Governing Factors in Mildly Acidic Zn/MnO2 Batteries: Interplay of Electrochemical Protocols, Electrolyte Composition, and Cell Configuration

Zn-MnO2 aqueous batteries are promising candidates for long duration energy storage systems due to their use of cost-effective materials and inherent safety profile. In aqueous mildly acidic electrolytes, both electrodes ideally undergo reversible electrodeposition during charge and electrodissolution during discharge. Here, we report on the investigation of the electrodeposition and electrodissolution mechanisms of the MnO2 cathode in mildly acidic Zn-MnO2 batteries with different electrochemical protocols (galvanostatic or chronoamperometric) in both an aqueous and a hybrid aqueous-sulfolane electrolyte. Ex situ characterization with Raman spectroscopy and scanning electron microscopy revealed substantial heterogeneity in electrodeposited MnO2 phases across both electrolyte systems. We used operando electrochemical optical spectroscopy (EC-OM) to further understand the electrochemical mechanisms including hydrogen evolution. We obtained the highest coulombic efficiency of the MnO2 cathode in a hybrid aqueous-sulfolane electrolyte, with short charging times of a few minutes using chronoamperometry, and a high surface area carbon scaffold. These parameters minimize the contribution from parasitic hydrogen evolution at the zinc anode and lead to the formation of dense but thin MnO2 films that undergo the most efficient electrodissolution during discharge. Our results validate the beneficial properties of hybrid aqueous-sulfolane electrolytes and underscore the need to further stabilize the zinc anode and develop novel high surface area carbon scaffolds for efficient MnO2 electrodeposition and electrodissolution for Zn-MnO2 batteries operating in mildly acidic electrolytes.锌-二氧化锰（Zn-MnO₂）水系电池凭借其低成本原材料与固有安全特性，成为长时储能系统的极具潜力的候选方案。在温和酸性水系电解液中，正负极理论上均可在充电时发生可逆电沉积、放电时发生电溶解反应。本研究针对温和酸性体系下的锌-二氧化锰电池，分别在水系电解液与混合水系-环丁砜电解液中，采用恒电流法或计时电流法两种电化学测试方案，探究了二氧化锰正极的电沉积与电溶解机制。通过拉曼光谱法与扫描电子显微镜开展的非原位表征结果显示，两种电解液体系中电沉积生成的二氧化锰相态均存在显著异质性。本研究进一步采用原位电化学光学光谱（EC-OM）对包括析氢反应在内的电化学机制进行解析。在混合水系-环丁砜电解液中，搭配高比表面积碳骨架时，二氧化锰正极可实现最高的库仑效率，且通过计时电流法可在数分钟内完成快速充电。该参数配置可最大程度抑制锌负极上的副析氢反应，同时形成致密且轻薄的二氧化锰薄膜，这类薄膜在放电过程中可实现最高效的电溶解。本研究结果验证了混合水系-环丁砜电解液的优良性能，并强调需进一步优化锌负极的稳定性，开发新型高比表面积碳骨架，以实现温和酸性电解液体系下锌-二氧化锰电池中高效的二氧化锰电沉积与电溶解过程。