Unconventional solid-state electrolytes for lithium-based batteries: Recent advances and challenges

Compared to traditional lithium-ion batteries, solid-state batteries (SSBs) are characterized by higher safety and energy density, so the key component, solid-state electrolyte (SSE) has received considerable attention. The most common SSEs can be divided into three types: oxides, sulfides, and polymers. This paper mainly presents and discusses the latest research on unconventional solid-state electrolytes (USSEs) such as halides, zeolites, etc., focusing on their properties, structures, costs, synthesis methods, etc. Although SSEs such as halides are less well known, they are becoming a very hot topic due to their numerous advantages, such as: a) ionic conductivity at room temperature of more than 0.6 mS cm- 1; b) oxidation potential exceeding 4V, which means that they are more stable than most other types of SSEs; c) high stability in dry air (i.e., easier handling during the fabrication process), unlike sulfides, which decompose to produce toxic gases; d) some halides can be synthesized by the liquid phase method, which is more suitable for mass production. Then, some typical USSEs were compared with conventional SSEs such as oxides and liquid electrolytes in various aspects. The paper concludes with a dis-cussion on future research directions for USSEs and the challenges to be overcome in practical applications.