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 LiCoO 2 electrode to investigate how protons at the interface contribute to the degradation. Protons were introduced to the LiCoO 2 by exposing the surface to water vapor. Electrochemical, compositional, 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 LiCoO 2; 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.