Recent Advances in Carboxymethyl Cellulose-Based Solid Polymer Electrolytes Incorporating Lithium Salts and Functional Additives: A Systematic Review

Carboxymethyl cellulose (CMC)-based solid polymer electrolytes (SPEs) have attracted significant attention as sustainable alternatives to conventional liquid electrolytes due to their biodegradability, non-toxicity, and excellent film-forming capability. However, pristine CMC suffers from inherent limitations, including low ionic conductivity, poor mechanical strength, and limited electrochemical stability. This systematic literature review comprehensively evaluates recent advances in CMC-based SPEs, focusing on the roles of lithium salts (e.g., LiCH3COO, LiClO4, LiI, LiBF4, and LiNO3) and functional additives, including plasticizers, ionic liquids, nanofillers, and cross-linking agents, in tailoring the physicochemical and electrochemical properties. The findings reveal that ionic conductivity can be significantly enhanced from ~10−7 to 10−2 S cm−1 through synergistic modifications that reduce crystallinity and promote segmental mobility. Electrochemical stability is improved by up to ~3.85 V with the incorporation of ionic liquids, while ion transference numbers approaching unity (t+ ≈ 0.96) indicate highly efficient Li+-dominated transport. Mechanical properties exhibit a trade-off between flexibility (elongation up to ~699%) and tensile strength (up to ~12.84 MPa), depending on the balance between plasticization and cross-linking. The degradation temperature is also strongly influenced by system composition, reaching ~508 °C in ionic liquid-modified systems. Overall, the performance of CMC-based SPEs is governed by the interplay between salt chemistry, polymer structure, and additive functionality. This review highlights key structure–property relationships and identifies critical research gaps, including salt-concentration optimization, long-term stability, and scalability, providing strategic insights for the rational design of high-performance, sustainable polymer electrolytes for next-generation energy storage applications.