The work performed for this project was carried out at the Department of Materials at the University of Oxford (UK). The focus has been on investigating how interfaces in battery cells behave during electrochemical cycling. X-ray spectroscopy together with modelling of the spectra as well as modelling of the electrochemistry have been used to study the side reactions taking place at the electrodes. This has included a study where the changes in degradation throughout the lifetime of the cells were investigated. It was found that the main source of capacity loss during the first 200 cycles in NMC811-graphite cells came from side reactions at the graphite electrode. However the side reactions cause a slippage of the potential profiles i.e. the graphite potential profile is shifted relative to the NMC’s so that the potential of both the NMC and the graphite becomes larger at end of charge. The increase in NMC electrode potential contribute to further degradation due to active material loss, transition metal dissolution as well as the reduction of transition metals at the NMC surface. The transition metal reduction is likely connected to the increase in impedance. Also the incorporation of transition metals into the SEI increases with cycle number and the ratio of transition metals does not follow the NMC stoichiometry, which has previously been assumed. Instead the Mn is much more prevalent during the initial cycles, but with the ratio of Ni increasing with cycle number. As the transition metals are incorporated into the SEI, the chemical surrounding around Ni changes.
Operando soft x-ray XAS was used to detect the SEI formation process on amorphous Si electrodes as a function of electrode potential using both a standard LP30 electrolyte and an LP30 electrolyte also containing the fluoroethylene carbonate FEC additive. The cells used in the study contained a silicon nitride membrane (100 nm thick) coated by a 20 nm Ni coating which in turn was coated with amorphous Si, which was thin enough to allow x-ray transparency. By using a modulated x-ray beam together with a lock in amplifier the total electron yield (TEY) could be extracted while the cell was held at an applied potential. It was possible to detect the formation of LiF and organic species as the electrode was lithiated.
NMC cathodes containing different concentrations of Ni were investigated how they reacted with common electrolyte solvents. It was found that the amount of lattice oxygen loss from the NMC depended on both the Ni concentration and the electrolyte solvent. Ni-rich NMCs were more prone to lattice oxygen release, especially in contact with ethylene carbonate (EC). The increased lattice oxygen release was found to be connected to higher cathode impedance, electrolyte decomposition and transition metal dissolution.
Direct recycling of NMC cathodes have been attempted through cell disassembly, dissolution of the cathode material in acetone followed by filtering and drying befor annealing the material together with lithium carbonate in a tube furnace under flowing oxygen atmosphere. After the recycling process the surface of the NMC particles remained reduced, indicating further steps are required for efficient recycling.
The reaction of NMC with various gasses was investigated to find out how the electrodes are affected by air exposure. The electrode material was sealed in pouches into which CO2 and or H2O was injected to allow differentiation of these with low level of contaminations. It was found that the NMC surface had little reactions with H2O, although the Ni became reduced. CO2 on the other hand caused more severe reactions leading to the formation of carbonates and the combination of H2O and CO2 lead to even more reactions. Photoemission electron microscopy was used to correlate the formation of carbonates with Ni reduction.
