Acetic acid additive in NaNO₃ aqueous electrolyte for long-lifespan Mg-air batteries

Mg-air batteries have attracted tremendous attention as a potential next-generation power source for portable electronics and e-transportation due to their remarkable high theoretical volumetric energy density, environmental sustainability, and cost-effectiveness. However, the fast hydrogen evolution reaction (HER) in NaCl-based aqueous electrolytes impairs the performance of Mg-air batteries and leads to poor specific capacity, low energy density, and low utilization. Thus, the conventionally used NaCl solute was proposed to be replaced by NaNO₃ and acetic acid additive as a corrosion inhibitor, therefore an electrolyte engineering for long-life time Mg-air batteries is reported. The resulting Mg-air batteries based on this optimized electrolyte demonstrate an improved discharge voltage reaching ~1.8 V for initial 5 h at a current density of 0.5 mA/cm² and significantly prolonged cells' operational lifetime to over 360 h, in contrast to only ~17 h observed in NaCl electrolyte. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry were employed to analyse the composition of surface film and scanning electron microscopy combined with transmission electron microscopy to clarify the morphology changes of the surface layer as a function of acetic acid addition. The thorough studies of chemical composition and morphology of corrosion products have allowed us to elucidate the working mechanism of Mg anode in this optimized electrolyte for Mg-air batteries.