Modification engineering of “polymer-in-salt” electrolytes toward high-stability solid-state lithium batteries

Solid-state lithium batteries have been regarded as a promising candidate to become the power supply for electric vehicles and smart grids due to their high energy density and reliable safety. The solid polymer electrolytes (SPEs) with light and thin features show distinctive potential in boosting the available energy density at battery level, whereas their ionic conductivity smaller than 10⁻⁴∼10⁻⁵ S cm⁻¹ at room temperature constrains the ionic transfer kinetics, leading to low power density and short cycling life. To overcome such problem, the increase of lithium-salt concentration over 50 wt% evokes the conversion from “salt-in-polymer” to “polymer-in-salt” (PIS) of SPEs, which can make additional ionic migration pathway and thus the improved ionic conductivity. However, the abundant lithium-salt may also cause the reduced electrochemical window as well as mechanical properties, which restricts the compatibility with high-voltage cathodes and lowers the operation safety. In this review, the structures and characteristics of PIS electrolytes have been elucidated through clarifying the correlation between lithium-salt and polymer matrix. Then, the recent modification engineering progresses on PIS electrolytes are addressed from the aspects of component regulations including polymer matrices, lithium salts and fillers, novel preparation techniques, and extended application scenarios. The crucial challenges and possible research directions are finally proposed for the PIS electrolytes regarding both science and practical perspectives.