The effect of fluorides in the TiO₂(B) anode on the hydrogen evolution reaction in aqueous electrolytes

For aqueous lithium-ion batteries (A-LIBs), the hydrogen evolution reaction (HER) poses a significant challenge, as it competes with the primary electrochemical processes of the anode, resulting in capacity loss and reduced cycling stability. In this study, we investigate the use of fluorine-based additives in anodes to mitigate HER in LIBs with aqueous electrolytes including low or high amounts of salt (water-in-salt electrolytes (WiSE)). We synthesized and incorporated three distinct materials into TiO₂(B) anodes: aluminum fluoride (AlF₃), lithium fluoride (LiF), and 1H,1H,2H, 2H-perfluorooctyltriethoxysilane (FAS) using a solution-based method. Among these fluorides containing composite anodes, FAS containing anodes delayed HER onset potentials of WiSE by 45-160 mV (1.2 m (molality) or 21 m (Lithium bis (trifluoromethanesulfonyl) imide in H₂O)) compared to the bare TiO₂ (B) anodes. Among these fluorides, FAS demonstrated the highest HER delay with the smallest amount of additives due to its hydrophobic nature. These findings underscore the effect of fluorine-based passivation layers in mitigating the HER, potentially expanding the energy density, and improving the operational stability of anodes in A-LIBs, thereby paving the way for their broader application in sustainable energy storage.