A review of interfaces within solid-state electrolytes: fundamentals, issues and advancements

All-solid-state lithium batteries (ASSLBs) have attracted worldwide attention due to excellent safety, wide electrochemical window and high energy density compared with liquid electrolyte (LE) systems. However, the solid nature of solid-state electrolytes (SSEs) results in high impedance during Li+ diffusion, especially on the electrode/electrolyte interface. Thus, many reported Reviews focus on the electrode/SSE interface, but seldom on the interfaces in SSEs themselves. Generally, the interface within SSEs also possesses sluggish ionic conductivity as compared to LE system, which is regarded as a key factor in developing ASSLBs. In this review, interface issues between the grain particles (grain boundaries (GBs)) in inorganic SSEs, along the crystal particles in organic SSEs, and between different phases in composite SSEs (CSSEs) are first discussed from the perspective of physical and electro/chemical aspects based on Li+ transport mechanism. The recent advancements in modifying these interfaces properties by tailoring the fabricating and processing method, interface engineering, structure design and regulation are also discussed as the focus. Finally, the main challenges on current demands and future development of high-performance SSEs and ASSLBs are prospected.