Synthesis, Electrochemical Properties and Crystal and Electronic Structure Changes in Charge/Discharge Process of Spinel Type Cathode-Materials Mg1.33-Y(V1.67-x+yMnx)O4 for Magnesium Secondary Batteries
Authors: Idemoto Yasushi , Takamatsu Mina , Ishibashi Chiaki , Kitamura Naoto 
Published: February 2024

Attention has therefore focused on rechargeable batteries using multivalent cations, which are expected to offer even higher energy densities than lithium ion batteries. One topic that has attracted attention is rechargeable batteries using Mg as a carrier. The advantage of using Mg is that it has a low electrode potential with a high capacity.
Although previous research into Mg rechargeable battery cathode materials started with a report by Aurbach1) et al., and included chevrel compounds, these have had problems in terms of battery voltage and cycle properties. In spinel-type Mg1+yCo2-x-yMnxO4, charge and discharge capacities were reported to increase by substituting Mn2). V is thought to have high reversibility due to having a broad mixed atomic valence. The authors synthesized Mg1.33V1.57Ni0.1O4, and reported that the discharge capacity after 14 cycles was 160 mAh/g and the capacity retention rate was high3). Furthermore, the authors investigated the spinel solid solution αMgCo1.5Mn0.5O4-(1-α)Mg(Mg0.33V1.57Ni0.1)O4 as a new cathode material and evaluated its battery properties, and as a result found that the initial discharge capacity at α = 0.3 was 180 mAh/g with the discharge capacity maintained after 60 cycles4
). However, the substitution products need to be investigated in order to improve the discharge potential of the Mg-V system and enhance the cycle properties.
The present study investigated the synthesis of a new spinel type cathode material Mg1.33-y(V1.67-x+yMnx)O4 by substituting Mn into Mg1.33V1.67O4 and different Mg content in order to increase the charge-discharge capacity and improve the cycle properties. The crystal and electronic structures and charge-discharge properties depending on the amount of Mn substitution and Mg content, and the structural changes accompanying charging and discharging.
Mg1.33-y(V1.67-x+yMnx)O4 was synthesized by a solid -phase method under high vacuum atmosphere. The product was assigned to the spinel structure with space group Fd-3m from the powder X-ray diffraction. Synthesized materials of uniform composition were confirmed from elemental mapping by STEM-EDS. Charge and discharge cycle tests showed that the deliverable discharge capacity depends on the cycle number, Mn composition, and working temperature. The charge and discharge cycle tests showed the discharge capacity increased with repeated cycles. Mg1.33V1.57Mn0.1O4 showed the largest discharge capacity of 256 mAh/g at 13th cycles while the initial capacity was only 73 mAh/g at 90°C. The local structure was analyzed based on EXAFS spectra at the V K-edge for the pristine and electrode materials, and the results suggested that Mg1.33V1.57Mn0.1O4 had the smallest lattice distortion due to Mn at 16d sites, and XAFS spectra at the V K-edge indicated the significant change in the oxidation state of V during the charge/discharge cycles. The particularly stable crystal structure and large contribution of charge compensation by V may jointly contribute to the superior charge-discharge property of Mg1.33V1.57Mn0.1O4
5).
Moreover, we investigated the different Mg, Mn content sample. Mg1.25(V1.55Mn0.2)O4 showed an increase in capacity from the first discharge capacity 71mAh/g to 199mAh/g after 34 cycles at 90℃ (Fig.1). The change in crystal structure due to charging and discharging was investigated. Focusing on the magnitude of M(16d)-O6 octahedral strain, the strain after 10th discharge is significantly reduced, so it is considered that the structure was stabilized by repeating the cycles and contributed to the improvement of the cycles characteristics. The electronic structure was analyzed by MEM, which showed that the electron density between the 16d-32e sites was higher and the covalent bond was higher than that of the diffusion sites after charge and discharge. From this, it is considered that the host structure was stabilized by charge and discharge, which contributed to the increase in charge and discharge capacity. From the result of XANES, it revealed that V is trivalent to tetravalent and Mn is divalent. Furthermore, from the results of EXAFS, it was clarified that the amount of reduction was larger in Mg1.25(V1.55Mn0.2)O4, especially in charge and discharge, because the amount of change in the distance between V and O was larger.

Acknowledgements

This work was supported by JST ALCA-SPRING Grant Number JPMJAC1301, Japan.
We are deeply grateful for the cooperation of Dr. T. Honma and Dr. H. Ofuchi of JASRI for the XAFS analyses (SPring-8, BL14B2), Dr. K. Osaka of JASRI for the measurement of the synchrotron X-ray diffraction (SPring-8, BL19B2).

References

1) D. Aurbach, et al., Adv. Mater., 19, 4260 (2007).
2) Y. Idemoto, et al., J. Power Sources, 482, 228920 (2021).
3) Y. Idemoto, et al., J. Power Sources, 455, 227962 (2020).
4) Y. Idemoto, et al, Electrochemistry, 90(2), 027002(2022).
5) Y. Idemoto, et al, J. Electroanal. Chem., 928, 1107064 (2023).

Figure 1