Investigating the Effects of Magnesium Doping in Various Ni-Rich Materials

As lithium ion battery technology expands into more demanding applications such as electric vehicles, attention has shifted towards nickel-rich positive electrode materials, namely LiNi1-x-yMnxCoyO2 (NMC) and LiNi1-x-yCoxAlyO2 (NCA).1 Aims to improve energy density and reduce costs of NMC and NCA can be achieved by increasing the Ni content of the material, but at the cost of shorter cell lifetimes. Besides Al, Co and Mn, research has tried substituting Ni with many different metals.1 Mg has repeatedly come up as a beneficial dopant from positive electrode material doping studies that try to improve material cycling performances. Mg has been shown to improve cycling performances of various positive electrode materials including LiCoO2 (LCO), NMC, and NCA when doped into the material in small amounts.1–4 Doping the material with Mg slightly reduces the specific capacity of the material since Mg is an inactive constituent, but it has been shown that a Mg content of as low as 1 mol% can improve cycling.3 LiNiO2 (LNO) and some other Ni-rich materials undergo phase transitions as Li gets removed during charging. In particular, when the material transitions from the H2 phase to the H3 phase at low Li, it experiences a large volume change. It is believed that this contributes to a poor lifetime of the material. Mg doping has been shown to suppress these phase transitions and related volume changes.2,4 Mg has also been shown to reduce the amount of Ni migration to the Li layer.1 As research continues to increase the Ni content of NMC and NCA, the compositions will invariable converge towards LNO. Additionally, efforts are being made to reduce the Co content of Ni-rich materials due to cost and sourcing issues.4 Co is believed to reduce the amount of Ni in the Li layer and also to stabilize cycling performance, which are similar benefits that Mg imparts. This work studies the effect of Mg doping in LiMO2 (M = Ni, Ni+Al, Ni+Co+Al, Ni content > 0.8) at a dopant level of less than 5 mol%. Structural and electrochemical characterization of the materials was carried out to understand how Mg affects the materials and whether the presence of Al or Co influences the dopant effects of Mg. Figure 1 shows the initial half-cell voltage vs capacity and differential capacity vs voltage curves for a series of LiNi0.88-xCo0.09Al0.03MgxO2 (x = 0, 0.01, 0.02, 0.04) materials, showing a trend of how Mg affects the electrochemical performance of the materials. (1) Kim, J.; Lee, H.; Cha, H.; Yoon, M.; Park, M.; Cho, J. Adv. Energy Mater. 2018, 8, 1–25. (2) Sasaki, T.; Godbole, V.; Takeuchi, Y.; Ukyo, Y.; Novák, P. J. Electrochem. Soc. 2011, 158, A1214–A1219. (3) Huang, B.; Li, X.; Wang, Z.; Guo, H.; Xiong, X. Ceram. Int. 2014, 40, 13223–13230. (4) Li, H.; Cormier, M.; Zhang, N.; Inglis, J.; Li, J.; Dahn, J. R. J. Electrochem. Soc. 2019, 166, A429–A439. Figure 1