Crystal Structures and Cathode Properties of Delithiated LixNi0.5Mn0.5O2 for Mg Rechargeable Batteries
Authors: Ishida Naoya , Yamazaki Naoto , Mandai Toshihiko , Kitamura Naoto , Idemoto Yasushi 
Published: December 2020

The Mg rechargeable batteries (MRBs) have attracted attention owing to the high volumetric capacity and safety of Mg metal anodes. A recent study of delithiated Li1-x(Mn1/3Ni1/3Co1/3)O2 revealed that an O3-type layered rock-salt structure was preferable for MRB cathode materials [1]. However, previous delithiated cathode materials showed poor cyclability. In the present study, we therefore focused on O3-type LiNi0.5Mn0.5O2, which could retain the crystal structure with delithiation. The purpose of this study was to examine the relationship between the cathode properties and Li contents in LixNi0.5Mn0.5O2.
Electrochemical delithiation for LiNi0.5Mn0.5O2 was performed by galvanostatically charging in a Li cell, where the cathode was consisted of LiNi0.5Mn0.5O2, carbon black and PTFE on an Al mesh. We also subjected the LiNi0.5Mn0.5O2 to chemical delithiation (CD) using NO2BF4 oxidizer to obtain x = 0.1 and 0.4 in LixNi0.5Mn0.5O2. The resulting materials were assessed using XRD and ICP-AES. The crystal structures were analyzed using the Rietveld method for synchrotron XRD patterns (BL19B2, SPring-8). Valence states were estimated by XAFS (BL14B2, SPring-8). Either an HS cell or a three-electrode cell (Hohsen Ltd.) was used in a Mg battery, where AZ31 alloy, a 0.5 mol/L Mg[TFSA]2/acetonitrile or a 0.3 mol/L [Mg(G4)][TFSA]2/[PYR13][TFSA] [2] and a glass-fiber filter were used as the anode, electrolyte and separator, respectively. A three-electrode cell construction was employed for the evaluation of the cathode properties without an overvoltage.
The extent of delithiation in LixNi0.5Mn0.5O2 was controlled by restricting the charging of the LiNi0.5Mn0.5O2/Li cell. The MRB cathodes fabricated from electrochemically delithiated Li0.5Ni0.5Mn0.5O2 (x=0.5ED) and Li0.1Ni0.5Mn0.5O2 (x=0.1ED) were evaluated by assessing their discharge/charge performance. The initial discharge capacity of the x=0.5ED/AZ31 cell was 22 mAh g-1, corresponding to the insertion of 0.04 Mg2+ ions per formula unit (pfu). We also examined the chemically delithiated (CD) LixNi0.5Mn0.5O2. Fig. 1 (a) presents the discharge and charge curves for x=0.45CD/AZ31/Ag three-electrode cell measured at 90 °C. Although the initial discharge capacity was low, the second one reached at 220 mAh g-1. Since the average potential showed 2.4 V vs. Mg/Mg2+, the high energy density about 500 Wh/kg was obtained. Furthermore, the cycle performance shown in Fig. 1(b) was characterized in the retention of more than 150 mAh g-1. The high capacity and high cycle performance must be derived from the ionic liquid electrolyte, 0.3 M [Mg(G4)][TFSA]2/[PYR13][TFSA], the three electrode-cell without overvoltage and the chemical delithiation without structural transformation.
This work was supported by JST ALCA-SPRING Grant Number JPMJAL1301, Japan.

Reference
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[1] N. Ishida, S. Ando, N. Kitamura, Y. Idemoto, Solid State Ionics, 343, 115080-1 (2019)
[2] T. Mandai, K. Tatesaka, K. Soh, H. Masu, A. Choudhary, Y. Tateyama, R. Ise, H. Imai, T. Takeguchi, K. Kanamura, Phys. Chem. Chem. Phys., 21, 12100 (2019)

Figure 1