Influence of Al₂(SO₄)₃ Electrolyte Additive on Cell Potential and Reaction Mechanism in Aqueous Acidic Zn–MnO₂ Batteries

Aqueous Zn–MnO₂ batteries are promising alternatives to lithium-ion batteries due to their inherent safety, low cost, and environmental compatibility. The incorporation of aluminum ions has been shown to enhance their performance by increasing operating voltage and cathode stability. However, the underlying mechanisms remain poorly understood due to complex cathode reactions involving the dissolution, migration, and redeposition of manganese species. Herein, the effects of pH and aluminum sulfate (Al₂(SO₄)₃) on the electrochemical performance and reaction mechanisms of Zn–MnO₂ batteries are investigated. The addition of Al₂(SO₄)₃ to the electrolyte increases the discharge voltage from 1.4 to ≈1.7 V (vs. Zn/Zn²⁺), primarily due to changes in the redox potential of the MnO₂ cathode rather than the Zn anode. This enhancement correlates with the maintenance of a highly acidic environment (pH ≈ 1.1), regardless of the presence of Al³⁺. Additionally, Al₂(SO₄)₃ suppresses the formation of zinc hydroxysulfate, typically observed during discharge in Al₂(SO₄)₃-free systems, while promoting the precipitation of aluminum hydroxysulfate (AlHS), which is influenced by the depth-of-discharge. The presence of AlHS improves the oxidation kinetics of Mn²⁺ during charging. These findings highlight electrolyte pH and AlHS formation as critical factors in enhancing the electrochemical performance of Zn–MnO₂ batteries.