Defect Rocksalt Oxides As Cathodes for Rechargeable Magnesium Batteries
Authors: Kawaguchi Tomoya , Yasuda Masaya , Nemoto Natsumi , Shimokawa Kohei , Li Hongyi , Okamoto Norihiko L , Ichitsubo Tetsu 
Published: December 2024

Oxide cathode materials are promising candidates for high-energy-density rechargeable magnesium batteries (RMBs) due to their high electrode potential. However, it is still challenging to reversibly insert/extract Mg into/from oxide cathodes since Mg insertion generally induces a partially irreversible phase transformation to a rocksalt structure (rocksalt transformation). This rocksalt transformation generally degrades the cyclability because the densely packed cations in the rocksalt structure impede the Mg migration. We reported that a defect spinel oxide of ZnMnO3 successfully suppressed the rocksalt transformation within a limited capacity of ~70 mAh g-1 [1]. However, the excessive Mg insertion resulted in the rocksalt transformation, limiting the available capacity. Elucidating the activation mechanism of the rocksalt oxide cathodes for RMBs is of great importance in circumventing this issue and ultimately utilizing rocksalt oxides themselves as promising cathode materials for RMBs.
To tackle this problem, we designed a disordered-rocksalt oxide of Mg0.35Li0.3Cr0.1Mn0.05Fe0.05Zn0.05Mo0.1O (M7O), in which initially contained Li is extracted in the first charging to yield substantial cation vacancies that facilitate the Mg migration in the structure, eventually activating the reversible Mg insertion/extraction in the subsequent cycles [2]. High configurational entropy derived from many constituent cations also plays a pivotal role. For example, high valence cations of Cr(III) and Mo(IV) do not solely form the rocksalt oxides. However, these elements were successfully incorporated into the structure. Furthermore, the formed vacancies were expected to be stabilized by high configurational entropy during the battery operation.
The single-phase oxide of the disordered rocksalt structure was successfully synthesized using the Pechini method. The reversible discharge capacity of ~80 mAh g-1 was constantly observed at a current density of 10.4 mA g-1 at 90 °C. Inductively coupled plasma optical emission spectroscopy confirmed the Li extraction in the 1st charging and subsequent reversible Mg insertion/extraction via the cation vacancies formed by the Li extraction, which was further corroborated by X-ray diffraction. X-ray absorption spectroscopy indicated that Mn, Fe, and Mo compensate for the charge upon charging and discharging by changing their valence states, while those of Cr and Zn hardly changed. The ab initio calculations demonstrated that adjacent two cation vacancies significantly decrease the activation energy of the Mg migration in the rocksalt structure from ~2000 to 800 meV. The composition range to ensure that such a fast diffusion path is thoroughly connected in the particle was evaluated using the percolation theory. The present study opens a new frontier of the oxide cathodes for RMBs.
[1] K. Shimokawa et al., Advanced Materials 33, 2007539 (2021). https://onlinelibrary.wiley.com/doi/10.1002/adma.202007539
[2] T. Kawaguchi et al., J. Mater. Chem. A (2024). https://pubs.rsc.org/en/content/articlelanding/2024/ta/d3ta07942b
Image caption: (Left) Schematic of the reversible Mg insertion/extraction mechanism in the present rocksalt oxide cathode. (Top right) Energy dispersive spectroscopy images of the constituent elements. (Bottom right) Charge/discharge curves of the present cathode material.

Figure 1