Toward Practical Li Metal Batteries: Importance of Separator Compatibility Using Ionic Liquid Electrolytes

Long-term cycling studies of high capacity Li-metal|lithium iron phosphate (LFP, 3.5 mAh/cm2) cells were carried out using two highly concentrated ionic liquid electrolytes (ILEs). Cells incorporating N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) or triethylmethylphosphonium bis(fluorosulfonyl)imide (P1222FSI), with 50 mol % lithium bis(fluorosulfonyl)imide (LiFSI) electrolytes were shown to operate for over 180 cycles at 50 °C at a rate of C/2 (1.75 mA/cm2). The choice of separator was identified as a critical factor to enable high areal capacity cycling, with the occurrence of cell failure through a short-circuiting mechanism being particularly sensitive to separator characteristics. Several commercial separators were characterized and tested, and their performance was related to membrane properties such as the MacMullin number, pore size, and contact angle. Celgard 3000 series separators were found to support long-term cycling due to their combination of desirable nanoporosity and wettability. The most compatible cell components were assembled into a pouch cell to further demonstrate the feasibility of ILE incorporation into high-capacity lithium metal batteries for commercial purposes.