Small components play a big role - fillers in composite solid-state electrolytes for lithium metal batteries

Lithium metal batteries (LMBs), the energy conversion and storage technologies that have been thoroughly investigated, are utilized in various areas. Most current commercial LMBs use liquid electrolytes, but their safety cannot be guaranteed. In contrast, all-solid-state LMBs with solid-state electrolytes (SSEs) are regarded as next- generation energy storage systems owing to their enhanced safety. In the examination of SSEs, the benefits of composite SSEs (CSSEs) are particularly prominent. CSSEs relying on multiphase composites have better flexibility to tailor electrolyte performance based on demand and, thus, the design of components. Based on existing research presentations, we herein review the development of CSSEs. Firstly, the essential components of CSSEs are introduced, focusing on explaining the transport channel of Li ions within the polymer matrix and the impact of various fillers on this channel. The key performance parameters of CSSEs are described in detail. Subsequently, the fillers in CSSEs are meticulously classified; the critical roles played by different fillers in the polymer matrix are clarified, and the hotspots of current research are summarized. Furthermore, the prevalent structural design methodologies in CSSEs are examined, and the impact of various structures on battery performance is elucidated. Finally, the current problems of CSSEs and the future development prospects are summarized. This review is significant because it provides ideas for the design of CSSEs and contributes to the development of all-solid-state LMBs to realize their wide application.