Basic Concept of Electrolyte Protection for Stable Operation of All-Solid-State Batteries—From the Perspective of Solid State Ionics

It is known that many of lithium-ion-conducting solid electrolytes used for all-solid-state batteries (ASSBs) do not have a wide enough electrochemical potential window to be thermodynamically stable against both positive and negative electrodes. Therefore, for stable operation of ASSBs, it is necessary to suppress decomposition of the electrolyte, for example, by inserting a ‘coating layer’ at the interface with the electrode. In fact, ASSBs often use electrode active materials coated with different kinds of oxide, and at least one function of this coating layer is thought to ensure thermodynamic stability of the electrolyte. However, basic design guidelines for the coating layer have not been well-established yet. In this chapter, we discuss how we can predict the properties and conditions required for the coating layer from viewpoints of the thermodynamic compatibility of the electrolyte and electrode active material.