Mitigating water effects in humid [Pyr14][TFSI] at electrified surfaces via lithium-salt engineering

Abstract

Ionic liquids (ILs) are attractive electrolytes for next-generation electrochemical devices; however, their hygroscopic nature leads to water uptake, which can reorganize the electrical double layer (EDL), promote interfacial water electrosorption, and contribute to the narrowing of the electrochemical stability window (ESW). In this work, we investigate lithium-salt engineering in humid 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Pyr14][TFSI]) using molecular dynamics (MD) simulations at fixed electrode surface charge densities. Corrected potential differences relative to the potential of zero charge (PZC), interaction energies, and number-density profiles of ions and water were systematically analyzed for dry, humid, and Li+-containing ILs. The simulations show that interfacial H2O interacts strongly with polarized electrodes and plays a central role in water-induced EDL restructuring. Upon Li+ addition, water molecules are increasingly coordinated by Li+, reducing the population of free water available for accumulation at electrified interfaces. At low surface charge densities, Li+ addition shifts the corrected potential differences toward those obtained for the dry IL, indicating partial mitigation of water-induced electrostatic screening. At higher polarizations and elevated Li+:H2O ratios, particularly 2:1, the calculated ΔδΦ values exceed those of the dry IL, suggesting an apparent overcompensation of water-screening effects within the classical MD framework. Importantly, the calculated potential differences are interpreted as electrostatic descriptors of EDL polarization and water-screening effects, rather than as direct measures of redox stability or absolute ESW. Overall, these results provide structural and electrostatic evidence that Li+ ions can sequester water and reduce free-water accumulation at electrified interfaces, suggesting that lithium-salt addition may help mitigate water-induced EDL perturbations in humid [Pyr14][TFSI]-based electrolytes