Lithium Borate Ester Salts for Electrolyte Application in Next‐Generation High Voltage Lithium Batteries

Abstract The atmospheric instability and the corrosive tendency of hexafluorophosphate [PF₆]⁻ and fluorosulfonylimide [FSI]⁻ based lithium salts, respectively, are among the major impediments towards their application as electrolytes in high voltage lithium batteries. Herein a new class of Li salts is introduced and their electrochemical behavior is explored. The successful synthesis and characterization are reported, including the crystal structure, of lithium 1,1,1,3,3,3‐(tetrakis)hexafluoroisopropoxy borate (LiBHfip). The oxidative stability of electrolytes of this salt in an ethylene carbonate:dimethyl carbonate mixture (v/v, 50:50) is found to be 5.0 V versus Li⁺/Li on various working electrodes, showing substantial improvement over a LiPF₆ based electrolyte. Moreover, a high stability of an aluminum substrate is observed at potentials up to 5.8 V versus Li⁺/Li; in comparison, a LiFSI based electrolyte shows prominent signs of Al corrosion above 4.3 V versus Li⁺/Li. Cells tested with high voltage layered LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) and spinel LiMn₂O₄ (LMO) cathodes show stable cycling over 200 cycles with capacity retention of 76% and 90%, respectively. The LMO|Li cell maintains this same low capacity fade rate for 1000 cycles even after the salt has been exposed for 24 h to atmospheric conditions (water content ≈0.57 mass%).