The crucial challenges of poly (ethylene oxide)-based solid polymer electrolytes are first presented and followed by a comprehensive review on the design concepts, different types of additives, and their corresponding working mechanisms. Finally, the future research directions of additive strategies are also highlighted to enable safer and longer-term all-solid-state lithium metal batteries.

Poly (ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are considered one of the most promising electrolytes for all-solid-state lithium metal batteries (ASSLMBs) due to their satisfactory mechanical properties, superior processability, excellent flexibility, and thermal stability. However, the low ionic conductivity at room temperature, the undesired growth of lithium dendrites and solid electrolyte interphase chemistry at the Li/PEO-based SPEs interface, as well as facile decomposition under high-voltage conditions impede their further application. Currently, most researchers devote their interest to the incorporation of various fillers or molecular modification strategies to overcome the mentioned obstacle. Nevertheless, none of them achieve the improvement of these properties meanwhile. Therefore, tailoring additive designs for PEO-based SPEs has been demonstrated to be a successful and operable strategy to simultaneously improve their ionic conductivity and regulate the interfacial chemistry. In this review, the crucial challenges of PEO-SPEs are first presented and followed by a comprehensive review on the design concepts, different types of additives, and their corresponding working mechanisms. Finally, the future research directions of additive strategies are also highlighted to enable safer and longer-term ASSLMBs.