Reasearch paper
Lithium Borate Ester Salts

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%).

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