Single-solvent ionic liquid strategy achieving wide-temperature and ultra-high cut-off voltage for lithium metal batteries

Bis(trifluoromethanesulfonyl)imide-based ionic liquid (IL) electrolytes hold the promise of achieving higher voltage (>5 V), wider temperature range (>80 °C), and non-flammability in lithium metal batteries (LMBs). However, its fatal Li compatibility severely restricts the battery's cycling lifespan. In this work, we propose a novel single-solvent dual anion ionic liquid electrolyte (HCILE) composed of lithium bis(fluorosulfonyl)imide (LiFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI), which exhibits an exceptionally elevated oxidation potential of 5.4 V and maintains a thermal stability up to 250 °C. The dual anion (FSI⁻ and TFSI⁻) in TFSI based-IL electrolyte synergistically constructs a robust solid electrolyte interphase, enabling dendrite-free Li deposition and reversible cathodic electrochemistry. The Li//Li cells with HCILE achieve reversible Li plating/stripping for 1900 h at 0.5 mA cm⁻²/0.5 mAh cm⁻², significantly surpassing the previously reported TFSI-based IL electrolytes. Additionally, paired Li//LiFePO₄ cells maintain a specific capacity of 143 mAh g⁻¹ with a high average Coulombic efficiency of 99 % after 300 cycles at 1 C under high voltages range of 2.5–4.95 V. Even at 80 °C, our LMBs with HCILE also exhibit excellent stability. We believe that our work opens up new possibilities for the design of high-voltage with wide-temperature electrolytes in energy storage systems.