Hyperbranched PCL/PS Copolymer-Based Solid Polymer Electrolytes Enable Long Cycle Life of Lithium Metal Batteries

Abstract

Solid polymer electrolytes (SPE) are of great importance for developing the next-generation all-solid-state lithium metal batteries, which are featured by their high safety and extraordinary energy density. Herein, by using the highly efficient azide-alkyne click chemistry, a series of long-subchain hyperbranched copolymers from various compositions of AB2 macromonomers of PCL-2N3 (polycaprolactone) and PS-2N3 (polystyrene) were synthesized and employed as SPE polymer host. Taking advantage of the hyperbranched structure, the crystallization of PCL was successfully suppressed, and endows the SPE with high ionic conductivity and high mechanical properties simultaneously. By doping with LiFSI, the SPEs thereof exhibit high ionic conductivity (1.59 × 10−4 S cm−1 at 80 °C), broad electrochemical stability window (>4.6 V) and high lithium-ion transference number (>0.40). The electrolyte film based on the HB-II (the hyperbranched PCL/PS copolymer with 70 wt% PCL and 30 wt% PS) shows an ionic conductivity of 1.36 × 10−5 S·cm−1 at 80 °C and a high lithium transference number of 0.49. The all-solid-state LiFePO4//Li cell with HB-II-based SPE delivers a high and stable discharge capacity (133 mAh·g−1 at 1 C, with 90% capacity retention after 300 cycles) and exhibits long lifetime up to over 1200 cycles, owing to its relatively high ionic conductivity and good interfacial stability.