Sulfonate-Based Single-Ion Conducting Polymer Electrolytes for Lithium Metal Batteries

Abstract

Solid-state batteries can offer improved energy density and safety relative to conventional liquid lithium-ion batteries. In particular, single-ion polymer electrolytes (SIPEs) offer high ionic conductivity, effectively suppressing lithium dendrite growth. In this study, a series of SIPEs composed of poly(ethylene oxide) methyl ether methacrylate (PEGMA) and poly(lithium 4-styrene sulfonate) (PLSS) were synthesized. These random copolymers (RCPs) were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization at six different compositions with PLSS content ranging from 12 to 75 mol%.

First, the chemical and thermal properties of the RCPs were fully characterized; all RCPs were found to have sufficiently high thermal degradation temperatures, with glass transition temperatures below room temperature. Next, PEGMA-𝑟-PLSS films were prepared and tested for ionic conductivity using electrochemical impedance spectroscopy (EIS); copolymers exhibited high conductivity of 2.2 x 10⁻⁵ S cm⁻¹ at room temperature, reaching a maximum of 2.3 x 10⁻⁴ S cm⁻¹ at 70 °C. As PEGMA is the flexible, ion-transporting polymer, while PLSS contributes lithium ions and rigidity from functionalized polystyrene, the RCP with the lowest PLSS content (i.e., PEGMA-𝑟-PLSS-12) exhibited the highest ionic conductivity but did not form free-standing films. Alternatively, films with more moderate amounts of PLSS resulted in both sufficient mechanical and ion transport properties. PEGMA-𝑟-PLSS-50, which has the reported optimal ethylene oxide to lithium-ion ratio (EO:Li⁺) of 9:1, achieved the second highest room temperature conductivity (1.6 x 10⁻⁶ S cm⁻¹) and second highest ion conductivity at 70 °C (1.4 x 10⁻⁴ S cm⁻¹), while also forming free-standing SIPEs.

Considering the need to balance conductivity with mechanical properties, two methods were explored to improve the RCP SIPEs: (1) blending RCPs with poly(lithium vinyl sulfonate) (PLVS) and (2) synthesizing block copolymer analogs of the RCP (PEGMA-𝑏-PLSS). Despite the low glass transition temperature of PLVS, it was not found to improve ion conductivity of PEGMA-𝑟-PLSS-12. Due to differences in polymer solubility, polymer miscibility may be an issue at higher PEGMA compositions. Block copolymer synthesis is ongoing. PEGMA-𝑟-PLSS copolymers show strong potential as a candidate for SIPEs for lithium metal batteries when synthesized with an optimal 9:1 ratio of EO:Li⁺ and conductivity may be further improved by transitioning to a block structure.