Structure solution of metal-oxide Li battery cathodes from simulated annealing and lithium NMR spectroscopy

Discerning the arrangement of transition metal atoms in Li­[Ni_<i>x</i>Mn_<i>y</i>Co_<i>z</i>]­O₂ 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[Li_1/3Mn_2/3]­O₂ and produces structures of the disordered Li­[Ni_<i>x</i>Mn_<i>x</i>Co_{1–2<i>x</i>}]­O₂ phases (where <i>x</i> = 0.02, 0.1, 0.33) that are verified by detailed ⁷Li NMR spectra. For each Li­[Ni_<i>x</i>Mn_<i>x</i>Co_{1–2<i>x</i>}]­O₂ 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

A Polymer-Rich Quaternary Composite Solid Electrolyte for Lithium Batteries

All-solid-state batteries continue to grow as an alternative to replace the traditional liquid-based ones not only because they provide increased safety but also higher power and energy densities. However, current solid-state electrolytes are either ceramics that are brittle but highly conducting (e.g. Li0.33La0.55TiO3, LLTO) or polymer electrolytes that are poorly conducting but form flexible films with desired mechanical properties (e.g. Poly(ethylene oxide):Lithium bis(trifluoromethanesulfonyl)imide, PEO:LiTFSI). In this work, we have developed quaternary composite solid-state electrolytes (CSEs) to combine the benefits of the two types along with Succinonitrile (SN) as a solid plasticizer. CSEs with different compositions have been fully characterized over the whole compositional range. Guided by neural network simulation results it has been found that a polymer-rich CSE film gives the optimal ionic conductivity (>10−3 S cm−1 at 55 °C) and mechanical properties (Tensile strength of 16.1 MPa; Elongation-at-break of 2360%). Our solid-state coin-type cell which employs our in-house made cathode shows good cycling performance at C/20 and 55 °C maintaining specific discharge capacity at 143.2 mAh g−1 after 30 cycles. This new approach of formulating quaternary CSEs is proven to give the best combination of properties and should be universal and be applied to other CSEs with different chemistry

<i>Ex Situ</i>²³Na 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 <i>ex situ</i> ²³Na 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 ²³Na 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 ∼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 ²³Na 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

Differentiating Lithium Ion Hopping Rates in Vanadium Phosphate versus Vanadium Fluorophosphate Structures Using 1D ⁶Li 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)₂F₂, using ⁶Li selective inversion (SI) NMR measurements. Li₅V­(PO₄)₂F₂ has six crystallographically distinct lithium sites giving the possibility of fifteen exchange partners between nonequivalent lithium environments. Here, ⁶Li 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 α-Li₃V₂(PO₄)₃ and α-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

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. Multinuclear 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. ¹³C 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 ¹H 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

Structure and ionic mobility of anti-perovskite compounds as solid-state electrolytes

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Synthèse et caractérisation de conducteurs ioniques Li3-xHxOCl de structure anti-pérovskite

International audienceLe remplacement de l’électrolyte liquide utilisé dans les accumulateurs Li-ion actuels .par un électrolyte solide ininflammable et plus stable (électro)chimiquement constitue une voie privilégiée pour l’amélioration de la sécurité des batteries..

Synthèse et caractérisation de conducteurs ioniques Li3-xHxOCl de structure anti-pérovskite

International audienceLe remplacement de l’électrolyte liquide utilisé dans les accumulateurs Li-ion actuels .par un électrolyte solide ininflammable et plus stable (électro)chimiquement constitue une voie privilégiée pour l’amélioration de la sécurité des batteries..

¹⁹F 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, ¹H 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. ¹⁹F 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 ¹⁹F–¹⁹F 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 β-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