Electrolyte Engineering in Aqueous Zn/MnO₂ Batteries: Effects of pH and ZnSO₄ on Electrochemical Behavior and Degradation Mechanisms

Aqueous Zn/MnO2 batteries offer a promising avenue for grid-scale energy storage, yet their performance and longevity are critically dependent on electrolyte composition. This work investigates the influence of initial electrolyte pH (via H₂SO₄ addition, 0-0.5 M) and ZnSO₄ concentration (0.5-3.0 M) on the electrochemical behavior and degradation pathways in Zn/β-MnO₂ and Zn/α-MnO₂ cells. A multi-faceted approach combining operando pH measurements, galvanostatic cycling, and physics-based continuum modeling coupled with parameter estimation was employed. Initial H₂SO₄ addition introduced a high-voltage discharge plateau, enhancing initial capacity; however, this benefit diminished rapidly over cycling with minimal impact on long-term stability. Parameter estimation linked capacity fade to decreasing fractions of active zinc manganese oxide (ZMO) material for both dissolution and insertion reactions, attributed to heterogeneous Zn hydroxysulfate precipitation, non-uniform MnO₂ utilization, and material redistribution. ZnSO₄ concentrations below 2.0 M significantly reduced discharge capacity, particularly for the MnO₂ dissolution/ZMO formation pathway, confirming Zn²⁺ depletion limitations. These findings highlight the critical role of electrolyte engineering and provide actionable design principles for optimizing electrolyte formulations to improve the performance and durability of aqueous Zn/MnO₂ batteries.