New Na0.7Mn0.6Ni0.3Co0.1O2 Na Layered Oxides As Electrode Materials for Na-Ion Batteries

Introduction

 Sodium–ion batteries are considered to be one of the favourite for the future hybrid and electric vehicles because of their low price and sodium abundant resources. Sodium layered oxides (Na_xMO₂) are the expectable candidates as cathode material in respect of potential and sodium–ion diffusion. Especially, O3 type is mainly focused, however, the O3 type structure can be transformed into the spinel type structure that leads to capacity decay in several cycles. From the structural stability point of view, the P2 type structure is able to avoid converting to the spinel type structure due to different oxygen stacking. We focus on synthesizing the new P2 material including three transition metals, such as Mn, Ni, and Co, in order to achieve higher capacity and cyclability.

 Experimental

 We adopted a co−precipitation method in order to make the aimed compound. A solution of transition metal nitrates, and another solution of sodium carbonate were dripped into a beaker with distilled water simultaneously. The washed precipitate was dried at 80 ̊C for two days. The obtained powder was mixed with sodium carbonate with an excess amount, and annealed at 900 ̊C in air atmosphere, and finally quenched to room temperature. The crystal structure was studied by XRD. The electrochemical performances were evaluated in two electrodes cell configuration. The positive electrode was a mixture containing 88 wt. % of the active material, 10 wt. % of graphite as the conductor and 2 wt. % of polytetrafluoroethylene (PTFE). The electrolyte was 1 M NaPF₆ in propylene carbonate (PC) with fluoroethylene carbonate (FEC) at 2 wt. %. Galvanostatic performances were carried out between 4.0 V and 1.5 V vs. Na⁺/Na at the current rate of 0.05 C and 25 ̊C. Cycle performance was evaluated between 3.8 V and 1.5 V at same condition.

Results

 Among all composition studied, a pure phase with the P2 structure was obtained for the Na_0.7Mn_0.6Ni_0.3Co_0.1O₂ composition. Fig. 1 shows the XRD pattern of Na_0.7Mn_0.6Ni_0.3Co_0.1O₂ phase. It indicates that the sample crystallizes in the hexagonal system (S.G., P6₃/mmc) and there were no impurities in the material. The cell parameters are a_hex. = 2.8847 Å, c_hex. = 11.0833 Å. The material has excellent electrochemical performance as shown in Fig. 2. At first, it was discharged until 1.5 V in order to insert sodium ions into the remaining vacancies. The amount of sodium at the end of the first discharge (x = 0.96) shows that almost all sites are occupied between MO₂ slabs. In the curves, there were two large potential drops around x = 0.7 and x = 0.5 (close to x = 1/2) identified with the single phase domain. The three sloping curves parts indicate solid solution behaviour. The specific capacity calculated from the sodium content was 153 mAh/g, which was similar to that of the O3 phase [1]. A good cycle retention was observed during 10 cycle test as shown in Fig. 3. In the presentation, we will discuss the crystal structural changes occurring during the sodium extraction deintercalation / intercalation.     

[1] M. Sathiya, et al., Chem. Mater. 2012, 24, 1846−1853.