Electrostatic aspect of the proton reactivity in concentrated electrolyte solutions

Water-in-salt electrolytes with a surprisingly large electrochemical stability window up to 3V revive the interest in aqueous electrolytes for rechargeable lithium-ion batteries. However, recent reports of acidic pH measured in concentrated electrolyte solutions appear to be in contradiction with the suppressed activity of hydrogen evolution reaction (HER). Therefore, the fundamental thermodynamics of proton reactivity in concentrated electrolyte solutions remain elusive. In this work, we have used density functional theory-based molecular dynamics (MD) simulations and the proton insertion method to investigate how the HER potential shifts in concentrated LiCl solutions under both acidic and alkaline conditions. Our results show that the intrinsic HER activity goes up significantly with the salt concentration under acidic conditions but keeps relatively constant under alkaline conditions. Moreover, by leveraging over finite- field MD simulations, it is found that a determining factor for the HER activity is the Poisson potential of liquid phase, which shifts positively in concentrated electrolyte solutions with comparable values from both DFT and point-charge models.