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

• Investigated Brønsted acid-base thermodynamics in concentrated aqueous LiTf 2 N solutions. • p K W increases to 16.2, indicating a more than 1 order decrease in [H + ] at 20 mol kg −1 LiTf 2 N. • Due to a high H + activity coefficient, pH decreases lower than 7, despite a lower [H + ] than in water. • The high H + activity coefficient originates from enthalpically unstable proton carriers. • Enthalpy-entropy compensation governs ionization in concentrated electrolyte solutions. We have studied proton-transfer reactions in highly concentrated aqueous electrolytes focusing on lithium bis(trifluoromethanesulfonyl)amide (LiTf 2 N) for potential applications in advanced aqueous lithium-ion batteries. In 20 mol kg −1 LiTf 2 N aqueous solution, the autoprotolysis constant p K W increased significantly to 16.2, indicating a reduction in ionization compared to bulk water (p K 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 LiTf 2 N concentration. Calorimetric measurements revealed that the increase in p K W is driven by an increase in the autoprotolysis enthalpy, suggesting that H + is enthalpically unfavorable in concentrated LiTf 2 N solutions. In contrast, the autoprotolysis entropy showed no significant contribution to the p K W variation. We extended this study to the acid-base reaction of acetic acid as a model system in highly concentrated electrolyte solutions. The p K a value of acetic acid (or the corresponding ionization Gibbs energy) showed little dependence on the LiTf 2 N concentration. In contrast, the corresponding ionization enthalpy and entropy increased significantly with increasing LiTf 2 N concentration. Due to the enthalpy–entropy compensation effect, the p K 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.