Understanding the Electrochemical Mechanism of K-αMnO2 for Magnesium Battery Cathodes

Batteries based on magnesium are an interesting alternative to current state-of-the-art lithium-ion systems; however, high-energy-density cathodes are needed for further development. Here we utilize TEM, EDS, and EELS in addition to soft-XAS to determine electrochemical magnesiation mechanism of a high-energy density cathode, K-alpha MnO2. Rather than following the typical insertion mechanism similar to Li+, we propose the gradual reduction of K-alpha MnO2 to form Mn2O3 then MnO at the interface of the cathode and electrolyte, finally resulting in the formation of K-alpha MnO2@(Mg,Mn)O core-shell product after discharge of the battery. Understanding the mechanism is a vital guide for future magnesium battery cathodes.