Quasi-Solid-State Polymer Electrolyte Based on Highly Concentrated LiTFSI Complexing DMF for Ambient-Temperature Rechargeable Lithium Batteries

Solid-state polymer electrolytes (SPEs) complexing a plasticizer is a valid strategy to improve the poor ionic conductivity of SPEs at ambient temperature. In this study, a quasi-SPE based on a polyurethane matrix (QSPE-PU) is constructed by regulating the contents of bis(trifluoromethanesulfonyl) imide (LiTFSI) salt and N,N-dimethylformamide (DMF) and shows high performance in ambient-temperature rechargeable lithium batteries. Highly concentrated LiTFSI is designed to anchor DMF, decreasing the free solvent molecules in the QSPE to improve the stability with lithium metal. Meanwhile, DMF can fully dissociate the highly concentrated LiTFSI, providing more carriers. The prepared QSPE-PU40 shows a high ionic conductivity of 1.12 × 10^-4S cm^-1at ambient temperature since DMF not only provides more carriers but also enhances the movement of the polymer segments and lowers the energy barrier of lithium ion migration. Density functional theory calculations further prove that DMF facilitates the conduction of lithium ions in the QSPE system. QSPE-PU40 shows good compatibility with the lithium metal electrode by forming a stable solid electrolyte interphase during lithium plating/stripping processes. The lithium ferrophosphate (LFP)/QSPE-PU40/Li battery exhibits a high specific capacity of 138 mA h g^-1with remarkable cycling stability at 0.5 C and 30 °C (94% capacity retention after 800 cycles). More impressively, the pouch cell based on QSPE-PU40 delivers good flexibility and high safety. Such a QSPE is expected to provide an effective strategy where SPEs are applied in solid-state lithium batteries at ambient temperature and even flexible batteries for next-generation wearable devices.