yO2 (NMC) cathode material in order to understand the battery performance deterioration driven by the cathode degradation upon cycling. Using high-resolution electron energy loss spectroscopy (HR-EELS) we clarify the role of transition metals in the charge compensation mechanism, particularly the controversial Ni2+ (active) and Co3+ (stable) ions, at different states-of-charge (SOC) under 4.6 V operation voltage. The cathode evolution is studied in detail from the first-charge to long-term cycling using complementary diagnostic tools. With the bulk sensitive Li-7 nuclear magnetic resonance (NMR) measurements, we show that the local ordering of transition metal and Li layers (R (3) over barm structure) is well retained in the bulk material upon cycling. In complement to the bulk measurements, we locally probe the valence state distribution of cations and the surface structure of NMC particles using EELS and scanning transmission electron microscopy (STEM). The results reveal that the surface evolution of NMC is initiated in the first-charging step with a surface reduction layer formed at the particle surface. The NMC surface undergoes phase transformation from the layered structure to a poor electronic and ionic conducting transition-metal oxide rock-salt phase (R (3) over barm -> Fm (3) over barm), accompanied by irreversible lithium and oxygen loss. In addition to the electrochemical cycling effect, electrolyte exposure also shows non-negligible influence on cathode surface degradation. These chemical and structural changes of the NMC cathode could contribute to the first-cycle coulombic inefficiency, restrict the charge transfer characteristics and ultimately impact the cell capacity.
A solid-state NMR study of hydrogen-bonding networks and ion dynamics in benzimidazole salts: On the basis of our solid-state NMR characterization of dynamics in two model salts, we draw the analogy to the fuel cell membrane candidate, phosphoric acid-doped poly(benzimidazole), and conclude that phosphate anion dynamics contribute to long-range proton transport, whereas the mobility of the polymer itself is not a contributing factor. This is contrasted with emerging membrane candidates, which rely on fully covalently bonded acid donors and acceptors, and target high-temperature PEM fuel cell operation in the absence of liquid electrolyte. The hydrogen-bonding structures of benzimidazolium phosphate and benzimidazolium methane phosphonate are established using X-ray diffraction paired with solid-state H-1 DQF NMR. By comparing the dynamics of the phosphate and methane phosphonate anions with the dynamics of imidazolium and benzimidazolium cations, the relative importance of these processes in proton transport is determined. The imidazolium cation is known to undergo two-site ring reorientation on the millisecond time scale. In contrast, it is shown here that the benzimidazolium rings are immobile in analogous salts, on a time scale extending into the tens of seconds. Therefore, we look to the phosphate anions and demonstrate that the time scale of tetrahedral reorientation is comparably fast (50 ms). Moreover, the P-31 CODEX NMR data clearly indicate a four-site jump process. In contrast, the methane phosphonate undergoes a three-site jump on a slower time scale (75 ms). A mechanism for a zigzag pathway of proton transport through the phosphonate salt crystallites is developed based on the P-31 CODEX and H-1 variable-temperature MAS NMR data.
Combined NMR and molecular dynamics modeling study of transport properties in sulfonamide based deep eutectic lithium electrolytes: LiTFSI based binary systems: The trend toward Li-ion batteries operating at increased (>4.3 V vs. Li/Li+) voltages requires the development of novel classes of lithium electrolytes with electrochemical stability windows exceeding those of LiPF6/carbonate electrolyte solutions. Several new classes of electrolytes have been synthesized and investigated over the past decade, in the search for LIB electrolytes with improved properties (increased hydrolytic stability, improved thermal abuse tolerance, higher oxidation voltages, etc.) compared with the present state-of-the-art LiPF6 and organic carbonates-based formulations. Among these are deep eutectic electrolytes (DEEs), which share many beneficial characteristics with ionic liquids, such as low vapor pressure and large electrochemical stability windows, with the added advantage of a significantly higher lithium transference number. The present work presents the pulsed field gradient NMR characterization of the transport properties (diffusion coefficients and cation transport numbers) of binary DEEs consisting of a sulfonamide solvent and lithium bis(trifluoromethanesulfonyl)imide salt. Insights into the structural and dynamical properties, which enable one to rationalize the observed ionic conductivity behavior were obtained from a combination of NMR data and MD simulations. The insights thus gained should assist the formulation of novel DEEs with improved properties for LIB applications.
