LiCoO2 Epitaxial Film Enabling Precise Analysis of Interfacial DegradationsSupported by the National Natural Science Fund for Innovative Research Groups (China) (Grant No. 51621003), the National Key Research and Development Program of China (Grant No. 2016Yu7FB0700700), the Beijing Municipal Fund for Scientific Innovation (Grant No. PXM2019 014204 500031) and the Beijing Municipal High Level Innovative Team Building Program (Grant No. IDHT20190503).

Abstract

Interfacial structure evolution and degradation are critical to the electrochemical performance of LiCoO2 (LCO), the most widely studied and used cathode material in lithium ion batteries. To understand such processes requires precise and quantitative measurements. Herein, we use well-defined epitaxial LCO thin films to reveal the interfacial degradation mechanisms. Through our systematical investigations, we find that surface corrosion is significant after forming the surface phase transition layer, and the cathode electrolyte interphase (CEI) has a double layer structure, an inorganic inner layer containing CoO, LiF, LiOH/Li2O and Li x PF y O z , and an outmost layer containing Li2CO3 and organic carbonaceous components. Furthermore, surface cracks are found to be pronounced due to mechanical failures and chemical etching. This work demonstrates a model material to realize the precise measurements of LCO interfacial degradations, which deepens our understanding on the interfacial degradation mechanisms.