An investigation of transition metal and oxygen redox in lithium-rich cathode materials

Cathode materials remain one of the main limiting factors for increasing the specific energy of lithium ion batteries. The reversible storage of charge on lattice oxygen, called oxygen redox (O redox), has been identified as a promising route for addressing the problem in layered and disordered rocksalt materials. Currently, several interrelated challenges, including slow kinetics, voltage and capacity fade, as well as structural changes, remain, that are investigated in this thesis. A dedicated operando cell, capable of applying significant stack pressure, was built to investigate layered lithium-rich materials. It is used to study Li1.2Ni0.13Co0.13Mn0.54O2 and Li1.2Ni0.2Mn0.6O2 using high energy-resolution fluorescence-detected X-ray absorption near edge structure (HERFD-XANES), as well as X-ray emission spectroscopy (XES). The complex interplay between bulk oxygen and transition-metal redox is analysed, and it is demonstrated that Ni is fully oxidised to Ni4+ in one compound but not the other. The structure of the disordered rocksalt Li2MnO2F is solved using a combination of neutron pair distribution function (PDF) and neutron diffraction, simultaneously fit using a Reverse Monte Carlo (RMC) approach. The capacity fade for the same material is investigated and found to be initially based on a large charge-transfer resistance, associated with the slow O redox kinetics on discharge.