Dual fluorine-free salt electrolytes for medium-to-high voltage lithium metal batteries

Flame-resistant and fluorine-free electrolytes based on (combining) the salts lithium saccharinate (LiSac) and lithium bis(oxalato)borate (LiBOB) in a single solvent triethyl phosphate (TEP) solvent and vinylene carbonate (VC) additive are presented and evaluated for lithium metal battery application. The dual salt electrolyte, 1.5 M LiSac + 0.2 M LiBOB in TEP w. 2 % VC, clearly outperforms the single salt ones in terms of electrochemical performance, especially vs. LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes, properties that originate in a Li+ cation first solvation shell mainly composed of Sac and BOB anions, promoting formation of a mechanically stable, inorganic-rich cathode electrolyte interphase layer, which by X-ray photoelectron spectroscopy was revealed to comprise Li3N, BxOy and SO32-species. Overall, this also results in stable cycling, and a capacity retention of 86 % in both Li||LiFePO4 and Li||NMC811 cells after 500 cycles at 1C rate-hence offering an intrinsically safer electrolyte that also enables the use of both lithium metal anodes and medium-to-high-voltage cathodes.