Large-scale energy storage necessitates inexpensive, safe and long life batteries. Present battery development is rapid but based on insufficient materials for large scale power grid applications. The Zn/MnO₂ battery has high energy density comparable to Li-ion batteries and delivers a lower price per kilowatt-hour. Nonetheless, the rechargeability of the battery is limited and the cathode suffers from an irreversible phase formed during cycling. In the traditional alkaline battery, the irreversibility arises from a spinel phase that is electrochemically inactive. In this work, we use a zinc sulfate electrolyte and thus the reaction mechanisms are different. The inactive phase corresponds to a basic zinc sulfate precipitate passivating the electrode surface. Noteworthy suppression of the inactive precipitate can be achieved by adding small amount of manganese sulfate into the electrolyte. Our current Zn/MnO₂ cells have achieved stable capacity retention over 100 cycles by using commercially available materials. Specific capacity of 230 mAh/g and coulombic efficiency around 99.9% of the MnO₂ electrode as well as a mid-point voltage of 1.35 V was achieved.

We describe the phase transformations and the failure mechanisms of the cathode in the aqueous zinc sulfate Zn/MnO₂ battery, with and without the manganese sulfate. The phase transformations occurring in the cathode are directly observed with in-house in-operando X-ray diffraction using a Mo-source and 2D detector for rapid data acquisition. The manganese dioxide electrode is reversible over several charge and discharge cycles. The basic zinc sulfate dissolves and precipitates facilely if manganese sulfate is added. We discuss the factors of this reversibility in perspective of pH and surface area. Furthermore, we describe the structural changes and the electrochemical performance of the cathode using X-ray radiation, analytical and electrochemical techniques.

Our results can make it possible to further improve this battery to make it a worthy contender to the Li-ion battery in large-scale storage applications.

Figure (left): Charge and discharge curves of Zn/MnO₂ cell at C/5. Figure (right): Specific discharge capacity and efficiency vs. cycle number of two Zn/MnO₂ cells over 100 cycles.