Investigation on the Overlithiation Mechanism of LiCoO2 Cathode for Lithium Ion Batteries

Overlithiation of lithium ion batteries often causes a structural transformation of the electrode and capacity degradation and may even lead to severe safety problems. In this study, the electrode structure, surface morphology and compositions at the different overlithiation depths of LiCoO 2 cathode material were investigated in detailed by examining the LCO/Li cells and anode-free cells, combined with post-mortem characterizations. When LiCoO 2 is found in a slight overlithiation state, the cycle capacity fades slightly and minority of LiCoO 2 reduces to CoO and Li 2 O. After LiCoO 2 undergoes a deep overlithiation reaction, a long flat voltage plateau appears at 1.2 V and the cycle performance is greatly deteriorated. SEM and EIS tests show that the single-crystal particles are fractured into small particles with thick-layer formation at the surface and the shape of the EIS curve changes strongly with increasing overlithiation depth. Additionally, as shown by XPS Ar-ion etching and TEM characterization, the LiCoO 2 surface is converted into greater amounts of Li 2 O, CoO and Co metal associated with the severe electrolyte decomposition, and LiF, Li 2 CO 3 and Li x PO y F z are the main electrolyte decomposition products. The detailed investigation of the overlithiation mechanism of LiCoO 2 provides comprehensive and thorough guidance for understanding the overlithiation mechanism of cathode materials as well as important reference information for practical all-solid-state battery applications.