Recent interest in the magnesium rechargeable batteries may be attributed both to their technological potential and to the fact that they represented an intercalation mechanism of magnesium into the oxide materials. The chevrel compound, Mo₆S₈, has been attracted much attention because of its high reversibility and excellent cycle life. The low stability and capacity, however, have led to the investigation of alternative materials. So far, the delithiated Li compounds, LiFePO₄ and Li₃V₂(PO₄)₃, have been studied to show high specific capacities of about 300 mAh g^-1 and 200 mAh g^-1, respectively. Although the theoretical capacity of delithiated Li_1-xNi_0.5Mn_0.5O₂ exceed 500 mAh g^-1, the examination of the use as the cathode of magnesium rechargeable battery has not been performed.

The synthesis of delihitated Li_1-xNi_0.5Mn_0.5O₂ has opened route for the electrochemically or chemically delihiated methods [1]. The advantage of such an electrochemically method is to control the deilthiation amount whereas a chemically method is to obtain the delithiated sample without the additives such as conductive carbon and PTFE binders. In this study, the delithiated Li_1-xNi_0.5Mn_0.5O₂ was synthesized and analyzed about the crystal structures and valence states of Ni and Mn before and after magnesiation.

Experimental

The LiNi_0.5Mn_0.5O₂ was synthesized via a traditional solid-state reaction route. The LiOH·H₂O, NiO and MnO₂ were thoroughly mixed by planetary ball milling. The obtained powders were heated at 600 °C in air for 5 hours and then pressed into a pellet to calcine at 1000 °C in air for 10 hours.

The delithiated Li_1-xNi_0.5Mn_0.5O₂ was prepared by electrochemical method and chemical one using NO₂BF₄ oxidizer. The LiNi_0.5Mn_0.5O₂ powders were stirred in NO₂BF₄ solution in acetonitrile for 12 hours. When the reaction was completed, the solution was filtered and washed with acetonitrile. Chemical analyses for Li, Ni and Mn were carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES) to evaluate the delithiation amounts. Powder X-ray diffraction data were collected on a Panalytical X’pert-pro diffractometer. The morphology was observed by scanning electron microscopy (SEM, Hitachi, S-2600N). The crystal structure analysis was performed by using the synchrotron X-ray diffraction at BL19B2 (SPring-8, JAPAN). The valence states of Ni and Mn before and after magnesiation were examined by the XAFS at BL14B2 (SPring-8, JAPAN).

Discharge and charge tests were performed at 60 °C using a positive electrode, which consisted of the delithiated Li_1-xNi_0.5Mn_0.5O₂, conductive carbon, and poly-tetrafluoroethylene powder between 0.5 and 3.0 V using an Mg-alloy negative electrode at a fixed current density of 5 mA g^-1. A solution of 0.5 M Mg(TFSI)₂ in a acetonitrile (Kishida Chemical Co., Ltd.) was used as the electrolyte. The cells were constructed in an argon-filled globe box.

Results and Discussion

The XRD pattern for the pristine LiNi_0.5Mn_0.5O₂ was attributed to the ordered rock-salt structure with R-3m space group. The electrochemically delithiated Li_0.1Ni_0.5Mn_0.5O₂ was transformed to different layered structure. On the other hand, the chemically delithiation of LiNi_0.5Mn_0.5O₂ resulted in the only peak shift of the XRD while retaining the original structure. The cathode performance of electrochemically delithiated Li_0.1Ni_0.5Mn_0.5O₂ showed about 35 mAh g⁻¹ at first and second discharges (Fig.). Although the first charge capacity was too large for first discharge, the agreement of the discharge capacities at first and second cycles indicated to demagnesiate from the magnesiated Mg_xLi_0.1Ni_0.5Mn_0.5O₂ at first charge. The synchrotron X-ray diffraction patterns of pristine and delithiated and magnesiated samples were analyzed by Rietveld method and were not fully fitted to the layered rock-salt structure for delithiated and magnesiated those. The refined lattice parameters showed to intercalate the magnesium in the Li_0.1Ni_0.5Mn_0.5O₂. In addition the XANES spectra of the pristine and delithiated and magnesiated samples were measured to show the valence change corresponding to the delithiation and magnesiation.

Reference

[1] N. Ishida, N. Tamura, N. Kitamura, and Y. Idemoto, J. Power Sources, 319, 255 (2016)