Recyclable Li‐Metal Battery Electrolytes via In Situ Cyclic Carbonate Polymerization

Abstract Enabling recycling and improving performance are key challenges for next‐generation electrolytes for rechargeable batteries. Here, an equilibrium polymerization: trimethylene carbonate (TMC) ring‐opening polymerization, in the presence of lithium difluoro(oxalato)borate salt, is utilized to form an electrolyte in situ during coin cell fabrication for lithium batteries. This process creates a semi‐solid poly(trimethylene carbonate) electrolyte with high ambient ionic conductivity (0.52 mS cm ⁻¹ ), thermal stability ( T _{d, 5%} = 160 °C), and oxidative stability up to 4.7 V. Using this electrolyte with commercial lithium iron phosphate cathodes, results in 97% capacity retention after 350 cycles at 2C, achieving theoretical capacities of 170 mAh g ⁻¹ at 0.1C. The cells retain excellent performance at high current densities (86 mAh g ⁻¹ at 4C). Post‐use, the polymer can be separated from the salt and selectively recycled to pure starting monomer (TMC) through a solid‐state chemical recycling process. The recycled monomer, when repolymerized to reform the polycarbonate electrolyte, yields cells with performance identical to the original. The exploitation of polymerization‐depolymerization equilibria offers a useful strategy for enhancing battery performance, ensuring effective material recycling, and advancing a circular economy.