Origins of capacity and voltage fading of LiCoO2 upon high voltage cycling

Abstract

Enhancing the high voltage cycling stability is critical for reviving LiCoO₂-based layered cathodes, and this requires in-depth understanding of various degradation mechanisms and their contributions to performance decay. Herein, we conduct systematic investigations into the failure mechanisms of LiCoO₂ upon high voltage cycling and reveal that the capacity loss is mainly due to bulk structure degradation and the voltage fading is mainly due to the blocking effect of the cathode/electrolyte interphase (CEI) layer. Furthermore, as the novel points of our characterization, we find that (i) the LiCoO₂ grains are in a highly delithiated state after high voltage cycling, causing poor thermal stability; (ii) the stripe-shaped O1 phase is verified in the grain bulk, demonstrating irreversible structural changes upon high voltage cycling; and (iii) the surface phase transition layer, though it can be as thick as tens of nanometers, plays a minor role in the deterioration of LiCoO₂ performance. Our findings stress that bulk structure stability and the blocking effect of the CEI layer are the two major challenges for high voltage usage of LiCoO₂-based layered oxides.