Strategies for Advanced Solid Electrolytes toward Efficient Lithium-Ion Conduction in All-Solid-State Lithium Metal Batteries

All-solid-state lithium metal batteries (ASSLMBs) have currently garnered significant academic and industrial interest, due to their great potential to overcome intrinsic shortages of poor energy density and unsatisfactory safety of liquid-state lithium-ion batteries. Recently, many efforts have been made to move the progress of solid electrolytes (SEs) forward for ASSLMBs, especially on the understanding and optimization of lithium-ion conduction in SEs. Herein, we summarize a review of recent design strategies for rational SEs that display enhanced lithium-ion conduction, as well as the discussion on design principles and working mechanisms for boosted performance and stability of ASSLMBs. Given the intimate relationship between the lithium-ion conduction mechanism and the composition of SEs, the reported SEs can generally be classified into single-phase SEs and composite SEs. In detail, single-phase SEs contain three typical categories, e.g., polymer-based, inorganic, and plastic crystal-based SEs. For composite SEs, there are also three main kinds, including polymer–inorganic, plastic crystal–polymer, and plastic crystal–polymer–inorganic ternary composite SEs. The state-of-the-art literature and representative materials have been carefully discussed and analyzed, with the corresponding factors of enhancing lithium-ion conduction highlighted. Finally, an outlook for future directions to design advanced SEs with efficient lithium-ion conduction is presented for the development of ASSLMBs.