Chemical Desodiation, Na/Mg Ion-Exchange and Mg Rechargeable Battery Cathode Properties in NaMn1/2Ni1/2O2

Abstract

1. Introduction In recent years, Mg rechargeable batteries have been studied instead of Li ion batteries to advance their energy densities and to reduce their costs. The layered rock salt structure has excellent battery characteristics in Na and Li ion batteries. Therefore, the layered rock-salt materials are also expected as high energy cathode materials in Mg rechargeable batteries [1]. However, since a (Li or Na)/Mg ion-exchange method has not yet been established, the present study examined to exchange the Na+/Mg2+ based on the soft chemical approach and to desodiate from NaMn1/2Ni1/2O2. We performed the crystal structure analysis and electrochemical measurements for the ion-exchanged and the desodiated NaMn1/2Ni1/2O2 to reveal whether these procedures were available method to synthesis the Mg rechargeable battery cathode materials. 2. Experimental NaMn1/2Ni1/2O2 was synthesized as the precursor for ion-exchange or desodiation via the coprecipitation method [2]. A molten salt or a reflux methods using organic solution were examined as the Na/Mg ion-exchange method. The phases and metal compositions of the synthesized samples were identified by XRD and ICP-AES, respectively. The valence states of the transition metals were evaluated by XAFS (BL14B2, SPring-8). The crystal structure analysis using the Rietveld method was performed for synchrotron XRD patterns (BL19B2, SPring-8). For electrochemical desodiation, the electrode consisted of NaMn1/2Ni1/2O2 : conductive carbon (Super C65) : binder (PTFE) = 5 : 5 : 1 and the metal Li as the negative electrode were assembled to the electrochemical cell, where the 1.0 mol/L LiPF6/EC:DMC (1:2 vol ratio) electrolyte was used. The cell was charged to 4.8 V at room temperature and then disassembled to extract the desodiated cathode in Ar atmosphere. The chemical desodiation was performed by using NO2BF4 oxidizer in the same manner as in the previous report [1]. The Mg battery cathode properties of the synthesized materials were measured using a three-electrode cell (Toyo System), with 1.0 mol/L Mg(TFSA)2/Triglyme as the electrolyte and AZ31 as anode and a Ag wire as reference electrode at 90 °C. 3. Results and Discussion The XRD showed that the precursor was assigned as a hexagonal system (R-3m). It was clarified that an unknown phase was formed after the ion exchange using the molten salt regardless the washing methods. In the ion exchange by reflux, when Mg(NO3)2・6H2O or MgCl2・6H2O was used as the Mg source, the product was attributed as a cubic system (Fm-3m). When MgBr2 was used as the Mg source, the (003) plane shifted to a lower angle. In chemical desodiation, XRD pattern of chemically desodiated Na1-xMn1/2Ni1/2O2 was similar to previously desodiated one by about 1.0 pfu [2]. The metal composition of the precursor was confirmed to be NaMn1/2Ni1/2O2 by ICP-AES. In the ion exchange due to reflux, when Mg(NO3)2・6H2O was used as the Mg source, Na and Mg were removed by washing, and in the case of using MgCl2・6H2O, it was insoluble in various acids. The valences of Mn and Ni decreased from XANES, suggesting that some Na ions were exchanged for Mg ions. In the charge/discharge test, since the theoretical capacity in electrochemical desodiation was achieved, the most of Na must be desodiated. In the discharge test of the Mg rechargeable battery, the initial discharge capacity was 100 mAh/g was obtained, it decreased to about 20 mAh/g after the 2nd cycle. The initial discharge capacity was 250 mAh/g as a result. In chemical desodiation, it was possible to obtain 210 mAh/g, and then to maintain the discharge capacity of 140 mAh/g for 5 cycles (Fig. 1). The capacity of chemically desodiated material was clearly improved compared with the electrochemically desodiated one. 4. Conclusion When MgI2 was used as the reducing agent, MgBr2 was used as the Mg source, and ethanol was used as the solvent, it was clarified that some Na/Mg ion-exchange may have occurred. In the electrochemically desodiated, and the initial high discharge capacity exhibited 250 mAh/g at room temperature. In the chemical desodiation, about 0.8 pfu desodiation and the initial discharge capacity of 209 mAh/g were obtained. It was revealed that the discharge capacity of about 140 mAh/g could be maintained for 5 cycles. Therefore, it is considered that the layered NaMn1/2Ni1/2O2 can be utilized as a cathode material for Mg rechargeable batteries by desodiation or ion-exchange. Acknowledgement This work was supported by JST ALCA-SPRING Grant Number JPMJAC1301, References 1) N. Ishida, S. Ando, N. Kitamura, Y. Idemoto, Solid State Ionics, 343, 115080 (2019).2) S. Komaba, N. Yabuuchi, T. Nakayama, A. Ogata, T. Ishikawa, I. Nakai, Inorg. Chem., 51, 6211-6220 (2012) Figure 1