Synthesis, Cathode Properties and Crystal and Electronic Structure Changes in Charge/Discharge Process of Spinel Type Cathode-Materials 0.3MgCo2-XMnxO4-0.7Mg(Mg0.33V1.67-yNiy)O4 for Magnesium Secondary Batteries
Authors: Idemoto Yasushi , Kumagai Shinichi , Ishibashi Chiaki , Kitamura Naoto 
Published: December 2024

In recent years, attention has focused on the development of next-generation rechargeable batteries using multivalent cations with even higher capacities and longer lifetimes. Among these, magnesium secondary batteries have attracted attention due to their high volumetric energy density, safety aspects, and low cost. In spinel-type Mg1+yCo2-x-yMnxO4, charge and discharge capacities were reported to increase by substituting Mn1). The authors synthesized Mg1.33V1.57Ni0.1O4, and reported that the discharge capacity after 14 cycles was 160 mAh g-1 and the capacity retention rate was high2). Mg1.33V1.57Mn0.1O4 showed the largest discharge capacity of 256 mAh g-1 at 13th cycles while the initial capacity was only 73 mAh g-1 at 90°C3). 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-1 with the discharge capacity maintained after 60 cycles4
).
In this study, with the aim of increasing the discharge capacity of these cathode materials, new spinel type cathode material with increased Mn and Ni substitution amount, the solid solution 0.3MgCo2-xMnxO4-0.7Mg(Mg0.33V1.67-yNiy)O4 (x= 0.6, y=0.1, 0.15, 0.2) were synthesized and their battery properties were evaluated. The well-characterized sample (x= 0.6, y=0.15) was also evaluated for battery properties as a function of operating temperature. In addition, the average, local and electronic structure analyses of the pristine samples and the electrodes after charge/discharge process were carried out to elucidate the relationship between the battery properties.
0.3MgCo2-xMnxO4-0.7Mg(Mg0.33V1.67-yNiy)O4 (x=0.6, y=0.1 0.15 0.2) was synthesized by the reverse co-precipitation method. The obtained samples were analyzed to identify the phase by powder X-ray diffraction, to determine the metal composition by ICP-AES. The charge-discharge tests were carried out at 90 °C and 40 °C in a 3-type cell. Synchrotron XRD (BL19B2, SPring-8) was performed on the pristine samples and some of the post-charge/discharge electrodes, and Rietveld analysis (RIETAN-FP) revealed the average structure; XAFS (BL14B2, SPring-8) investigated the valence of transition metal elements in the pristine and after charge-discharge electrodes.
The products were assigned to the spinel structure with space group Fd-3m by X-ray powder diffraction. The Mg and Co and Mn and Ni contents were higher and the V content was lower than the nominal composition by ICP-AES. The charge/discharge cycling tests showed that the sample (x=0.6, y=0.15) exhibited the highest battery characteristics, with an initial discharge capacity of ca. 280 mAh g-1 and a discharge capacity of ca. 220 mAh g-1 after 19 cycles at 90°C with a cut-off voltage from 1.500 to -0.900 V vs. Ag/Ag+ (4.000 V ~ 1.445 V vs. Mg/Mg2+) (Fig.1). The sample (x=0.6, y=0.15) also showed good battery properties with a maximum discharge capacity of ca. 128 mAh g-1 at 40°C.
Based on Rietveld analysis results, the pristine samples showed good fitting. The cation mixing decreased with increasing Mn and Ni substitution content. The electronic structure was analyzed by maximum entropy method (MEM) based on Rietveld analysis results, which showed that Mg insertion was easier than in αMgCo1.5Mn0.5O4-(1-α)Mg(Mg0.33V1.57Ni0.1)O4 (α=0.3)1).
Rietveld analysis of the electrode (90°C) after the charge-discharge test of the sample with the best properties (x=0.6, y=0.15) showed that the ratio of spinel phase to rock salt phase was almost the same after the first discharge and after the 5th discharge. This suggests that a reversible phase transition occurs with the charge/discharge process.
From the XANES spectra, the valence of all the transition metals shift to the lower valence side after discharge, with Mn in particular showing a significant shift. The EXAFS analysis revealed a relaxation of the distortion between Ni and the other metals, which may be one of the reasons for the improved battery performance due to the increased Ni substitution.

Acknowledgements

This research was financially supported by JST ALCA-SPRING project (JPMJAL1301) and GteX Program Japan Grant Number JPMJGX23S1. 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) Y. Idemoto, et al., J. Power Sources, 482, 228920 (2021).
2) Y. Idemoto, et al., J. Power Sources, 455, 227962 (2020).
3) Y. Idemoto, et al, J. Electroanal. Chem., 928, 1107064 (2023).
4) Y. Idemoto, et al, Electrochemistry, 90(2), 027002(2022).

Figure 1