Spinel-to-Rocksalt Transition on Mg Insertion into Spinel-Oxide Cathodes in Magnesium Rechargeable Batteries

Abstract

Modern society strongly demands high energy-density rechargeable storage batteries. Although lithium ion batteries (LIBs) are widely used for a lot of practical applications, the growing rate of the energy densities tends to be saturated recently. If Li metal itself could be adopted as an active material of the anode, LIBs would have exhibited extremely high energy densities, but this cannot be currently realized because of the fatal problem, “dendritic growth” of Li metal on charge. Therefore, in order to further enhance the energy density of storage batteries, we have to develop new type of metal-anode battery systems. As an alternative to Li metal-anode battery, polyvalent-metal storage batteries have recently attracted much attention owing to their large capacities; for example, the capacity of Mg metal (ca. 2200 mAh/g) largely exceeds that of the current carbonaceous anode materials (ca. 370 mAh/g). What is the most important for use as an anode material is that the dendritic formation can be avoided on charging, and in terms of this Mg-metal anode is superior to Li-metal anode. Thus, the Mg rechargeable battery (MRB) field has been currently attracting much attention, but the progress rate is at present quite gradually. Namely, the MRB research is still a very challenging field and not established yet, and hence we have to make much effort to accomplish MRBs. For example, there are few cathode materials for MRBs except for Chevrel compound.[1] Therefore, unless more talented cathode materials that can accommodate polyvalent cations are sought out, MRBs comparable to LIBs in terms of the energy density would not be realized. Here, we focus Mg spinel oxides as candidates for cathode materials of MRBs. The lattice sites in the spinel structure are generally denoted as 8a, 16d (cation sites), and 32e (oxygen sites) in the Wyckoff position in the space group No. 227, while those in the rocksalt structure are denoted as 16c, 16d (cation sites) and 32e (oxygen sites) when it is assigned to the same space group. Thus, a spinel structure can be regarded as a similar rocksalt structure, whose 16c sites are vacant and instead the 8a sites are usually occupied by cations. Therefore, it is expected that Mg cations can be inserted onto 16c vacant sites in the spinel lattice, as well as the Li insertion mechanism in spinel oxide materials.[2,3] Actually, the future Mg battery is expected to be operated at moderately high temperatures in that Mg insertion and extraction can be facilitated at such temperatures. In this work, on the basis of the similarity between spinel and rocksalt structures, with several spinel oxides MgCo2O4, MgMn2O4, MgFe2O4, MgCr2O4, and Co3O4, we demonstrate that some of spinel oxides can allow the insertion of Mg cations at high potentials (about 2-3 V vs. Mg2+/Mg) via “intercalation and push-out” mechanism to form a rocksalt phase in the spinel mother phase.[4] The electrochemical-test temperature was set at 150 °C in the present study by the following two reasons: i) the melting temperature (about 120 °C) of the CsTFSA based ionic liquids[5,6] used here and ii) the enhancement of Mg diffusion in the active materials. For example, especially in MgCo2O4, by utilizing the valence change from Co(III) to Co(II) in MgCo2O4, Mg insertion occurs at a considerably high potential of about 2.9 V vs. Mg2+/Mg, and similarly it occurs around 2.3 V vs. Mg2+/Mg with the valence change from Mn(III) to Mn(II) in MgMn2O4, being comparable to the ab initio calculation. The feasibility of Mg insertion would depend on the phase stability of the counterpart rocksalt XO of MgO in Mg2X2O4 or MgX3O4 (X = Co, Fe, Mn, and Cr). This “intercalation and push-out” process would provide a safe and stable design of cathode materials for polyvalent cations.