Lithium-rich layered oxides have drawn much attention as a positive active material in the future automotive applications because of a high gravimetric capacity of more than 250 mAh g^-1,¹ which make the high energy density LiBs. However, this positive active material suffer from several problems such as low initial coulombic efficiency, poor capacity retention and voltage fading upon charge/discharge cycling.² While a lot of efforts have been made to mitigate these problems, the problems have not sufficiently improved. This situation makes the industrial application difficult. According to previous works, a transition metal composition (Ni/Co/Mn ratio) and a Li/Metal ratio have much influence on electrochemical properties of this positive active material.^3-5 However the specific chemical composition of this positive active material has not been appeared to show the high discharge capacity, the high initial coulombic efficiency and good charge/discharge cycle life performance. In this work, the various compositions of lithium-rich layered oxide positive active materials were prepared with a co-precipitation process and a calcination process. The electrochemical properties of these materials were investigated. The relation between compositions and electrochemical properties of these positive active materials were discussed.

The twenty different kinds of transition metal compositions carbonate precursors were prepared by a co-precipitation method. The lithium-rich layered oxide positive　active materials were made from these carbonate precursors with the different Li/Me ratio and the different calcination temperature. The materials were characterized by means of X-ray diffraction　and inductively coupled atomic plasma emission spectroscopy measurements. The materials showing the high discharge capacity were chosen in each transition metal composition. The relation between the composition and electrochemical properties of these materials was organized. Then several compositions were picked up to evaluate the charge/discharge cycle life performance by means of the half-cell with a metallic lithium counter electrode. The material showing the high energy density and good charge/discharge cycle life performance was evaluated the long-term charge/discharge cycle life performance for the cell with graphite anode.

Various compositions of lithium-rich layered oxide positive active materials successfully synthesized. The chemicla compositions showed the specific relation with the electrochemical properties. Discharge capacities and energy densities showed the local maximum value in the relation to the x Li₂MnO₃ - (1-x) LiTMO₂ (TM = transition metal). On the other hand, the average potentials showed gradually decreased with the increasing the x in x Li₂MnO₃ - (1-x) LiTMO₂. There was the threshold value of x that discharge capacity retentions suddenly decreased in the relation to the x Li₂MnO₃ - (1-x) LiTMO₂. The average potentials retention showed gradually decreased with the increasing the x in x Li₂MnO₃ - (1-x) LiTMO₂. These results suggest that the chemical composition is extremely important for electrochemical properties of this positive active material. The critical chemical composition has been also found out to show the high discharge capacity (> 260 mAh g^-1), the high initial coulombic efficiency (> 90 %) and excellent charge/discharge cycle life performance as well as high energy density (Figure 1). Energy density retention is 78 % after 1000 cycles with 4.6-2.0 V at 1 I_tA. This critical composition is based on the hypothesis that valences of Ni and Mn should be only 2+ and 4+, and the ratio of Li₂MnO₃ and LiTMO₂ is predetermined value in order to show stable crystal structure. Moreover, this is composed of less cobalt, so that this has to be one of the promising candidates for xEV-use LiBs.

Reference

1. M. S. Whittingham, Chemical Reviews, 104, 4271-4301 (2004).
2. J. R. Croy et al., J. Phys. Chem. C, 117, 6525–6536 (2013).
3. Z. Lu et al., J. Electrochem. Soc., 149, A778-A791 (2002).
4. G. Y. Kim et al., Materials Research Bulletin, 43, 3543-3552 (2008).
5. Q. C. Zhang et al., J. Power Sources, 250, 40-49 (2014).

Caption

(a) The initial charge/discharge profile of the critical chemical composition lithium-rich layered oxide positive active material for the cell with metallic lithium counter electrode.

Charge: CC-CV 0.1 I_tA 4.7 V (vs. Li/Li⁺), 0.02 I_tA cut-off at 25 °C.

Discharge: CC 0.1 I_tA 2.0 V (vs. Li/Li⁺) at 25 °C.

(b) The cycle performance of the critical chemical composition lithium-rich layered oxide positive active material for the cell with graphite anode. The energy density means the value that calculated the cell energy in a gram of positive active material.

Cycling condition

Charge: CC-CV 0.5 I_tA 4.6 V, 0.1 I_tA cut-off at 25 °C.

Discharge: CC 1 I_tA 2.0 V at 25 °C.

Capacity check every 50 cycles

Charge: CC-CV 0.1 I_tA 4.6 V, 0.05 I_tA cut-off at 25 °C.

Discharge: CC 0.1 I_tA 2.0 V at 25 °C.