A Bis(2-fluoroethyl) Carbonate as a New Electrolyte Additive for Enhancing the Long-Term Cycle Performance of Li-Metal Batteries

To meet the demand for high energy density, Li metal is considered a next-generation anode material owing to its high theoretical specific capacity and low electrode potential. However, conventional LiPF₆-based electrolytes form a thick and porous solid electrolyte interphase (SEI) on Li metal, resulting in poor cycle performance. One of attempts to resolve these is to optimize the electrolyte composition because the Li metal reacts most actively with electrolyte. Here, bis(2-fluoroethyl) carbonate (B-FC), as a new fluorine-based linear carbonate, was added to a LiTFSI-LiBOB-based dual-salt electrolyte. To confirm the effect of B-FC on the electrochemical properties, Li || Li symmetric cells and LiNi_0.6Co_0.2Mn_0.2O₂ (NMC622) || Li metal full cells with or without B-FC were evaluated. The addition of B-FC forms LiF-rich SEI and significantly reduced Li dendrite growth, leading to the thin dead Li layer formation. Furthermore, high-voltage performances of NMC622 || Li metal full cells with B-FC were effectively improved compared to the pure DSL (capacity retention of 73.1% vs 62.4% after 300 cycles and a capacity of 117 mAh g⁻¹ vs 87 mAh g⁻¹ at 21 mA cm⁻²). Consequently, herein, we demonstrated that the dual-salts with B-FC can stabilize the SEI even under the 4.5 V cut-off condition.