UC San Diego

The rapid growth of the electric vehicle market requires the development of Li-ion batteries (LIBs) with higher energy density and longer cycle life. The classical layered nickel, manganese, and cobalt oxides (NMC) and lithium-rich layered oxides (LRLO) have attracted great interest as high-energy LIB cathode materials due to their high theoretical capacity. However, their inherent structure instability at the highly-delithiated state and the electrolyte degradation induced at high voltage cause cell degradation as cycling proceeds. In this thesis, different degradation mechanisms and the corresponding mitigating strategies are studied for both NMC and LRLO materials. Firstly, twin boundary defect engineering was adopted in a series of NMC cathodes to improve the structure and cycling stability. The radially aligned twin boundaries with the formation of rocksalt-like phase along the boundaries are observed through STEM, acting as a rigid framework that mitigates the anisotropic changes during charge and discharge, as confirmed by operando XRD. The reduced microcrack formation is also confirmed by FIB and SEM. Secondly, an in-depth understanding of the heat treatment induced structure and voltage recovery in cycled LRLO is provided. The transition metal layer reordering is identified as the key factor under the structure recovery of degraded LRLO. The reappearance of the honeycomb superlattice during heat treatment is captured through NPD, PDF, and EXAFS. In addition, an ambient-air relithiation combined with heat treatment is proved to effectively recover both the voltage and capacity of cycled LRLO. Lastly, lithium bis-(oxalate)borate (LiBOB) is studied as an electrolyte additive in protecting cathode-electrolyte interphase (CEI) from hydrofluoric acid (HF) corrosion induced by electrolyte decomposition at high voltage. Analytical EM under cryo-condition confirms the formation of a uniform CEI and less phase transformation on the LRLO particle surface. The formation of B-F species is identified in the cycled electrolyte with NMR, elucidating the HF scavenger effect of LiBOB. Due to less HF corrosion on both CEI and SEI, a reduced amount of transition metal dissolution and redeposition has been proved by EDX and XPS. The prevention of cell crosstalk thereby mitigates the capacity decay in LRLO/graphite full cells.