Organic–Inorganic Composite Electrolytes Optimized with Fluoroethylene Carbonate Additive for Quasi-Solid-State Lithium-Metal Batteries

Composite solid electrolytes (CSEs) are considered crucial materials for next-generation solid-state lithium batteries with high energy density and reliable safety, and they make full use of the advantages of both organic and inorganic solid-state electrolytes. However, few CSEs have sufficiently high ionic conductivity at room temperature for practical applications. Here, a traditional CSE consisting of poly(ethylene oxide) (PEO) matrix and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) fillers was optimized by introducing a fluoroethylene carbonate (FEC) additive, resulting in an improved high ionic conductivity of 1.99 × 10^-4 S cm^-1 at 30 °C. The symmetric Li||Li cell assembled with the optimized CSE exhibited a low overpotential and a good cycling stability of more than 1500 h at room temperature. Moreover, the Li||LiFePO₄ battery with the optimized CSE delivered a discharge capacity of 132 mAh g^-1 at 0.2 C after 300 cycles at room temperature. Comparisons between the LATP-containing CSE and control electrolytes indicated that the enhanced ion conductivity of the former resulted from the synergistic effect of LATP and FEC. Comprehensive characterizations and DFT calculations suggest that with the presence of LATP, FEC additives in the precursor could transform into some other species in the preparation process of CSE. It is believed that these FEC-derived species improve the ion conductivity of the CSEs. The results reported here may open up new approaches to developing composite electrolytes with high ionic conductivity at room temperature by introducing organic additives in the precursor and converting them into species that facilitate ion conduction in the CSE preparation process.