New Insights to Electrolyte Design for Aqueous Zinc-Manganese Dioxide Batteries: The Role of Anion Complexes and pH Dynamics

Electrolytic zinc manganese dioxide batteries (ZMB) are promising alternatives to lithium-ion systems due to their cost-effectiveness, material abundance, and safety, but their adoption is hindered by challenges in energy density and stability. To overcome these hurdles, a deeper understanding of the underlying electrochemical processes is required. This study investigates the role of acetate and sulfate anions in electrolyte performance, focusing on pH buffering capacity and complexation effects at the electrode interface. A thermodynamic model was developed to simulate speciation and reaction equilibria, highlighting that proton transport is the rate-limiting step in sulfate-based electrolytes. Acetate-containing electrolytes exhibited superior buffering capacity and facilitated proton-coupled electron transfer, leading to enhanced manganese dioxide deposition at moderate pH levels. For the first time, a systematic study was conducted in which sulfate was progressively substituted by acetate while maintaining consistent pH conditions. Although acetate ions enable cyclability under mild pH conditions, their strong complexation with Mn³⁺ promotes manganese diffusion into the electrolyte, ultimately reducing reversibility as acetate content increases. Moreover, contrary to common assumptions, acetate anions do not inherently enhance anode stability under identical pH conditions. These insights provide a foundation for optimizing electrolyte design to overcome the limitations of zinc-manganese dioxide batteries.