With the widespread adoption of lithium-ion batteries (LIBs), safety concerns associated with flammable organic electrolytes have become increasingly critical. Solid-state lithium batteries (SSLBs), with enhanced safety and higher energy density potential, are regarded as a promising next-generation energy storage technology. However, the practical application of solid-state electrolytes (SSEs) remains hindered by several challenges, including low Li⁺ ion conductivity, poor interfacial compatibility with electrodes, unfavorable mechanical properties and difficulties in scalable manufacturing. This review systematically examines recent progress in SSEs, including inorganic types (oxides, sulfides, halides), organic types (polymers, plastic crystals, poly(ionic liquids) (PILs)), and the emerging class of soft solid-state electrolytes (S³Es), especially those based on “rigid-flexible synergy” composites and “Li⁺-desolvation” mechanism using porous frameworks. Critical assessment reveals that single-component SSEs face inherent limitations that are difficult to be fully overcome through compositional and structural modification alone. In contrast, S³Es integrate the strength of complementary components to achieve a balanced and synergic enhancement in electrochemical properties (e.g., ionic conductivity and stability window), mechanical integrity, and processability, showing great promise as next-generation SSEs. Furthermore, the application-oriented challenges and emerging trends in S³E research are outlined, aiming to provide strategic insights into future development of high-performance SSEs.