Investigation of Stable Structures, Electronic States and Mechanism of Mg Intercalation of MgCo2−ZNi0.5MnAlzO4 (z = 0, 0.3) during Charge and Discharge Process as Cathode Materials for Magnesium Rechargeable Batteries Using First-Principles Calculations
Authors: Ishibashi Chiaki , Takeuchi Ryo , Kitamura Naoto , Idemoto Yasushi 
Published: December 2024

Introduction

In recent years, clean energy materials such as storage battery materials have been attracting attention in order to reduce greenhouse gas emissions. Li ion batteries (LIB) are already widely used as batteries for laptop and smartphones. However, Li metal is expensive and new storage battery materials with higher volumetric energy density are needed to be developed for application in large power supplies. Therefore, storage battery materials using Mg metal with divalent ions are currently attracting attention. Our group has been developing spinel Mg-(Co, Ni, Mn, Al)2-O4 cathode materials with three-dimensional diffusion pathways1). Spinel MgCo2-zNi0.5MnAlzO4 (z=0, 0.3) were synthesized and electrochemical tests revealed that the Al substituted material has particularly improved battery properties. Furthermore, the stable structure and electronic state of pristine MgCo2-zNi0.5MnAlzO4 (z=0, 0.3) were calculated using first principles calculations to clarify the effect of substituted species2). The results predicted that the Mg-O bonding around Al was specifically weakened, allowing Mg to diffuse easily move the crystal.
In this study, we clarify the stable structure of spinel MgCo2-zNi0.5MnAlzO4 (z=0, 0.3) after discharge and charge, and clarify the electronic structure and the mechanism of Mg intercalation during charge and discharge processes.

Calculation

The structural relaxation calculation was performed by generalized gradient approximation (GGA) exchange-correlation potential used with a plane-wave cutoff energy of 550 eV, and 1×2×2 k-point mesh was applied, corresponding to the inverses of the lattice constants on Vienna Ab-initio Simulation Package (VASP) code. The convergence criterion for structural optimization for pristine model and during discharge and charge models was that the energy difference per iteration be less than or equal to 0.02 eV/Å. 2×1×1 supercell was created and structural relaxation calculation was performed.

Result and discussion

In this study, the stable structure of MgCo2-zNi0.5MnAlzO4 (z=0.3) during discharge was first obtained from the stable pristine structure. Figure 1 a) and b) show the stable structures of pristine and discharge model obtained by structural relaxation calculation, respectively. Fig. 1 b) is the stable structure after discharge in which Mg atoms are inserted into the vacancy 16c site in the stable pristine structure shown in a) (Mg insertion amount is 0.375). After insertion of Mg, the atoms on the 8a site move into the direction of the arrow in shown in b) to vacant 16c site, resulting in a vacancy at the 8a site. It is clear that the structural change from a spinel type structure to a rocksalt type structure partially occurs during the discharge.
Therefore, we further show in Fig. 1 c) the stable structure of z=0.3 after charging in which the same amount of Mg insertion on the 16c site in the stable structure during discharge shown in b) is desorbed. After structural relaxation, Mg atoms on 16c site indicated by the dashed circle migrates to vacant 8a site during charge. During charging, it is apparent that the partial rock-salt type structure reverses to spinel type due to the desorption of Mg. The desorption of Mg from the 16c sites around the 8a vacancy site reduces the repulsion between cations, allowing the remaining 16c site Mg to move to the 8a site and return to the spinel type structure. Therefore, if a large structural change occurs during discharge, in which Mg insertion increases and almost all 8a site Mg becomes vacancies, it is not possible to return to the spinel type structure during charging. However, some 8a sites remain vacant in Fig. 1 c). When Co with cation mixing on the 8a site moves to the 16c site during discharge, Co cannot move to vacant 8a site because the bond between Co-O is much stronger than the bond between Mg-O. Therefore, it is clear that the Co influence at the 16c site also prevents the adjacent Mg from migrating to the 8a site, resulting in vacancies. Therefore, it is clear that due to the influence of Co at the 16c site, Mg adjacent to Co also cannot migrate to the 8a site. The cation mixing between Mg and transition metals is suppressed, and the change to a rocksalt type structure during discharge is suppressed during the charge-discharge process.

Acknowledgement

This research was financially supported by JST, GteX Program Grant Number JPMJGX23S1, Japan.
1) Y. Hirata, Y. Idemoto et al., ECS Meeting Abstracts, MA2020-02
, 3451 (2020)
2) C. Ishibashi, R. Takeuchi, N. Kitamura, Y. Idemoto, J. Phys. Chem. C., 127, 10470-10489 (2023).

Figure 1