Enhanced single-ion conduction and free-standing properties of solid polymer electrolyte by incorporating a polyelectrolyte

Abstract

A free-standing solid polymer electrolyte (SPE) composed of poly(ethylene oxide) (PEO), bis (trifluoromethanesulfonyl)imide (LiTFSI), and poly(lithium 4-styrenesulfonate) (PLSS)was developed in this work. Thermal analysis indicated that the melting points of PEO were depressed with increasing the salt additives, LiTFSI and PLSS. At the composition of[EO]:LiTFSI:[LSS] = 14:1:1, the SPE exhibited a crystallinity of 7.75%, and a crystallite size of 30.62 nm on the (120) crystallographic plane. [EO] and [LSS]represent the structural unit of PEO and PLSS, respectively. This SPE also exhibited an ionic conductivity (σ) of 1.70 × 10−5 S cm−1 at 25 °C and 1.04 × 10−4 S cm−1 at 45 °C. For analyzing the temperature dependence ofσ, the Vogel-Tammann-Fulcher equation was employed. Resultantly, a pseudo activation energy (Ea = 0.1552 eV), a prefactor(A = 206.0338 S cm−1K1/2 ), and an empirical constant(B = 1800.5879 K)were obtained using the optimized [EO]:[Li+] = 7:1 complex. The SPE showed an electrochemical stability window of∼4.7 ± 0.1 V versus Li/Li+. Through DC polarization and AC impedance, the Li-ion transference number of 0.66 was obtained at 70 °C. Finally, when a Li/SPE/LiFePO4 cell was prepared, the device exhibited a discharge capacity of 121 mAh g−1 at 50 °C with a coulombic efficiency close to 100%.