Solid-state composite electrolytes: turning the natural moat into a thoroughfare

Solid-state composite electrolytes (SCEs) bridge the gap between solid-state polymer electrolytes (SPEs) and solid-state inorganic electrolytes (SIEs), which play an important role in developing the expected solid-state Li-ion batteries (SSLIBs). SCEs can be divided into “ceramic-in-polymer” (CIP), “intermediate” and “polymer-in-ceramic” (PIC) types, according to the components. For the CIP type, SCEs mainly consist of three parts, namely, a polymer matrix, fillers, and lithium salts. The polymer matrix as the main component of SCEs largely determines their performance. Therefore, the choice of polymer matrix is the most critical step among all the preparation processes. Moreover, the fillers as functional components in SCEs are beneficial for enhancing the ionic conductivity, promoting the mechanical properties, and addressing the electrolyte–electrode interface problems. Hence, the promising fillers hold the key to improving the comprehensive performance of the resulting SCEs. In addition, the construction of stable interfacial structures and the interfacial electrochemistry are equally important as the selection of polymer matrix and fillers. Last but not least, understanding the ion migration mechanism is a critical step in the design of SCEs for future high-performance solid-state Li-ion batteries.