Benchmarking Corrosion with Anionic Polarity Index for Stable and Fast Aqueous Batteries Even in Low‐Concentration Electrolyte

Abstract Despite aqueous electrolyte endowing batteries with the merits of safe operation, low‐cost fabrication, and high ionic conductivity, water‐induced corrosion, including spontaneous chemical and electrochemical hydrogen evolution corrosion, adversely affects lifespan and rate capability. There is still a lack of selection criteria for benchmarking corrosion behavior qualitatively. Through theoretical simulation, an anionic polarity index (API) tactic is proposed to resist corrosion by manipulating interfacial and solvated water concomitantly, thus realizing stable and fast Zn aqueous batteries (ZABs). As proof of concept, a low‐cost zinc salt of 0.5 m zinc bis(4‐hydroxybenzenesulphonate) (Zn(HBS)₂) with low‐API anion is prioritized. Combined in situ spectroscopic and electrochemical analyses reveal that, even in a low‐concentration electrolyte, the low‐API anion reduces interfacial water in the inner Helmholtz plane, shielding the chemical water dissociation. Meanwhile, their entering into the solvation sheath of Zn²⁺ lowers the solvent‐separated ion pair, suppressing the electrochemical corrosion. The elaborated API‐screened zinc salt endows fast plating kinetics of 50 mA cm⁻² (119.1 mV polarization), high coulombic efficiency of 99.8%, dendrite‐free cycling over 1600 h, and prolonged lifespan over 5000 cycles for the Zn‐V cell. The results provide new metrics that can benchmark the success of ZABs for large‐scale energy storage.