High-Efficiency Lithium Metal Batteries with Fire-Retardant Electrolytes

Rechargeable lithium metal batteries are regarded as the “holy grail” of energy storage systems, but their practical applications have long been hindered by poor cyclability and severe safety concerns. In this work, we report a fire-retardant localized high-concentration electrolyte consisting of 1.2 M lithium bis(fluorosulfonyl)imide in a mixture of flame-retardant triethyl phosphate/bis(2,2,2-trifluoroethyl) ether (1:3 by mol) for 4-V class lithium metal batteries. This electrolyte enables stable, dendrite-free cycling of lithium metal anodes with high Coulombic efficiency of up to 99.2%. Moreover, it exhibits excellent anodic stability even up to 5.0 V and greatly enhances the cycling performance of lithium metal batteries. A Li||LiNi_0.6Mn_0.2Co_0.2O₂ battery using this electrolyte can retain >97% capacity after 600 cycles at 1 C rate (ca. 1.6 mA cm⁻²), corresponding to a negligible capacity decay of <0.005% per cycle. Therefore, this new electrolyte can enable safe operation of high-energy lithium metal batteries for practical applications. A safe electrolyte for 4-V class lithium metal batteries (LMBs) was reported by diluting a fire-retardant high-concentration electrolyte (HCE) with an electrochemically “inert” and poorly solvating fluorinated ether. Named localized high-concentration electrolyte (LHCE), it inherits the merits from the HCE but dramatically overcomes its disadvantages. The fire-retardant LHCE enables dendrite-free and stable cycling of a Li metal anode with high Coulombic efficiency of up to 99.2% and greatly enhances the cycling stability of Li||NMC622 batteries for more than 600 cycles. The excellent electrochemical performances of the LHCE is ascribed to the well-reserved, locally concentrated solvation structures and its improved interfacial reaction kinetics and stability. These findings open up a new avenue for developing highly stable and safe electrolyte systems for high-energy-density LMBs for practical applications. A fire-retardant localized high-concentration electrolyte (LHCE) inherits the merits from the high-concentration electrolyte (HCE) (non-flammability, wide electrochemical stability windows etc.) and dramatically overcomes the disadvantages (high viscosity, high cost, poor wettability) of HCE. Its unique properties lead to dendrite-free and high-Coulombic-efficiency cycling of Li metal anode that supports safe 4-V class Li||NMC622 batteries with excellent cycling stability.