Fluorinated Boron-Based Anions for Higher Voltage Li Metal Battery Electrolytes

Lithium metal batteries (LMBs) require an electrolyte with high ionic conductivity as well as high thermal and electrochemical stability that can maintain a stable solid electrolyte interphase (SEI) layer on the lithium metal anode surface. The borate anions tetrakis(trifluoromethyl)borate ([B(CF₃)₄]^-), pentafluoroethyltrifluoroborate ([(C₂F₅)BF₃]^-), and pentafluoroethyldifluorocyanoborate ([(C₂F₅)BF₂(CN)]^-) have shown excellent physicochemical properties and electrochemical stability windows; however, the suitability of these anions as high-voltage LMB electrolytes components that can stabilise the Li anode is yet to be determined. In this work, density functional theory calculations show high reductive stability limits and low anion-cation interaction strengths for Li[B(CF₃)₄], Li[(C₂F₅)BF₃], and Li[(C₂F₅)BF₂(CN)] that surpass popular sulfonamide salts. Specifically, Li[B(CF₃)₄] has a calculated oxidative stability limit of 7.12 V vs. Li⁺/Li⁰ which is significantly higher than the other borate and sulfonamide salts (≤6.41 V vs. Li⁺/Li⁰). Using ab initio molecular dynamics simulations, this study is the first to show that these borate anions can form an advantageous LiF-rich SEI layer on the Li anode at room (298 K) and elevated (358 K) temperatures. The interaction of the borate anions, particularly [B(CF₃)₄]^-, with the Li⁺ and Li anode, suggests they are suitable inclusions in high-voltage LMB electrolytes that can stabilise the Li anode surface and provide enhanced ionic conductivity.