Role of LiTFSI Salt Content and Salt Crystallization on Carboxymethyl Chitosan-Based Solid Polymer Electrolytes for Stable Li Metal Batteries

Solid polymer electrolytes (SPEs) have attracted tremendous attention owing to their beneficial mechanical properties and compatibility with lithium metal anode for high-energy-density lithium metal battery applications. However, the ionic conductivities of SPEs are generally lower than those of conventional liquid electrolytes. Herein, a high ionic conductive SPE with excellent mechanical and thermal stability, derived from carboxymethyl chitosan (CMCS) and lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI), was prepared for stable Li metal batteries. The LiTFSI salt content in the CMCS matrix was varied to obtain the CMCS/LiTFSI SPE with superior properties. The CMCS-based SPE with the highest room-temperature ionic conductivity and tensile strength was achieved with a 25 wt % LiTFSI addition, that is, 7.81 × 10–5 S cm–1 and 35.6 MPa, respectively. At higher temperatures, the ionic conductivity of the SPE reached 5.20 × 10–4 S cm–1 (80 °C). The SPE exhibited an excellent thermal stability up to 244 °C and a stable Li|SPE|Li cycling for over 350 h. The addition of more than 30% LiTFSI induced the formation of salt crystallites, which decreased the mechanical properties and room-temperature ionic conductivity. Density functional theory calculations revealed that the strong interaction between Li+ and TFSI– ions was the main factor contributing to the formation of salt crystallites, which caused network inhomogeneity in the SPE.