Multivalent batteries are a promising alternative to Li-ion batteries due to their potential to provide higher energy density. Mg²⁺ has achieved relative success through Chevrel-structured and thio-spinel cathodes, which can be cycled against a Mg metal anode in a full-cell arrangement. However, the low voltage and capacity of the sulfide cathodes limit the energy density of this system, necessitating the search for more energy-dense Mg cathodes.¹ Recent theoretical² and experimental³ studies indicate that the oxide spinel family presents a set of promising Mg cathodes. One of these, the Mg_xCr₂O₄ spinel, has reasonable Mg migration barriers and unprecedented energy density for an intercalation cathode. Using first-principles calculations, we investigate the voltage profile for Mg insertion at room temperature and the activation barriers for Mg diffusion at different Mg concentrations in the Cr₂O₄ structure. Based on our results, we identify a potential limitation to Mg intercalation in the form of a stable Mg-vacancy ordering in the Cr₂O₄ lattice, which exhibits high migration barriers for Mg diffusion in addition to a steep voltage change. Hence, for the practical usage of Cr₂O₄ as a cathode in multivalent batteries, steps must be taken to avoid the formation of the stable ordered phase.

¹Canepa et al, Chem. Rev. 2017, 117 (5), pp 4287-4341

²Rong et al, Chem. of Mat. 2015, 27 (17), pp 6016-6021

³ Kim et al, Adv. Mat. 2015, 27 (22), pp 3377-3384