Electrochemical Reduction of a Spinel-Type Manganese Oxide Cathode in Aqueous Electrolytes with Ca2+ or Zn2

In this report, the feasibility of reversible Ca2+ or Zn2+ intercalation into a crystalline cubic spinel Mn2O4 cathode has been investigated using electrochemical methods in an aqueous electrolyte. A combination of synchrotron XRD and XANES studies identified the partial structural transformation from a cubic to a tetragonally distorted spinel Mn3O4, accompanied by the reduction of Mn4+ to Mn3+ and Mn2+ during discharge. TEM/EDX measurements confirmed that practically no Ca2+ was inserted upon discharge. However, non-negligible amounts of Zn were detected after Mn2O4 was reduced in the Zn2+ electrolyte, but through the formation of secondary phases that, in some cases, appeared adjacent to the surface of a cathode particle. This report aims to identify bottlenecks in the application of manganese oxide cathodes paired with Ca or Zn metal anodes and to justify future efforts in designing prototype multivalent batteries.