Understanding the interfacial reactions of LiCoO2 positive electrodes in aqueous lithium-ion batteries

Abstract

Aqueous lithium-ion batteries (LIBs) have been highlighted as being applied for low-cost and safe energy storage. However, a conventional positive electrode used in LIBs, representatively layered LiCoO₂ (LCO), has low compatibility with water. Reports on the interfacial reactions in aqueous LIBs are superficial, which means that finding suitable methods to improve cell stability is a challenge. In this study, we investigated the interfacial degradation of the LCO electrode using various pH conditions and surface and bulk X-ray spectroscopic analyses. We found that the insertion of protons (H⁺) into the LCO surface caused distortion of the local LCO structure and reduced lithiation efficiency, which significantly impacted the cell performance. In addition, the absence of a protective layer for LCO expedites capacity fading. We showed that a lithiated Nafion layer coating the LCO electrode provided both a hydrophobic matrix and an ion channel domain. This polymeric layer delayed capacity fading by limiting the movement of water, suggesting the need for a tolerant ion channel that controls water inflow.