Enhancing safety and performance of magnesium batteries through novel aqueous electrolytes

Abstract

Metal-based batteries represent a leading energy storage technology with diverse practical applications. Lithium-ion batteries are widely applied in portable electronics and EVs today, but they exhibit significant shortcomings including limited energy density, safety hazards, and scarce material supply. As a result, the emergence of rechargeable magnesium batteries has garnered substantial attention because of their excellent theoretical capacity, abundance and safety of materials, and cost-effectiveness. However, the passivation of magnesium metal, particularly in aqueous solutions, poses a significant challenge, limiting the practicality of magnesium metal anodes. To date, magnesium batteries reported are either primary aqueous batteries with limited discharge times or rechargeable non-aqueous batteries with poor ionic conductivity, high cost, and safety concerns. Such limitations highlight the need for innovative research solutions. This thesis addresses these challenges by developing advanced aqueous electrolytes. Initially, a dual-electrolyte Mg-air battery is introduced to mitigate magnesium passivation and hydrogen evolution reactions. By employing an acid-salt aqueous electrolyte system, magnesium passivation is inhibited and the voltage window is expanded, achieving 50% higher peak power density and 46% higher open circuit voltage than traditional single-electrolyte Mg-air batteries. Subsequently, a novel MgCl2 water-in-salt (WIS) electrolyte is developed to enable reversible and stable Mg stripping and plating, creating the first rechargeable aqueous Mg metal battery reported. The transformation of the native oxidation layer into a metallic oxide complex was enabled by Cl-triggered dissolution and the restraint of water activity. To further enhance battery performance, a quasi-solid-state electrolyte is formulated to alter cathodic charge storage mechanisms, enabling true multivalent metal ion insertion which promotes a high energy density. Combining the benefits of aqueous and non-aqueous systems, the battery adopting this non-flammable electrolyte achieves a voltage plateau of 2.6-2.0 V and a remarkable energy density of 264 Wh kg−1, which is nearly five times higher than the energy density of current aqueous Mg-ion batteries. This breakthrough unlocks the potential of aqueous Mg-ion batteries and offers a safe and high-performing alternative to current Li-ion battery technology. This study demonstrates significant progress in improving the safety and performance of magnesium-based batteries through electrolyte advancements, addressing challenges at the Mg metal anode, and enabling favorable multivalent ion storage at the cathode.