Manganese Molybdate Cathodes with Dual-Redox Centers for Aqueous Zinc-Ion Batteries: Impact of Electrolyte on Electrochemistry

Although bimetallic oxides have been shown to be beneficial as electrode materials in battery systems over their monometallic oxide counterparts, the implementation of metal molybdates with dual-redox centers has not been widely studied in aqueous rechargeable Zn-ion batteries. Manganese molybdate (MnMoO4) was synthesized via facile cosolvent coprecipitation and implemented as a cathode material for the first time in this system using 3 M ZnSO4 and 3 M ZnCl2 electrolytes to investigate the impact of the anion. In the two electrolytes, both manganese and molybdate metal centers were determined to be redox active using Mn and Mo K-edge operando X-ray absorption spectroscopies (XAS), with the corresponding voltage plateaus at 1.4 and 0.5 V, respectively. The difference in anions resulted in a preference regarding the active redox center, with the sulfate based preferring Mo redox and chloride based preferring Mn redox. Additionally, the redox reactions also differ in rate dependency, with Mn and Mo redox reactions preferring slow and fast current rates, respectively. In both systems, Mn redox was seen to be the more stable mechanism over prolonged cycling. The preference was related to the dissolution of the MnMoO4 material by applying Pearson's hard–soft acid–base theory and considering the solubilities of the respective salts.