Influence of the Cyclic versus Linear Carbonate Segments in the Properties and Performance of CO₂-Sourced Polymer Electrolytes for Lithium Batteries

Polycarbonates bearing linear carbonate linkages and polyether segments have demonstrated to be highly attractive solid electrolyte candidates for the design of safe energy storage devices, for example, lithium metal batteries. In this contribution, we are studying the influence of the introduction of some cyclic carbonate linkages within the polymer backbone on the electrolyte properties. We first describe the synthesis of polycarbonates/polyethers containing different contents of both linear and cyclic carbonate linkages within the chain by the copolymerization of a highly reactive CO2-based monomer (bis(α-alkylidene cyclic carbonate)) with poly(ethylene glycol) diol and a dithiol at room temperature. We then explore the influence of the content of the cyclic carbonates and the loading of the polymer by lithium bis(trifluoromethane) sulfonimide (LiTFSI) on the electrolyte properties (glass transition and melting temperatures, ion conductivity, and diffusivity). The best electrolyte candidate is characterized by a linear/cyclic carbonate linkage ratio of 82/18 when loaded with 30 wt % LiTFSI. It exhibits an ion conductivity of 5.6 × 10–5 S cm–1 at 25 °C (7.9 × 10–4 S cm–1 at 60 °C), which surpasses by 150% (424% at 60 °C) the conductivity measured for a similar polymer bearing linear carbonate linkages only. It is also characterized by a high oxidation stability up to 5.6 V (vs Li/Li+). A self-standing membrane is then constructed by impregnating a glass fiber filter by this optimal polymer, LiTFSI, and a small amount of a plasticizer (tetraglyme). Cells are then assembled by sandwiching the membrane between a C-coated LiFePO4 (LFP) as the cathode and lithium as the anode and counter electrode. The cycling performances are evaluated at 0.1 C at 60 °C and room temperature for 40 cycles. Excellent cycling performances are noted with 100% of the theoretical capacity (170 mAh g–1) at 60 °C and 73.5% of the theoretical capacity (125 mAh g–1) at 25 °C.