Investigating Barriers to Mg Intercalation in Oxide Spinel Cathodes through First-Principles Calculations

Abstract

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 MgxCr2O4 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