Degradation mechanisms of C₆/LiNi_0.5Mn_0.3Co_0.2O₂ Li-ion batteries unraveled by non-destructive and post-mortem methods

The ageing mechanisms of C₆/LiNi_0.5Mn_0.3Co_0.2O₂ batteries at various discharging currents and temperatures have systematically been investigated with electrochemical and post-mortem analyses. The irreversible capacity losses (ΔQ_ir) at various ageing conditions are calculated on the basis of regularly determined electromotive force (EMF) curves. Two stages can be distinguished for the degradation of the storage capacity at 30 °C. The first stage includes SEI formation, cathode dissolution, etc. The second stage is related to battery polarization. The various degradation mechanisms of the individual electrodes have been distinguished by dV_EMF/dQ vs Q_out and dV_EMF/dQ vs V plots. The Solid-Electrolyte-Interface (SEI) formation as well as the electrode degradation has been experimentally confirmed by XPS analyses. Both Ni and Mn elements are detected at the anode while Co is absent, indicating that the bonding of Co atoms is more robust in the cathode host structure. A Cathode-Electrolyte-Interface (CEI) layer is also detected at the cathode surface. The composition of the CEI layer includes Li salts, such as LiF, LiCOOR, as well as transition metal compounds like NiF₂. Cathode dissolution is considered to be responsible for both the NiF₂ detected at the cathode and Ni at the anode.