Multivalent Electrochemistry of Spinel Mg_xMn_{3- x}O₄ Nanocrystals

Oxides undergoing reversible electrochemical cycling of Mg²⁺ ions would enable novel battery concepts beyond Li⁺, capable of storing large amounts of energy. However, materials showing this chemical reactivity are scarce. Suitable candidates require small particles to shorten transport lengths, together with chemically complex structures that promote cation mobility, such as spinel. These goals pose a challenge for materials chemists. Here, nanocrystals of spinel-type Mg_0.5Mn_2.5O₄ were prepared using colloidal synthesis, and their electrochemical activity is presented. Cycling in an aqueous Mg²⁺ electrolyte led to a reversible transformation between a reduced spinel and an oxidized layered framework. This reaction involves large amounts of capacity because of the full oxidation to Mn⁴⁺, through the extraction of both Mg²⁺ and, in the first cycle, Mn²⁺ ions. Re-formation of the spinel upon reduction resulted in enrichment with Mg²⁺, indicating that its insertion is more favorable than that of Mn²⁺. Incorporation of water into the structure was not indispensable for the transformation, as revealed by experiments in non-aqueous electrolytes and infrared spectroscopy. The findings open the door for the use of similar nanocrystals in Mg batteries provided that electrolytes with suitable anodic stability are discovered, thereby identifying novel routes toward electrode materials for batteries with high energy.