New Sulfur-Containing Gel Polymer Electrolytes for Sodium–Metal Batteries

A novel sulfurated telechelic bis(methacrylate) monomer was synthesized and evaluated for its potential use in photopolymerization to yield gel polymer electrolytes (GPEs). The difunctional monomer was obtained via a two-step synthesis by the radical bismonoaddition of telechelic sulfurated dithiol onto allyl alcohol followed by the methacrylation of the resulting diol. It was prepared in 60% overall yield and thoroughly characterized by FTIR, Raman, and NMR spectroscopies. Several photopolymerizable formulations were then proposed by combining telechelic bis(methacrylate) with sodium bis(fluorosulfonyl)imide and tetraethylene glycol dimethyl ether. Upon UV initiation, a series of soft GPE films of various thicknesses were obtained and analyzed by FTIR spectroscopy. Their physicochemical, thermal, and preliminary electrochemical properties were then systematically studied. The resulting GPEs exhibit very low glass transition temperatures (−80 to −40 °C) and high electrical conductivity (1.2 × 10^–4 S·cm^–1) at room temperature, with electrochemical stability up to 4.5 V vs. Na⁺/Na. Sodium plating-stripping experiments in Na/GPE/Na symmetric cells show stable polarization voltage curves with no significant fluctuations, indicating homogeneous sodium deposition and dissolution up to a current density of 0.2 mA/cm². These preliminary and promising results support the potential application of the developed GPEs in sodium batteries.