Lithium-nickel-manganese ternary oxides have attracted great attention as cathode materials owing to their high capacity and high voltage. Li_2/3Ni_1/3Mn_2/3O₂ with an O3-type layered structure exhibits high capacity; however, the capacity retention upon subsequent cycling is poor. The phase structure of O3-Li_2/3Ni_1/3Mn_2/3O₂ is known to be metastable, but is readily prepared via ion-exchange reaction and P3-Na_2/3Ni_1/3Mn_2/3O₂ as precursors [1]. O3-type structures are known to undergo phase transformation to spinel structures upon electrochemical cycling [1]. Therefore, the crystal structure is considered to be stabilized in the spinel phase state. In this study, by using an additional heat treatment during the synthesis procedure, we have synthesized spinel-like Li_2/3Ni_1/3Mn_2/3O₂ which exhibits excellent capacity retention and sustains respectable rate capabilities [2].

The P3-Na_2/3Ni_1/3Mn_2/3O₂ precursor was synthesized via a conventional solid-state reaction, and was then mixed with LiNO₃ and LiCl and annealed in air at 260 °C for 1 h to obtain O3-Li_2/3Ni_1/3Mn_2/3O₂. The samples were reannealed at 500 °C for 5 h in air to obtain ‘thermally-treated’ O3-Li_2/3Ni_1/3Mn_2/3O₂, hereafter denoted as O3₅₀₀-Li_2/3Ni_1/3Mn_2/3O₂. The material was characterized by XRD, ICP, and ⁶Li-MAS-NMR. Electrochemical lithium charge/discharge cycling was performed at 25 °C using coin-type lithium cells.

Figure 1 shows the initial charge/discharge voltage curves for O3-Li_2/3Ni_1/3Mn_2/3O₂ and O3₅₀₀-Li_2/3Ni_1/3Mn_2/3O₂, starting from Li extraction. Two distinct plateaus are observed in the discharge curve at around 4.2 and 3.0 V, as previously reported [1]. The average discharge voltage and the initial discharge capacity of the O3-Li_2/3Ni_1/3Mn_2/3O₂ are 3.41 V and 194 mA h g⁻¹, respectively. Furthermore, the discharge capacity is maintained at 175 mA h g⁻¹ after 30 cycles, and its efficiency is 90.4%, as shown in Fig. 2. In the case of O3₅₀₀-Li_2/3Ni_1/3Mn_2/3O₂, plateaus in the charge and discharge curves are observed just below 4.8 V, and the capacity of the 4.8 V plateau is about 80 mA h g⁻¹. Since the spinel phase transition is formed at temperatures higher than 260 °C, the heat treatment of O3-Li_2/3Ni_1/3Mn_2/3O₂ stabilizes the crystal structure, leading to the high initial capacity and improved cyclability. In addition, a gradually sloping voltage profile between 3.0 and 4.8 V is apparent, which is ascribed to the layered rock salt structure. The average discharge voltage is 3.21 V, and the initial discharge capacity is 257 mA h g⁻¹. Moreover, the capacity is retained at 247 mA h g⁻¹, even after 30 cycles, as shown in Fig. 2. This translates to an efficiency of 96.3%.

This study demonstrates that the thermal treatment of the O3-Li_2/3Ni_1/3Mn_2/3O₂ cathode material is an effective method to attain not only good cycle performance, but also high voltage and high capacity.

Acknowledgement

This work was supported by the “Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan.

References

1. J. M. Paulsen et al., J. Electrochem. Soc., 147, 2478 (2000).

2. K. Chiba et al., J. Power Sources, 304, 60 (2016).