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

Abstract

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||LiNi0.6Co0.2Mn0.2O2 (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.