Copolymers comprising poly(ethylene glycol) acrylate soft unit and sulfobetaine methacrylate zwitterionic unit and its application in solid-state lithium metal battery at ambient temperature

A series of copolymers comprising poly(ethylene glycol) methyl ether acrylate (PEGMEA) and sulfobetaine methacrylate (SBMA) were synthesized. The PEGMEA unit acted as a soft segment to lower the glass transition temperature of the copolymers, while the SBMA unit acted as a zwitterionic unit. Such a novel design greatly improved the electrochemical and mechanical properties of the copolymers. Subsequently, solid polymer electrolytes (SPEs) consisting of copolymers and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were prepared. The SPE exhibited a maximum ionic conductivity of 19.3 x 10-5 S cm-1, lithium-ion transference number of 0.31, and oxidation stability of 5.23 V (vs. Li/Li+) at ambient temperature. The selected SPE with the best electrochemical properties was placed between a Li metal electrode and a LiFePO4 electrode to assemble a solid-state lithium metal battery (SSLMB). The discharge capacity at 0.1 C-rate and room temperature was 169 mA h g-1. Furthermore, the discharge capacity at a 0.5 C-rate was 146 mA h g-1, steadily decreasing to 103 mA h g-1 (i.e., capacity retention of 70 %) after 450 cycles. By contrast, the electrochemical properties of the SSLMB with a SPE prepared by a homopolymer, synthesized from PEGMEA, and LiTFSI (as a reference battery sample) were inferior to those of the copolymer-based SPEs.