Primary Magnesium-air Batteries Performance Comparison: 3.5 wt.% NaCl vs Natural Seawater as the Electrolyte

• The electrochemical and battery performance of Mg-air batteries with natural seawater and 3.5 wt.% NaCl electrolytes was systematically compared. • When discharged at 2 mA/cm2, nature seawater electrolyte doubles the specific energy density of a Mg-air battery to 1267.1 mWh/g. • While the Mg-air battery exhibits a higher initial discharge voltage in a 3.5 wt.% NaCl solution, the seawater electrolyte provides superior voltage retention over time. • The presence of Mg²⁺ and Ca²⁺ ions in natural seawater alters the anode-electrolyte interface, resulting in high anodic efficiency at low current densities. • A simple filtration process was found to be detrimental to the performance of the Mg-air battery. The utilization of seawater as an electrolyte in magnesium-air (Mg-air) batteries represents an exciting opportunity for harnessing Earth's abundant resources to solve critical energy storage challenges in the energy sector. Whilst a few prior studies have explored the feasibility of Mg-air batteries with seawater electrolytes, comprehensive understanding of battery performance and underlying operating mechanisms in seawater remain unclear. In this work, the electrochemical and battery performance of Mg-air batteries constructed using 3 distinct electrolytes (3.5 wt.% NaCl in water, natural seawater, and filtered seawater) were systematically compared. For Mg-air batteries fabricated with recast Mg as the anode, the highest specific energy density was found using seawater, reaching 1267.1 mWh/g at 2 mA/cm 2 , compared to 642.5 mWh/g using a 3.5 wt.% NaCl solution and 462.03 mWh/g using filtered seawater. The anodic efficiency using the seawater electrolyte reached 47% at a current density of 10 mA/cm 2 . In addition, the discharge mechanism of the Mg-air battery with the natural seawater electrolyte was investigated. The results suggest that the formation of dolomite (CaMg(CO₃)₂) on the Mg anode surface in the seawater electrolyte contributes to the enhanced battery discharge performance observed over the 3.5 wt.% NaCl electrolyte.