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.