An in-depth understanding of the effect of aluminum doping in high-nickel cathodes for lithium-ion batteries

High-nickel layered oxides, LiNixM1-xO2 (x ≥ 0.6), are regarded as highly promising materials for high-energy-density Li-ion batteries, yet they suffer from short cycle life and thermal instability. Tuning these cathodes for improved performance via elemental doping is an effective approach, and Al has proven to be the most popular and commercially successful of all the dopants. Despite the popularity of Al doping, the practical effects of varying Al content on high-nickel cathodes are still unknown. Herein, a series of high-Ni cathodes, LiNi0.85Co0.15-xAlxO2 (NCA, x = 0, 0.023, 0.056, 0.096, and 0.15), are synthesized and investigated. It is found that the cycling, air stability, lattice structure, and interphase stability of NCA are gradually improved with increasing Al content from 0 to 5.6%. Further increase in Al content beyond 5.6% triggers adverse effects, including significantly increased residual lithium on the cathode surface and compromised initial capacity resulting from the formation of impurity phases (LiAlO2 and Li5AlO4). A 5.6% Al doping is suggested to be optimal, which suppresses the secondary-particle pulverization and reduces residual lithium species as well as enhances the electrode/electrolyte interphases, thereby improving both rate capability and cycling performance at ambient and high temperatures. Overall, this study delivers fundamental insights into Al doping and provides guidance for the optimal use of Al dopants in developing advanced cathodes.