High Energy Density Layered Cathodes for Lithium Ion Batteries: New Morphologies and New Compositions

Abstract

The road transport must be significantly electrified to meet the policy requirements regarding carbon dioxide emission in the next decade. The need has become unprecedentedly urgent in the context of rapid climate change. Serving as the mainstream power source for electric vehicles, the lithium ion batteries are widely recognised for their high energy density compared to other electrochemical energy storage systems. To be able to compete with the traditional internal combustion engines that consume fossil fuels, however, the energy density of lithium ion battery systems must be further increased. Another key performance metric is the time needed to charge a battery to pre-designed state of charge, known as the rate performance. One intuitive approach to realise higher rate performance is to reduce lithium ion diffusion path by downsizing active electrode material particles to nano metre scale, which often sacrifices the energy density due to lower tap density. The first part of this PhD thesis aims at synthesising nanorod shaped particles, which can potentially be densely packed at the electrode level by aligning and self-assembling. The aim is thus to profit from the short diffusion length and power density of nanoparticles while at the same time maintaining high energy density by organising the materials into a close packed structure. This work started from a classic cathode material LiNi0.33Mn0.33Co0.33O2, known as NMC111, then extended to higher Ni-containing formulations such as LiNi0.4Mn0.4Co0.2O2 (NMC442) and LiNi0.5Mn0.3Co0.2O2 (NMC532). The second contribution from this PhD focussed on the study of new cathode materials with intrinsic high energy density. More specifically, Li1.2Ni0.2Mn0.6O2 was studied, which shares a similar layered structure as the NMCs, but store more Li ions per unit cell and therefore offer higher energy densities. The commercialisation of this material is impeded, however, due primarily to several complex degradation processes, and the mechanisms behind are poorly understood. Our contribution here is a comprehensive degradation study with an array of both long range and short range ordering sensitive characterisation techniques such as synchrotron X-ray diffraction, X-ray absorption spectroscopy, 7Li magic angle spinning solid-state nuclear magnetic resonance, Raman spectroscopy and scanning transmission electron microscopy coupled with advanced electrochemical tests. We unambiguously show that degradation is not due to a bulk layered-to-spinel phase transformation, as proposed in some prior studies, but that several degradation processes in these materials, such as voltage fade are primarily attributed to the severe reduction of the particle surfaces. The discrepancies in elucidating degradation mechanisms of this class of materials in previous studies were largely resulted from different material synthesis approaches, particle morphology and electrochemical testing protocols. This problem was addressed by synthesising the same material via three routes (i.e. hydrothermal, microwave hydrothermal and ball milling) and testing under strictly controlled electrochemical protocols. It was found the materials show similar degradation patterns regardless of synthesis methods, hence shedding light on overarching degradation mechanisms. With deeper understanding of the surface driven degradation mechanisms in Li1.2Ni0.2Mn0.6O2, the third contribution from this thesis seeks to alleviate the degradation processes. In this case, surface-specific Nb doping with varying dopant concentrations was carried out for surface protection. In parallel, Nb bulk and host dopings of the same materials were also carried out as a part of a systematic study that reveals the differences in crystal structures and electrochemical performances. One key finding here is that dopant concentration must be kept at low levels in order not to trigger the formation of electronically and ionically insulating impurity phases that may offset the benefits brought by the doping itself. To further improve the electrochemical performance and alleviate the degradations of Li- and Mn-rich materials in the future, it is suggested to apply an integrated protection strategy, such as combining surface-specific doping, coatings, and use of advanced electrolytes where appropriate.