Proton-transfer reaction thermodynamics in highly concentrated electrolytes for advanced aqueous lithium-ion batteries

Abstract

We have studied proton-transfer reactions in highly concentrated aqueous electrolytes focusing on lithium bis(trifluoromethanesulfonyl)amide (LiTf₂N) for potential applications in advanced aqueous lithium-ion batteries. In 20 mol kg⁻¹ LiTf₂N aqueous solution, the autoprotolysis constant pK_W increased significantly to 16.2, indicating a reduction in ionization compared to bulk water (pK_W = 14 at infinite dilution), and the H⁺ concentration is lower than in dilute aqueous solutions. This was mainly attributed to the increase in the activity coefficient of H⁺ (γ_H) with increasing LiTf₂N concentration. Calorimetric measurements revealed that the increase in pK_W is driven by an increase in the autoprotolysis enthalpy, suggesting that H⁺ is enthalpically unfavorable in concentrated LiTf₂N solutions. In contrast, the autoprotolysis entropy showed no significant contribution to the pK_W variation. We extended this study to the acid-base reaction of acetic acid as a model system in highly concentrated electrolyte solutions. The pK_a value of acetic acid (or the corresponding ionization Gibbs energy) showed little dependence on the LiTf₂N concentration. In contrast, the corresponding ionization enthalpy and entropy increased significantly with increasing LiTf₂N concentration. Due to the enthalpy–entropy compensation effect, the pK_a value resulted in limited variation even under highly concentrated conditions. The obtained enthalpy values for the ionization processes support that proton carriers lie in an enthalpically unstable state. These findings provide new insights into the fundamental behavior of protons in highly concentrated aqueous electrolytes, which is critical for the design and optimization of aqueous lithium-ion batteries and other electrochemical systems.