Solvent Molecule Decomposition at the Spinel Cathode Surface of Mg-Battery: A First-Principles Study
Authors: Sodeyama Keitaro , Kaneko Tomoaki , Fujihara Yui , Mandai Toshihiko , Kobayashi Hiroaki 
Published: December 2024

Mg-based spinels (SP) such as MgM2O4 (M=Mn, Fe, Co) attract much interest as cathode material of future Mg-battery owing to those high capacities and high voltage. In the discharging process, Mg-based SP transforms into a rocksalt (RS) phase,
MgM2O4 + Mg2+ + 2e- → Mg2M2O4.
Due to low reversibility and slow Mg2+ kinetics, however, the Mg-battery with Mg-based SP cathode is not used for the practical applications. For the reversibility problem, the oxidative decomposition of solvent molecules is a crucial issue, which is the purpose of the present study.
In this study, we performed theoretical simulations based on the density functional theory using Quantum Espresso code. The cutoff energy of a plane wave basis set is 35 Ry. The PBE functional of generalized gradient approximation with D3-version of van der Waals correction was employed. The isotropic on-side Coulomb term U was included for the 3d orbitals of Fe, Mn and Co. Both (001) orientation of SP and RS surfaces were considered since these surfaces have the lowest surface energy. While the long glyme molecules (such as G3, and G4) are used in the practical experiments, we employed the simplest ether molecule, i.e., diethyl ether as a model solvent molecule.
First, we briefly mention the surface reconstruction of the SP surface. For the (001) surface of SP, the topmost Mg2+ ions which originally belong to the 16c site of bulk SP move to the 8a site which the Mg2+ ion sites of RS phase. Then, the surface of SP increases the RS-like feature and stabilizes. The growth of the RS phase on the SP surface was observed by TEM for MgMn2O4. In the following calculations, we used such reconstructed surface models for the adsorption and decomposition calculations.
Next, we will discuss the solvent molecule decomposition. We consider the reaction energies of C-H dissociation and C-O dissociation for both SP and RS surfaces, which are defined by the difference of adsorption energies between the reactants and products.
For the reactant, we put diethyl ether with the O atom of diethyl ether being the Mg or M ion sites. For the products, the H atom and -CH3 group are placed at the top of surface oxygen sites. We found that the C-H dissociation reactions on SP surfaces are exothermic irrespective of M elements, while the other reactions, i.e., the C-H dissociation reactions on SP and C-O dissociation on both SP and RS are endothermic.
According to the charge analysis using Bader charge, the C-H dissociation can be understood as deprotonation rather than dehydrogenation. Then, an electron used in the C-H bond transfers to the M3+ ion. For the RS surface which does not contain M3+ ions, the electron cannot transfer to the RS surface. Therefore, the presence of M3+ in SP is important for this reaction. We also investigated the electronic structures of reactants and products by calculating LDOS and PDOS. The HOMO level of solvent is located 1.4-2.6 eV below the valence band maxima, suggesting that the adsorbates are hard to be oxidized directly. For the C-H dissociation reaction on SP surfaces, on the other hand, the valence band maxima shifts upward due to the reduction of M3+ ion. Since such energetically unstable occupied states can be easily oxidized, the electron at this level can be extracted as an oxidative current.
[1] T. Kaneko, Y. Fujihara, H. Kobayashi and K. Sodeyama, Appl. Surf. Sci. 613, 156065 (2023).
[2] T. Kaneko, Y. Fujihara, T. Mandai, H. Kobayashi and K. Sodeyama, Electrochemistry 23, 00087 (2024).

Figure 1