Thermoplastic polyurethane (TPU) based high-performing solid polymer electrolytes for solid-state lithium metal batteries

Abstract

Polymer electrolytes are favored in solid-state batteries due to their mechanical strength and improved electrode-wetting properties compared to conventional solid electrolytes. However, achieving a balance between key properties such as mechanical stability, thermal stability, ionic conductivity, electrochemical stability, and room-temperature operation, remains challenging for most state-of-the-art polymer electrolytes. This study explores Thermoplastic Polyurethane (TPU) as a versatile, cost-effective polymer with tunable elastic and thermal properties that have been under-explored for electrolyte applications. The objective was to optimize TPU-based solid polymer electrolytes (SPEs) for ionic conductivity while considering cost and demonstrating room-temperature cycling. Membranes were fabricated using TPU, LiTFSI, and SCN via solution casting. Four different TPU-based membranes (TPULxySzz, where x, y indicate the weight ratios of TPU and LiTFSI, and zz indicate the weight percentage of SCN in the SPE) were screened based on ionic conductivity and cost. Electrochemical characterization was performed in symmetric, half-cell, and full-cell configurations, evaluating the stability of the Li-metal anode and LiFePO4 cathode interfaces with the SPEs. Among the membranes tested, TPUL23S50 exhibited the highest ionic conductivity (1.09 mS/cm) and lowest interfacial resistance with lithium metal. TPUL11S50 showed superior current capability (496.56 $\mu$A/cm²) and lithium transference number (0.25). Linear Sweep Voltammetry (LSV) revealed that TPUL23S60 had the best voltage stability. In full-cell tests, all membranes demonstrated excellent room-temperature cycling performance, with capacities close to theoretical values and strong retention. TPUL23S50 achieved an initial discharge capacity of 169.94 mAh/g with 93.74 % retention after 20 cycles, while TPUL11S50 showed 169.92 mAh/g and 97.51 % retention. This study demonstrates that TPU-based polymers have the potential to surpass current polymer electrolytes, offering efficient lithium-ion conduction and robust electrochemical performance, making them promising candidates for commercial solid-state batteries.