Synthesis, Crystal Structure Analysis and Electrochemical Properties of MgxNi1-XO2.

INTRODUCTION Currently, the secondary battery having a high energy density as a high performance secondary battery has been developed. Although polyvalent cations secondary batteries such as Mg and Ca batteries are a promising candidate, polyvalent cations secondary battery as alternative to lithium ion batteries has not been reported. In the lithium-ion battery, LiNi 0.5 Mn 1.5 O 4 was well known to show two-electron reaction from Ni 2 + to Ni 4+ and high potential of 4.7 V. Interestingly, the electrochemical characteristic of LiNi 0.5 Mn 1.5 O 4 was due to firing temperature changes. Therefore, the oxides consisted of Mg and Ni can be expected to be high potential and capacity from a viewpoint of the combination with divalent cations and two-electron reaction. However, synthesis of the oxides consisting of Mg and Ni has been only reported in the limited chemical composition, MgNiO 2 and Mg 0.67 Ni 1.33 O 2 , and not reported the synthesis of other Mg / Ni ratio 1,2) . The present study was aimed at examination of difference in the electrochemical properties due to the difference in the firing temperature, controlling the rock-salt type Mg x Ni 1-x O 2 to Mg/Ni ratio, and evaluation as a magnesium ion battery cathode material. EXPERIMENTAL Mg x Ni 1-x O 2 was synthesized by a reverse coprecipitation synthesis 1) . Mg x Ni 1-x O 2 synthesized by nitrate aqueous solution of Mg and Ni were mixed to Na 2 CO 3 solution while heating to 80° C, and the precursor dried (100 ° C, Air, 24h), and firing (550 - 950° C, Air, 24h). It was examined that the change in Mg/Ni ratio by changing the concentration of Na 2 CO 3 solution. The samples were characterized by powder X-ray diffraction and ICP measurements. We also examined the valence of Ni by XAFS analysis (BL14B2, SPring-8). Before Charge-discharge test, active material and conductive agent in a weight ratio of 10:3 carbon coating (300rpm, 2h) by a planetary ball-mill. The positive electrodes were mixed active material and a conductive carbon and a binder (PTFE) at 5: 5: 1 weight ratio. Negative electrode used Mg metal or AZ31, electrolyte used 0.5 mol/L-Mg(N(CF 3 SO 2 ) 2 ) 2 /Acetonitrile (AN) or 1.0 mol/L-Mg(N(CF 3 SO 2 ) 2 ) 2 /Triglyme (G3). We attempted the Rietveld analysis (RIETAN-FP) and MEM analysis (Dysnamia) using the measured synchrotron X-ray diffraction data(BL19B2, SPring-8) RESULTS AND DISCUSSION The synthesized samples were identified as Rock-salt structure with space group Fd-3m by powder X-ray diffraction. To refine the crystal structure of the samples, we performed the Rietveld analysis using the synchrotron X-ray diffraction (e.g. R wp =5.60%, R e =2.35%). In addition, the results of ICP analyses for the compounds revealed that Mg/Ni ratio of the samples were ranged from 0.28 to 0.48 by changing Na 2 CO 3 aq. concentration. The XAFS analysis, the valence state of Ni was determined to be divalent (Fig.1). The intensity ratio of d 111 / d 200 increased with the increasing the Mg/Ni ratio in the X-ray diffraction. We performed a charge-discharge test. Results of the charge-discharge test using AN showed 217mAh/g for the sample fired at 950 °C . The sample being higher Mg/Ni ratio tended to show higher capacity. However, the capacity might increase by a side reaction because of the electrochemical reaction occurred at extremely low voltage. To solve this reaction, electrolyte was changed from AN to G3 and the current collector was changed from Pt to Ni. As a result for such condition, it showed high reversibility and suppressed the side reactions. We examined the elimination of Mg by an oxidizing agent of NO 2 BF 4. Since the lattice constants did not change before and after treatment, it was suggested elimination of Mg by chemical treatment was difficult. Therefore, we partly replaced Mg with Ca for the improvement of electrochemical property.We confirmed that (Mg,Ca) x Ni 1-x O 2 was synthesized from the increase of the lattice parameter. As a result of the charge-discharge test for (Mg,Ca) x Ni 1-x O 2 , discharge potential was increased from 0.5V to 0.8V. Therefore, it may be possible to significantly improve the properties of Mg secondary battery of rock-salt type by Ca substitution. It was performed by help of ALCA-SPRING and shows thanks to the members concerned. References 1)S. Yagi et al., Jpn. J. Appl. Phys., 52, 025501 (2013). 2)T. Ichitsubo, T. Adachi, S. Yagi, T. Doi, J.Mater.Chem. , 21 , 11764(2011). Figure 1