LiNi_1-x-yCo_xAl_yO₂ (NCA) was developed from LiNiO₂ by partially substituting Ni with Co and Al, and it has been successfully commercialized and used in electric vehicles by Panasonic and Tesla, respectively. Because Co has a relatively high price of $29.98 USD/lb as of Aug. 3, 2018, while the prices of Ni and Al are only $6.00 and $0.92 USD/lb, respectively, as reported on InfoMine (http://www.infomine.com/investment/metal-prices), reducing the Co content in NCA materials has become a priority.^1,2

For NCA materials, substitution of Al for Ni was shown to improve the thermal stability and safety.^3,4 Partial replacement of Ni with Co was thought to improve structural stability by hindering the mixing between Ni²⁺ and Li^{+ 5,6}, and suppressing the multiple phase transitions during charge and discharge. Partially substituting Ni with other elements such as Mn and Mg has been investigated as well.^7,8 However, it is hard to make a head to head comparison between the different substituents because of various synthesis conditions and analysis methods chosen by different researchers. With the increasing demand for reducing Co content, it is important to go “back to basics” and systematically study the impact of different cation substitutions.

In this work, cations including Al, Co, Mn, and Mg, were selected for investigations. Li_xNi_1-nM_nO₂ (M=Al, Co, Mn or Mg, n=0.05 or 0.1) were synthesized and studied with differential capacity versus voltage (dQ/dV vs. V) methods. In-situ X-ray diffraction (XRD) measurements were carried out on selected samples, and the unit cell parameters and unit cell volumes were carefully measured versus x. Figure 1a shows the clipped dQ/dV vs. V of LiNiO₂, in which the peaks corresponding to the phase transitions have been circled. Figures 1b – 1d show that 5% Al, 5% Mn, or 5% Mg substitutions diminish these dQ/dV vs. V peaks, suggesting an effective suppression of the multiple phase transitions observed in LiNiO₂. Figure 1e shows dQ/dV vs. V of 5% Co substitution, and it shows almost identical peak features as LiNiO₂ (Figure 1a), indicating the existence of multiple phase transitions. However, the dQ/dV curves for the 5% Co and LiNiO₂ samples are partially off-scale in Figures 1e and 1a, respectively. Therefore, Figures 1f – 1g show the same dQ/dV vs. V with a larger y-axis scale. Figures 1f and 1j show that both LiNiO₂ and LiNi_0.95Co_0.05O₂ have similar sharp and intense dQ/dV peaks compared to the other samples. The conclusions from dQ/dV vs. V analysis were supported by in-situ XRD measurements. The studies on 5% and 10% cation doped series showed trends in the changes in material structure and specific capacities. Based on the observed trends, a preliminary theory of how the various cations impact LiNi_1-xM_xO₂ has been developed and will be reported.

References

1. Y. K. Sun, D. J. Lee, Y. J. Lee, Z. Chen, and S. T. Myung, ACS Appl. Mater. Interfaces, 5, 11434–11440 (2013).
2. K. Ghatak, S. Basu, T. Das, V. Sharma, H. Kumar, and D. Datta, Phys. Chem. Chem. Phys., 120, 22805-22817 (2018).
3. T. Ohzuku, A. Ueda, and M. Kouguchi, J. Electrochem. Soc., 142, 4033–4039 (1995).
4. M. Guilmard, A. Rougier, M. Grüne, L. Croguennec, and C. Delmas, J. Power Sources, 115, 305–314 (2003).
5. S. T. Myung, F. Maglia, K. J. Park, C. S. Yoon, P. Lamp, S. J. Kim, and Y. K. Sun, ACS Energy Lett., 2, 196–223 (2017).
6. C. Delmas, I. Saadoune, and A. Rougier, J. Power Sources, 44, 595–602 (1993).
7. H. Arai, S. Okada, Y. Sakurai, and J. Yamaki, J. Electrochem. Soc. , 144 , 3117–3125 (1997).
8. C. C. Chang, J. Y. Kim, and P. N. Kumta, J. Electrochem. Soc., 147, 1722–1729 (2000).

Figure 1. Cell voltage as a function of specific capacity (V vs. Q) of LiNiO₂ (A), LiNi_0.95Al_0.05O₂ (B), LiNi_0.95Mn_0.05O₂ (C), LiNi_0.95Mg_0.05O₂ (D), and LiNi_0.95Co_0.05O₂ (E); Differential capacity as a function of cell voltage (dQ/dV vs. V) of 2^nd charge and discharge of LiNiO₂ (a), LiNi_0.95Al_0.05 O₂ (b), LiNi_0.95Mn_0.05O₂ (c), LiNi_0.95Mg_0.05O₂ (d), and LiNi_0.95Co_0.05O₂ (e); The same dQ/dV vs. V curves with larger Y axis scale (f – j).