Li-ion batteries (LIBs) are being widely used in various consumer electronics applications and battery electric vehicles. In current and next-generation LIBs, particularly Ni-rich NCMs (e.g. Li_(1+x)[Ni_(0.6)Co_(0.2)Mn_(0.2)]_(1-x)O_2) are considered as a promising class of cathode active materials (CAMs) due to their high specific capacities, their high power and energy densities, and their good structural stability. In order to meet cycle-life requirements and to maintain a safe state of operation, LIBs are only cycled within a specific potential range, significantly limiting the usage of the specific capacity achievable with these materials. A major problem for NCMs are over-charge and/or harsh cycling conditions, which lead to highly aggravated capacity fading and ultimately to cell-failure. One fundamental degradation mechanism for NCM materials is the deleterious oxygen evolution from the NCM surface at high degrees of delithiation (>80% state of charge, SOC), which results in the formation of an oxygen-depleted spinel and/or rock salt phase on the NCM particle surface as well as in a concomitant oxidation of the electrolyte. A thorough understanding of the exact structures of the O-depleted phases produced by lattice oxygen release from the NCM materials and of their effects on the cycling performance of LIBs are still missing, which hampers the development of countermeasures. In the literature, oxygen release from partially delithiated NCMs has been frequently studied by thermal treatment, showing that with increasing Ni content the temperature at which oxygen is released decreases while the amount of released oxygen increases.
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