Mg anode interface engineering in KNO₃ electrolyte with sodium 5-sulfosalicylate as an additive for enhanced performance of Mg-air batteries

The Mg-air batteries face limitations with pronounced hydrogen evolution and low anodic utilization efficiency from Mg anodes in conventional NaCl electrolytes. The corrosion performance, surface composition, and discharge properties of commercial purity Mg anodes were thoroughly investigated in KNO₃ electrolytes with and without sodium 5-sulfosalicylate and compared to NaCl electrolyte. The addition of sodium 5-sulfosalicylate to KNO₃-based electrolyte results in efficient inhibition of H₂ evolution, consequently enhancing anodic utilization efficiency to 84% and specific capacity to 1844 mAh/g, compared to NaCl (24% and 534 mAh/g, respectively) under discharge condition of 10 mA/cm² in half cell. Furthermore, the chelating ability of sodium 5-sulfosalicylate can significantly improve the Mg surface dissolution kinetics and discharge product deposition rate at the Mg anode / electrolyte interface, yielding formation of a thinner discharge layer as confirmed by time-of-flight secondary ion mass spectrometry. The discharge voltage is increased to 1.60 V, compared to 1.35 V in KNO₃ at 0.5 mA/cm² in full cell. However, higher concentration of sodium 5-sulfosalicylate can accelerate Mg anode dissolution, impeding the improvement of anodic utilization efficiency, specific capacity, and energy density. Hence, determining optimal additive concentration and current density is crucial for enhancing the discharge properties of Mg-air batteries and mitigating excessive Mg dissolution in chloride-free electrolytes.