Degradation Mechanisms of C6/LiNi0.5Mn0.3Co0.2O2 Li-ion Batteries Unraveled by Non-destructive and Post-mortem Methods

The ageing mechanisms of C6/LiNi0.5Mn0.3Co0.2O2 batteries at various discharging currents and temperatures
have systematically been investigated with electrochemical and post-mortem analyses. The irreversible capacity
losses (ΔQir) 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 dVEMF/dQ vs Qout and dVEMF/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.