Cosolvent Electrolyte Design for High-Voltage Aqueous Zinc–Sulfur Batteries

Aqueous zinc-sulfur batteries (AZSBs) offer cost and safety advantages but face challenges related to severe cell polarization. Herein, we introduce a cosolvent, N,N-diethylformamide (DEF), in the ZnSO4 electrolyte to facilitate the sulfur cathode conversion reaction. Compared to Zn(H2O)62+, the reconstructed solvated [Zn(H2O)5DEF]2+ in the cosolvent electrolyte exhibits a narrower HOMO-LUMO gap (0.74 eV vs 1.31 eV), leading to faster charge transfer kinetics at the electrolyte-electrode interface. Due to the strong interaction between DEF and sulfur electrode, the electrons donated by DEF occupy the antibonding orbitals of sulfur, increasing the electron density and electrostatic repulsion between sulfur atoms, thereby weakening the S-S bond interaction. This electron injection effect also reduces the band gap and enhances the intrinsic electrical conductivity of sulfur. As a result, this cosolvent electrolyte design for AZSBs achieves an elevated discharge plateau of 0.8 V vs Zn2+/Zn and reduced cell polarization of 0.32 V, outperforming most reported AZSBs. Additionally, we present design principles for cosolvent electrolytes in AZSBs, with machine learning results suggesting that the donor number and HOMO energy of the cosolvents are critical descriptors for predicting the discharge voltage of AZSBs. This work offers new insights into the development of high-performance AZSBs.