Improving Cycling Electrochemical Performance of P2-Type Na2/3Mn2/3Ni1/3O2 by Controlling Its Crystallinity
Authors: Kataoka Riki , Taguchi Noboru , Tada Kohei , Machida Akihiko , Takeichi Nobuhiko 
Published: February 2024

1. Introduction
Na2/3Mn2/3Ni1/3O2 with P2-type structure is a positive electrode material for the sodium ion battery with relatively high reversible capacity of 170 mAh/g and average potential of 3.6 V (vs. Na/Na+). On the other hand, it is known that there is a problem in cycle stability due to the P2-O2 phase transition associated with Na insertion/removal and the structural change caused by O2 desorption at the high state of charge. In this study, we focus on the crystallinity of the layered structure to improve the electrochemical property of P2-type Na2/3(Mn2/3Ni1/3)O2 .The effect of crystallinity on the electrochemical property of the P2-Na2/3(Mn2/3Ni1/3)O2 was explored using the ball milling approach.

2. Experiments
The Na2/3(Mn2/3Ni1/3)O2 was prepared by mixing NaOH, Mn2O3, and Ni(OH)2 in a molar ratio of 2 : 2 : 1 and firing at 950°C for 3 h in air (sintered sample). The sintered sample was ball-milled with 4 mmφ ZrO2 balls in an 80 ml ZrO2 vial using a planetary ball mill (Fritsch, P-6) at 500 rpm for 2 h to obtain a sample with low crystallinity (milled sample). The resulting samples was mixed with Ketjen black and PTFE at a weight ratio of 84:8:8, and the sheet was pressed onto an Al mesh to form an electrode. The electrodes were dried under vacuum at 220 °C for 5 hours, and then half-cells were assembled using Na foil as the counter electrode and 1M NaPF6/EC-DEC as the electrolyte to evaluate electrode characteristics. The cell was galvanostatically charged and discharged using a BLS5500 (Keisokuki Center) with a current density of 10 mA g−1, in which the denominator represents the mass of the active material in the positive electrode. The crystal structure of the samples was evaluated using XRD (Rigaku UltimaIV, Cu K-α), XAS (BL14B2 at SPring-8, BL-11, Ritsumeikan SR center), X-ray total scattering measurement (SPring-8, BL22XU, 70 keV) and TEM. In addition, the electronic structure was investigated using the first-principles calculation program (VASP) based on density functional theory.
3. Results and Discussion
Fig. 1 shows XRD of Na2/3(Mn2/3Ni1/3)O2 after calcination and ball milling. The sintered sample showed a diffraction pattern attributed to the P2 structure, but the milled sample showed a broad profile and the detailed crystal structure could not be identified. However, a peak in the 002 plane around 2θ = 16° could be confirmed, and from the results of TEM observation and PDF analysis, it is considered that the milled sample also has a layered structure.
Fig. 2 shows a comparison of charge-discharge profiles and cycle characteristics. In the charge-discharge profile of the sintered sample, potential flat areas due to Na ordering and P2-O2 phase transition can be observed, but that for the milled sample, these potential flat areas are not observed and the profile is on a slope, which indicates that the crystallinity degradation due to milling suppresses structural changes during the charge-discharge process. The initial charge-discharge capacities of the milled samples were 146 and 141 mAh/g, respectively, which were smaller than those of the calcined samples (175 and 146 mAh/g, respectively), indicating that the milling process did not significantly affect the Na insertion/desorption characteristics. Comparing the cycle characteristics shown in Fig. 2, the discharge capacity of the milled sample maintained more than 80% while the sintered sample deteriorated to the 37% of the initial capacity after 50 cycles, confirming that the milling treatment was effective in improving cycle characteristics.
Detailed mechanism of the milling effect and redox process will be discussed in the presentation.

Figure 1