In Situ Visualized Cathode Electrolyte Interphase on LiCoO₂ in High Voltage Cycling

Charging lithium ion battery cathode materials such as LiCoO₂ to a higher voltage may simultaneously enhance the specific capacity and average operating voltage and thus improve the energy density. However, battery cycle life is compromised in high voltage cycling due to lattice instability and undesired oxidation of electrolyte. Cathode solid-electrolyte interphase (SEI), or cathode-electrolyte interphase (CEI), in situ formed at the cathode-electrolyte interface under high voltage, is critically important in understanding the cathode degradation process and crucial in improving high voltage cycle stability. Here we present in situ atomic force microscopy (AFM) investigation of CEI on LiCoO₂ at high voltage. The formation of CEI is only observed at the LiCoO₂ edge plane, not at the basal plane. The thin layer of Al₂O₃ coating completely suppresses the formation of CEI at the edge planes, and is shown to significantly improve coin cell high voltage cycle stability.