Protective coatings on cathode active materials have become paramount for the implementation of solid-state batteries; however, the development of coatings lacks the understanding of the necessary coating properties. In this study, guidelines for the design of solid electrolytes and electrode coatings in all-solid-state batteries are proposed from the viewpoint of the steady-state Li chemical potential profile across the battery cell. The model calculation of the (electro)chemical potential profile in all-solid-state batteries is established by considering the steady-state mixed ionic and electronic conduction in the solid electrolyte under the assumption of local equilibrium. For quantitative discussion, the potential profiles within oxygen ion conductors are calculated instead of Li/Na ion conductors as their partial electronic conductivities have not been reported so far in sufficient detail. Based on the calculated chemical potential profile, two main conclusions are obtained: (1) the decisive factor for the formation of the chemical potential profile of the neutral mobile component (e.g., oxygen or lithium) in the solid electrolyte is its electronic conductivity (and the activity dependence), and (2) a particularly large potential drop is formed in a region where the electronic conductivity becomes small. While these conclusions are valid and general for any solid electrolyte device, they are particularly important for the design of protective coatings and the understanding of the functionality of self-assembled solid electrolyte interphases in all-solid-state batteries. To protect the solid electrolyte from decomposition by reduction/oxidation at the anode/cathode interfaces, a sufficient chemical potential drop is necessary within the coating layer or directly at the interphase layer. To achieve this situation, the coating/interphase materials need to have a lower electronic conductivity than the solid electrolyte.
Keywords: all-solid-state batteries; buffer layer; cathode coating; potential distribution; potential profile.