Protons Inside the LiCoO2 Electrode Largely Increase Electrolyte-Electrode Interface Resistance in All-Solid-State Li Batteries

Formation of an electrolyte-electrode interface that allows smooth Li-ion transport is essential for the further development of all-solid -state Li batteries. Water vapor is recognized as one critical origin of increased resistance at the electrolyte-electrode interface. However, the detailed mechanism of the degradation remains unclarified. This study uses a thin-film battery with a LiCoO2 electrode to investigate how protons at the interface contribute to the degradation. Protons were introduced to the LiCoO2 by exposing the surface to water vapor. Electrochemical, composi-tional, and structural investigations reveal that the protons induce the mixing of Li and Co during the first charging process. The mixing induces the formation of low-temperature-phase LiCoO2; this layer acts as an interphase layer that degrades battery performance. Understanding these interfacial phenomena contributes to increasing the power density of all-solid-state Li batteries.