Reorientation phenomena in imidazolium methyl sulfonate as probed by advanced solid-state NMR: Evidence for reorientation of imidazolium rings in imidazolium methylsulfonate is demonstrated using solid-state NMR. This material is a model system for exciting new proton-conducting materials based on imidazole. Two advanced NMR methods, including H-1-C-13 and H-1-N-15 recoupled polarization transfer with dipolar sideband pattern analysis and analysis of the coalescence of C-13 lineshapes are used to characterize the ring reorientation. The process is found to occur at temperatures well below the melting point of the salt, between 240 and 380 K, and is described by a single activation energy, of 38 +/- 5 kJ/ mol. This material is considered as a model system for quantifying the ring reorientation process, which is often proposed to be the rate-limiting step in proton transport in imidazole-based proton conducting materials. (C) 2003 Elsevier Inc. All rights reserved.
Bayesian Uncertainty Quantification in Inverse Modelling of Electrochemical Systems: This study proposes a novel approach to quantifying uncertainties of constitutive relations inferred from noisy experimental data using inverse modelling. We focus on electrochemical systems in which charged species (e.g., Lithium ions) are transported in electrolyte solutions under an applied current. Such systems are typically described by the Planck-Nernst equation in which the unknown material properties are the diffusion coefficient and the transference number assumed constant or concentration-dependent. These material properties can be optimally reconstructed from time- and space-resolved concentration profiles measured during experiments using the Magnetic Resonance Imaging (MRI) technique. However, since the measurement data is usually noisy, it is important to quantify how the presence of noise affects the uncertainty of the reconstructed material properties. We address this problem by developing a state-of-the-art Bayesian approach to uncertainty quantification in which the reconstructed material properties are recast in terms of probability distributions, allowing us to rigorously determine suitable confidence intervals. The proposed approach is first thoroughly validated using "manufactured" data exhibiting the expected behavior as the magnitude of noise is varied. Then, this approach is applied to quantify the uncertainty of the diffusion coefficient and the transference number reconstructed from experimental data revealing interesting insights.
Elucidating the Li-Ion Battery Performance Benefits Enabled by Multifunctional Separators: The dissolution of transition metal ions from positive electrodes and loss of (both electroactive and transport) Li + ions seriously impair the durability of lithium ion batteries. We show herein that the improvement in the cycle life of lithium manganate spinel-graphite cells effected by multifunctional separators results from smaller interfacial resistances at both positive and negative electrodes, that can in turn be traced back to thinner, more uniform, and chemically different surface films, due to lessened parasitic reactions and a decreased accumulation of parasitic reaction products at electrode surfaces, as evidenced by HR-SEM, FIB-SEM, EDX, 19 F MAS NMR, and ICP-OES data.
A Magnetic Resonance and Electrochemical Study of the Role of Polymer Mobility in Supporting Hydrogen Transport in Perfluorosulfonic Acid Membranes: Perfluorosulfonic acid (PFSA) materials have been used in polymer electrolyte membrane fuel cells (PEMFCs) as electrolyte materials due to their mechanical durability and high proton conductivity. To understand the fundamental chemistry at a molecular level in material performance properties, we have developed and validated method for evaluating local dynamics using 19 F double-quantum solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The local dynamics information can be separated and analyzed in terms of fluorine interactions with respect to the different temperatures and hydration levels. The polymer side chain is proven to be more locally mobile which is reflected by the lower apparent dipolar coupling constant ( D app ) compared to the backbone. This observation agrees with the micro-phase separation morphology evolution. In the current study, different types of PFSA materials were explored and compared. The dynamics investigation of the PFSA materials has been conducted at various conditions. In operando membrane performance analyses were performed in parallel at Ballard Power Systems. PFSA membranes were prepared into membrane electrode assemblies (MEAs), with catalyst layers and gas diffusion layers. From the cyclic voltammetry measurements, the H 2 crossover values were extracted. These data reveal a strong correlation between the proton conductivity and the site-specific PFSA side chain local dynamics. Moreover, a correlation was drawn between increasing side chain mobility (lower D app ), and increased H 2 permeability. The link between the fundamental dynamics study and this key PFSA performance analysis provides insight into proton transport mechanisms.
