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Lithium-metal batteries (LMBs) are promising alternatives to state-of-the-art Lithium-ion batteries (LIBs) to achieve higher energy densities. However, the poor cyclability of LMBs resulting from Li metal anode (Li0) irreversibility and concomitant electrolyte decompositions limits their practical applications. In this study, we reported a per-fluorinated salt, lithium tetrakis(perfluoro-tertbutyloxy)borate (abbreviated as Li-TFOB) as an electrolyte additive for Li-metal batteries, which contains 36 F atoms per molecule. This newly designed ionic additive tuned the chemical composition of the solid-electrolyte interphase (SEI) on Li0 by increasing the amount of LiF and Li-B-O inorganic species. DFT calculations and Molecular dynamics (MD) simulations indicated the preferential reduction of the TFOB anions at Li0, which occurs with a lower free energy change than PF6anions. The designed ionic additive enables the 4.6 V Li||LiNi0.6Mn0.2Co0.2O2 (NMC622) cell to achieve an average CE of 99.1 % and a high-capacity retention of >50 % after 500 cycles. This experiment-simulation joint study illustrated an attractive approach to accelerating the design of electrolytes and interphases for LMBs. 

Abstract Ferrocyanide, such as K₄[Fe(CN)₆], is one of the most popular cathode electrolyte (catholyte) materials in redox flow batteries. However, its chemical stability in alkaline redox flow batteries is debated. Mechanistic understandings at the molecular level are necessary to elucidate the cycling stability of K₄[Fe(CN)₆] and its oxidized state (K₃[Fe(CN)₆]) based electrolytes and guide their proper use in flow batteries for energy storage. Herein, a suite of battery tests and spectroscopic studies are presented to understand the chemical stability of K₄[Fe(CN)₆] and its charged state, K₃[Fe(CN)₆], at a variety of conditions. In a strong alkaline solution (pH 14), it is found that the balanced K₄[Fe(CN)₆]/K₃[Fe(CN)₆] half‐cell experiences a fast capacity decay under dark conditions. The studies reveal that the chemical reduction of K₃[Fe(CN)₆] by a graphite electrode leads to the charge imbalance in the half‐cell cycling and is the major cause of the observed capacity decay. In addition, at pH 14, K₃[Fe(CN)₆] undergoes a slow CN^‐/OH^‐exchange reaction. The dissociated CN^‐ligand can chemically reduce K₃[Fe(CN)₆] to K₄[Fe(CN)₆] and it is converted to cyanate (OCN^‐) and further, decomposes into CO₃^2‐and NH₃. Ultimately, the irreversible chemical conversion of CN^‐to OCN^‐leads to the irreversible decomposition of K₄/K₃[Fe(CN)₆] at pH 14.