Multinuclear NMR Investigations of Local Structure, Distortions and Redox Mechanisms in Layered Lithium Ion Battery Cathode Materials
Creators: name: Reeves, Philip James
Publisher: name: Apollo - University of Cambridge Repository
Year: 2020
Subjects: Battery
batteries
NMR
Li ion
Multinuclear
cathode

description: Lithium ion (Li ion) battery technology has enabled a complete revolution in consumer electronics and is beginning to have a similar impact on transport. Increased adoption of electric vehicles is essential to reduce anthropogenic carbon emissions and combat climate change. For vehicular applications, improvements in specific and volumetric capacity are desirable, if they can be achieved without sacrificing safety, cost or cyclability. The redox mechanisms of many of the cathode materials with the highest capacities, particularly the “Li-excess” family, are poorly understood however, and such materials typically show accelerated degradation that makes commercial implementation impractical. Solid state NMR is a powerful tool to study local structure and ^{6/7}Li NMR has been used extensively to probe many aspects of local structure and dynamics in Li-ion batteries. Less commonly studied nuclei, such as ^{59}Co and ^{17}O can also offer complementary information, although their implementation and interpretation can be challenging. In this thesis the local structure, distortions and delithiation behaviours of two intriguing compounds are investigated: LiNi_{0.8}Co_{0.15}Al_{0.05}O_{2} (NCA)—a commercial cathode material with complex redox behaviour—and Li_2RuO_3—a model compound for the highly promising Li excess family of compounds. Firstly, the structure of pristine NCA is characterised. The complex dynamics of the Ni^{3+} Jahn-Teller (JT) distortion are probed using ^7Li, ^{17}O, ^{27}Al and ^{59}Co NMR spectroscopies and, by comparison with the expected statistical distribution of environments, a model emerges in which the JT distortions are dynamically disordered but the average structure is weighted towards thermodynamically favoured arrangements. This study is then extended to electrochemically delithiated NCA samples, which reveals enhanced Li mobility on delithiation from variable temperature (VT) ^7Li NMR measurements. Using an extension of the statistical model employed for the pristine material, the lineshapes are modelled and hopping rates for Li are estimated. At the onset of fast Li motion, two populations of Li are observed, indicating heterogeneous delithiation; this may suggest a kinetic origin for the reaction heterogeneity and poor first cycle coulombic efficiency observed in NCA. Finally, the redox mechanisms and electronic structure of NCA are investigated. ^{59}Co NMR reveals a population of Co^{3+} is present at the end of charge, demonstrating a deviation from the conventional cation redox model. The unintuitive evolution of the ^{59}Co peak position further reveals an evolution of the Co^{3+} electronic structure which is consistent with the observed long-range structural changes. Li^{2}RuO^{3} and its doped analogues are commonly employed as model compounds to understand redox mechanisms in Li excess cathode materials. Despite its single redox centre and well-ordered Ru-layer, ^7Li and ^{17}O NMR, along with magnetic susceptibility measurements confirm that that the Ru^{4+} ions form dimers and the effect of dimerisation on the observed ^{17}O NMR shifts is elucidated. The dimers, which are ordered at room temperature, lose their long-range ordering above 260 °C when Li_2RuO_3 undergoes a phase transition. This phase transition is characterised using laser-heated VT NMR and the changes observed, via ^7Li and ^{17}O NMR, reflect the changes in susceptibility and confirm the room temperature assignments. The changes in the structure, dimerisation of Ru and electronic structure in Li_2RuO_3 on delithiation are then investigated, which have implications for the understanding of the redox mechanisms in this highly studied compound and other Li-excess compounds. Notably, it is found that the Ru-dimerisation, which is highly influential in the pristine material, appears to remain throughout the discharging process.
description: I thank the North East Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award DE-SC0012583 for funding.