Porous cellulosic substrates for lithium ion battery electrodes: An electrode material for an electrochemical cell is provided. The electrode includes a porous hydrophilic substrate, an electroactive material, and a binder. The porous hydrophilic substrate includes a plurality of voids and may be formed from cellulose or cellulosic derivative material. The electroactive material is dispersed in at least a portion of the voids of the hydrophilic substrate. In other aspects, another electrode material for an electrochemical cell is provided. The electrode includes a porous hydrophilic substrate, an electroactive material, an electrically conductive particle, and a binder. The porous hydrophilic substrate includes a plurality of voids and may be formed from cellulose or cellulosic derivative material. The electroactive material and the electrically conductive particle are dispersed in at least a portion of the voids of the hydrophilic substrate. In still other aspects, the porous hydrophilic substrate comprises a coating that is electrically conductive.
Operando Mapping of Li Concentration Profiles and Phase Transformations in Graphite Electrodes by Magnetic Resonance Imaging and Nuclear Magnetic Resonance Spectroscopy: An innovative combined magnetic resonance imaging and nuclear magnetic resonance methodology, which enables the visualization and spatially resolved quantification of the lithiation/delithiation process in porous Li-ion battery electrodes in real time, is reported. We demonstrate that polarization of the thick graphite electrode correlates with the appearance of energy barriers during the graphite phase transformations and the resulting significant reduction of the lithium chemical diffusion, which must be addressed in any attempt at the fast charging of Li-ion batteries. It is also shown that a portion of the transported Li+ is temporarily stored in the electrode surface film prior to its intercalation into graphite, indicating reversible lithium storage within the surface electrolyte interface. The inclusion of a short current reversal, which could be considered as part of a shaped charging process, facilitates the complete lithiation of thick graphite electrodes, an attractive strategy for increasing usable cell capacities.
Bayesian uncertainty quantification in inverse modeling of electrochemical systems
Multifunctional Separators: A Promising Approach for Improving the Durability and Performance of Li-Ion Batteries: Electrified vehicles require Li-ion batteries (LIBs) with 10-year useful life. We review herein our progress since 2016 in the understanding of dissolved Mn species and LIB performance degradation by multifunctional materials. Multifunctional separators (MFSs), can trap Mn cations, scavenge acid species, and/or dispense alkali metal ions, with significant battery performance benefits: increased capacity retention during electrochemical cycling and improved rate performance. Cells with LiMn2O4 (LMO), LiNi0.6Mn0.2Co0.2O2 (NCM622), or LiNi0.5Mn1.5O4 (LNMO) positive electrodes, graphite negative electrodes, and LiPF6/mixed organic carbonate solutions were investigated. XRD, ICP-OES, XANES, HR-SEM, FIB-SEM, and MAS-NMR on harvested cell components complemented and aided the interpretation of electrochemical test results. While in LIBs with positive electrodes affected by Mn dissolution cell performance improvements can be enabled by delaying and then impeding the deposition of transition metal (TM) ions deposition at negative electrodes through their trapping by MFS, acid scavenging separators provide a more general approach that can enable performance benefits in cells with non-spinel positive electrode materials such as NCM622, as well as with 5 V class positive electrode materials such as LNMO. We illustrate our discussion with both previously published and new data. We conclude with a discussion of remaining challenges, new opportunities, and recommendations for future work.
Optimization of a parallel-plate Rf probe for high resolution thin film imaging

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