2,2,2-Trifluoroethyl nonafluorobutanesulfonate as bifunctional electrolyte additive for high-energy-density 4.5 V LiNi0.8Co0.1Mn0.1O2||Li batteries

Combining a high cut-off voltage cathode with a high specific-capacity lithium metal anode offers a promising path toward ultra-high-energy-density batteries (> 400 Wh kg^-1). However, the practical application of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)||Li batteries with a 4.5 V cut-off voltage is challenged by severe interface deterioration. This study introduces a novel bifunctional electrolyte additive called 2,2,2-Trifluoroethyl Nonafluorobutanesulfonate (TNS), designed to form a thin, robust electrolyte interface layer on both cathode and anode surfaces. Detailed analysis reveals that TNS decomposes and modifies the solid-electrolyte-interface layer. Through the synergistic effects of fluoride and sulfonate functional groups, the resulting cathode-electrolyte-interface (CEI) enhances the structural stability of NCM811 while providing a high-efficiency lithium-ion transport channel to accelerate Li⁺ insertion and extraction. At a cut-off voltage of 4.5 V, NCM811||Li batteries with 1 % TNS (by weight) exhibit high discharge capacity retention of 74.2 % after 150 cycles at a 1C rate and 60.1 % after 150 cycles at a 2C rate. This study offers valuable insights for designing high-voltage electrolyte additives in ultra-high-energy-density lithium metal batteries.