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

## TL;DR

This paper explores how adding fluorine-based materials to anodes in aqueous lithium-ion batteries can reduce hydrogen evolution and improve battery performance.

## Contribution

The study introduces fluorine-based additives that effectively delay hydrogen evolution in aqueous electrolytes, enhancing battery stability.

## Key findings

- FAS-containing anodes delayed HER onset potentials by 45–160 mV in aqueous electrolytes.
- FAS showed the highest HER delay with minimal additive amounts due to its hydrophobic properties.
- Fluorine-based passivation layers can improve anode stability and energy density in aqueous lithium-ion batteries.

## Abstract

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.