Salt Anion Amphiphilicity-Activated Electrolyte Cosolvent Selection Strategy toward Durable Zn Metal Anode

One effective solution to inhibit side reactions and Zn dendrite growth in aqueous Zn-ion batteries is to add a cosolvent into the Zn(CF₃SO₃)₂ electrolyte, which has the potential to form a robust solid electrolyte interface composed of ZnF₂ and ZnS. Nevertheless, there is still a lack of discussion on a convenient selection method for cosolvents, which can directly reflect the interactions between solvent and solute to rationally design the electrolyte solvation structure. Herein, logP, where P is the octanol-water partition coefficient, a general parameter to describe the hydrophilicity and lipophilicity of chemicals, is proposed as a standard for selecting cosolvents for Zn(CF₃SO₃)₂ electrolyte, which is demonstrated by testing seven different types of solvents. The solvent with a logP value similar to that of the salt anion CF₃SO₃^- can interact with CF₃SO₃^-, Zn²⁺, and H₂O, leading to a reconstruction of the electrolyte solvation structure. To prove the concept, methyl acetate (MA) is demonstrated as an example due to its similar logP value to that of CF₃SO₃^-. Both the experimental and theoretical results illustrate that MA molecules not only enter into the solvation shell of CF₃SO₃^- but also coordinate with Zn²⁺ or H₂O, forming an MA and CF₃SO₃^- involved core-shell solvation structure. The special solvation structure reduces H₂O activity and contributes to forming an anion-induced ZnCO₃-ZnF₂-rich solid electrolyte interface. As a result, the Zn||Zn cell and Zn||NaV₃O₈·1.5H₂O cell with MA-involved electrolyte exhibit superior performances to that with the MA-free electrolyte. This work provides an insight into electrolyte design via salt anion chemistry for high-performance Zn batteries.