Non-flammable electrolytes based on a fluorine-free salt for safe and high-voltage lithium metal batteries

Lithium nitrate (LiNO₃) is the preferred additive for achieving high efficiencies with improved lithium deposition morphologies due to its ability to form stable solid electrolyte interface (SEI) layers. However, LiNO₃ is not employed as its pure salt owing to its low solubility in common organic solvents. Here, we propose a non-flammable electrolyte (NF-LiNO₃/GBL) consisting of LiNO₃ in γ-butyrolactone (GBL), wherein functional additives such as pentafluoro(phenoxy)cyclotriphosphazene (FPPN) and vinylene carbonate (VC) are added as a flame retardant and the solid electrolyte interphase (SEI)/cathode electrolyte interphase (CEI)-forming additive, respectively. Due to the strong interactions between Li⁺ and NO₃⁻ in the GBL, contact ion pairs and aggregates are the dominant ionic species even at a concentration of 0.8 M. Experimental and computational investigations demonstrated that the combination of LiNO₃, GBL, FPPN, and VC forms SEI/CEI layers that are rich in conductive inorganic species, which can ultimately enhance the interfacial stability at the anode/cathode. The Li||Cu cells based on NF-LiNO₃/GBL electrolyte exhibited a high average Coulombic efficiency of 98.3%, and Li||LiNi_0.6Co_0.2Mn_0.2O₂ (4.3 V) cell based on NF-LiNO₃/GBL maintained excellent capacity retention of 90% after 200 cycles. This work provides a promising non-flammable electrolyte for the production of safe and high-energy-density rechargeable lithium metal batteries.