Synthesis, Cathode Properties and Crystal and Electronic Structural Change in Charge-Discharge Process of Spinel Type Cathode Materials Mg4V5-XNixO12 for Magnesium Secondary Battery

In recent years, research has been conducted into high-performance next-generation batteries due to problems with stability and limits on capacity. The applications of rechargeable batteries can be categorized into three main directions: automotive and transport equipment, the smart grid, and mobile equipment. The functions and performance required for each of these is different, and the design considerations are important. Although research into multivalent cation batteries has been conducted on rechargeable batteries with calcium and aluminum as the mobile ions, discharge capacity and battery characteristics matching those of lithium have not been obtained, and the devices have not reached practical application. Among these, currently the most widely researched is rechargeable batteries that use divalent Mg 2+ as the mobile ion. The road to magnesium rechargeable batteries started in 2000 with research by Aurbach et al. 1) . The existence of a spinel compound MgM 2 O 4 has been reported for M=Co, Mn, and V. The battery characteristics when MgCo 2 O 4 , MgCo 2-x Mn x O 4 2) and MgMn 2 O 4 are used as the cathode material have been reported, and research is also progressing into substitution into these. The synthesis and crystal structure of MgV 2 O 4 3) has been reported, and research is being conducted into the battery characteristics of substitution into Mg(Mg 0.5 V 1.5 )O 4 4) made with an excess of Mg. V is thought to have high reversibility due to its broad mixed valence. A wider mixed valence can be used by synthesizing V with a low valence. V can be synthesized at the desired valence by adjusting the sintering temperature in a vacuum atmosphere. In this research, we synthesized Ni-substituted Mg 4 V 5-x Ni x O 12 based on Mg(Mg 0.33 V 1.67 )O 4 with the aims of increasing the discharge capacity and improving the battery characteristics, and investigated the crystal and electron structure, and the charge and discharge characteristics depending on the amount of substitution and the structural changes accompanying charging and discharging. Mg 4 V 5-x Ni x O 12 was synthesized by solid phase method under high vacuum atmosphere 5) . The product was assigned to spinel structure with the space group of Fd-3m from the powder X-ray diffraction. Synthesized materials showed the particles in uniform composition confirmed from elemental mapping by STEM-EDS. The charge and discharge cycle tests were showed the discharge capacity of exceeding 150 mAh g -1 at 90 °C. Cyclability was reversible up to 14 cycles. Mg 4 V 4.7 Ni 0.3 O 4 can delivered to high capacity of 194 mAh g -1 at a low rate of 1 mA g -1 . From the Rietveld analysis of the electrode, the occupancies of 8a and 16c increased after charging. Since the Mg occupancy at 16c site greatly changes after charging at the second cycle and after discharging, it is considered that the discharge capacity is largely responsible for Mg occupancy of 16c sites. From the XANES spectra, energy shifts from low energy to high were observed in the spectra of V and Ni. Therefore, the charge-discharge mechanism was revealed to be the redox of V and Ni involved in charge and discharge. Acknowledgements This work was supported by JST ALCA-SPRING Grant Number JPMJAC1301, Japan. We are deeply grateful for the cooperation of Dr. Keiichi Osaka of JASRI for the measurement of the synchrotron X-ray diffraction (SPring-8,BL19B2), Dr. Tetsuo Honma and Dr. Hironori Ofuchi of JASRI for the XAFS analyses (SPring-8, BL14B2). References 1) D. Aurbach, et al., Adv. Mater. , 19 , 4260 (2007). 2) Y. Idemoto, Y. Mizutani, C. Ishibashi, N. Ishida and N. Kitamura, Electrochemistry , 87 , 220(2019). 3) H.Uchida, et al. Amer. Min. , 92 , 1031(2007). 4) S. Ikeda, N. Ishida, N. Kitamura, and Y. Idemoto, Abstract of the 56th Battery Symposium in Japan, 546 (2015). 5) Y. Idemoto, N. Kawakami, N. Ishida, N. Kitamura, Electrochemistry, 87 , 281(2019). Figure 1