Rechargeable Zn batteries, consisting of abundant, environmentally benign electrode materials that can be manufactured economically and theoretically provide decent energy density (around 400 Wh/kg), are regarded as promising alternatives to lithium-ion batteries. Combined with a Zn metal anode and an aqueous ZnSO4 electrolyte, MnO2 and the Prussian blue hexacyanoferrates have been shown to work as cathode materials for rechargeable Zn batteries. However, the significant capacity fade during long-term cycling tests and insufficient energy density largely hamper their applications. Meanwhile, there are ongoing debates on reaction mechanisms of the Zn/MnO2 system.
In this study, our goal is to investigate the possibility of spinel ZnMn2-xTMxO4 as cathode materials for rechargeable Zn batteries, verify the reaction mechanisms, and eventually to achieve high voltage. Single phase ZnMn2O4 nanopowders with good crystallinity were synthesized by a combination of co-precipitation reactions and subsequent calcination technique, and the synthesis parameters were systematically optimized. We found that the calcination temperature has a significant influence on the crystallinity and morphology, thus electrochemical behavior of ZnMn2O4 nanopowders. The optimized ZnMn2O4 with ZnSO4 as electrolytes shows an initial specific capacity of 125 mAh/g at C/10. To elucidate the electrochemical reaction mechanism of ZnMn2O4, phase & chemical compositions, and microstructure of cathodes before and after cycling were systematically investigated. In parallel, ZnMn2-xFexO4 (0≤x≤2) has been synthesized and characterized.