Valorization of ethylene carbonate recovered from spent lithium-ion battery electrolytes: synthesis and properties of biodegradable poly ethylene succinate-co-ethylene glycol (PESG) copolyesters

Abstract

This study proposes a green closed-loop strategy aimed at valorizing ethylene carbonate (EC), a high-boiling organic component found in spent lithium-ion battery (LIB) electrolytes, into degradable polyester materials. First, EC was recovered through vacuum distillation at 150 °C and 0.09 MPa, yielding the target product with an 86.5% yield and 96.9% purity. Subsequently, benchmark polyester polyethylene glycol succinate (PES) was synthesized from EC and dimethyl succinate (DMSu) via a polycondensation reaction catalyzed by dibutyltin oxide (Bu₂SnO). The resulting material exhibited a viscosity of 0.42 dL g⁻¹, a thermal weight loss of 5% at 315 °C, and a glass transition temperature (T_g) of −9.5 °C. To further enhance the flexibility and biodegradability of PES, short-chain diol ethylene glycol (EG) was introduced as a third monomer, and a one-pot random copolymerization reaction was conducted to prepare six PESG copolymers with EC : EG molar ratios ranging from 10 : 0 to 5 : 5. The esterification–polycondensation process conditions were optimized using the Box–Behnken response surface method, yielding optimal parameters: a catalyst addition of 0.97 wt%, esterification/polycondensation temperatures of 224 °C/216 °C, and a reaction time of 2.3 hours. Under these conditions, the copolymers exhibited an intrinsic viscosity of 0.57 dL g⁻¹. Structural characterization results (FTIR, NMR, XRD) confirmed the successful incorporation of EG into the polyester backbone without altering its crystalline structure. GPC measurements of PESG revealed a molecular weight of 2.2 × 10⁴ g mol⁻¹ and a dispersity index of 1.95. As the EG content increased, the T_g decreased to −18.8 °C, crystallinity declined, and segmental mobility improved. Notably, PESG demonstrated optimal hydrophilicity and biodegradability, with a 75% mass loss within 24 days under lipase treatment and a contact angle reduction to 53.6°. This study validates the feasibility of recycling electrolyte waste to synthesize degradable copolyesters, offering a novel pathway for the high-value utilization of electrolyte resources and the development of environmentally friendly materials.