Molecular regulated polymer electrolytes for solid-state lithium metal batteries: Mechanisms and future prospects

Solid polymer electrolytes (SPEs) have been widely adopted in solid-state lithium metal batteries (SSLMBs) recently due to their excellent flexibility, superior processability and outstanding safety. However, balancing excellent ionic conductivity and mechanical robustness of SPEs is still a bottleneck. Currently, most researches focus on the incorporation of various fillers into polymer matrix to overcome the mentioned obstacle. Nevertheless, the large specific surface area of fillers makes them easy to agglomerate, resulting in uneven dispersion in SPEs, and this would interrupt the continuity of lithium ion migration and cause unstable filler/polymer interfaces. Therefore, direct regulation of polymer electrolytes at molecular level instead of incorporating fillers will effectively avoid the above obstacle while obtaining excellent performance. In this review, three kinds of mechanisms of electrolyte regulation at the molecular level are presented, including designing polymer matrix, adding soluble additives, and building molecular interactions. Through molecularly regulating the polymer matrix, constructing three-dimensional (3D) networks, grafting of special functional groups, and coordination interactions in SPEs, the creation of second phases and unstable filler/polymer interfaces are all avoided. It is hoped that this review can inspire an in-depth understanding on direct regulation of SPEs at the molecular level, further improving ionic conductivity and mechanical robustness of SPEs.