Electrospun poly(ionic liquid) nanofiber separators with high lithium-ion transference number for safe ionic-liquid-based lithium batteries in wide temperature range

Abstract

Ionic-liquid electrolytes (ILEs) promise high-safety lithium-metal batteries (LMBs) because of their wide electrochemical stability window and nonflammability. However, existing commercial polyolefin separators for LMBs are incompatible with ILEs owing to poor wettability and thermal stability. Also, conventional ILE/separator systems suffer from the inherent sluggish Li+ transport kinetics. Here we report on a novel highly Li+-conductive poly(ionic liquid) nanofiber membrane (PILNM) separator. It exhibits nonflammability, excellent thermal stability, and remarkable wettability with ILEs, achieving high ionic conductivity (3.81 mS cm−1) and significantly improved Li+ transference number (0.47). Benefiting from desirable interactions between the charged PILNM and ions of the ILE, the first solvation shell of [lithium-bis(trifluoromethane)sulfonimidex]1−x ([Li-TFSIx]1−x) complex is regulated. Fast Li+ transport and dendrite-free lithium metal anode are achieved. The PILNM combined with the ILE enables Li||LiFePO4 cells with enhanced capacities and rate capability in a wide temperature range from 25 °C to 120 °C. Notably, the thermal safety of Li|PILNM/ILE|LiFePO4 pouch cells at elevated temperatures has been further verified by adiabatic accelerating rate calorimetry. This work provides a new strategy to design and fabricate functionalized, thermally durable separators towards high-safety ILE-based LMBs.