CO2-sourced polycarbonates as solid electrolytes for room temperature operating lithium batteries

Abstract

In the last few years, polycarbonates have been identified as alternatives to poly(ethylene oxide) as polymer electrolytes for lithium battery applications. In this work, we show the design of CO₂-sourced polycarbonates for their use in room temperature operating lithium batteries. Novel functional polycarbonates with alternating oxo-carbonate moieties and polyethylene oxide segments are synthesized by the facile room temperature (rt) organocatalyzed polyaddition of CO₂-sourced bis(α-alkylidene carbonate)s (bis-αCCs) with polyethylene oxide diols. The effect of the molar mass of polyethylene oxide on the ionic conductivity and thermal properties of poly(oxo-carbonate)s is investigated. The best candidate shows a low glass transition temperature of −44 °C and a high ionic conductivity of 3.75 × 10⁻⁵ S cm⁻¹ at rt when loaded with 30 wt% bis(trifluoromethanesulfonyl)imide salt (LiTFSI) without any solvent. An all-solid semi-interpenetrated network polymer electrolyte (SIN-SPE) is then fabricated by UV cross-linking of a mixture containing specifically designed poly(oxo-carbonate) bearing methacrylate pendants, diethylene glycol diacrylate and the previously described poly(oxo-carbonate) containing LiTFSI. The resulting self-standing membrane exhibits a high oxidation stability up to 5 V (vs. Li/Li⁺), an ionic conductivity of 1.1 × 10⁻⁵ S cm⁻¹ at rt (10⁻⁴ S cm⁻¹ at 60 °C) and promising mechanical properties. Assembled in a half cell configuration with LiFePO₄ (LFP) as the cathode and lithium as the anode, the all-solid cell delivers a discharge capacity of 161 mA h g⁻¹ at 0.1C and 60 °C, which is very close to the theoretical capacity of LFP (170 mA h g⁻¹). Also, a stable reversible cycling capacity over 400 cycles with a high coulombic efficiency of 99% is noted at 1C. Similar results are obtained at rt provided that 10 wt% tetraglyme as a plasticizer was added to the SIN-SPE.