Research progress of Ni-rich ternary cathode materials

Ni-rich ternary cathode materials (LiNi_xCo_yMn_zO₂, x ≥ 0.8) have significant potential for use in lithium-ion batteries because of advantages that include their high energy density, long cycle life, low cost, and environmental sustainability. In particular, they have great application prospects in electric vehicles and renewable energy storage systems. However, with an increase in Ni content, materials face numerous challenges in terms of their structural and interfacial stability, cycling, and safety. The fundamental reason for these issues is the hybridization of the 3d orbitals of transition metals with the 2p orbitals of oxygen. During the charge and discharge processes, the oxidation state of transition metals such as Ni varies between +2 and +4 valences, and hole states are spontaneously generated at the O 2p energy level. The density of the hole states in the O 2p orbitals increases with the state of charge (SOC), ultimately leading to the release of lattice oxygen. With this increase in the amount of released lattice oxygen, layered phases transition to spinel or rock salt phases, which affect the electrochemical activity of the material. In addition, the H2–H3 phase transition at approximately 4.2 V can lead to intergranular slip and the formation of intergranular microcracks. These exacerbate the harmful reactions at the cathode-electrolyte interface, leading to a significant decrease in cycling stability. These issues become even more serious if the Ni content is greater than 90%, which seriously restricts its large-scale industrialized application. This paper first reviews the various challenges currently faced by Ni-rich ternary cathode materials, including lithium-nickel mixing and irreversible phase transitions, surface residual alkali and interfacial side reactions, stress-strain and microcracking, and transition metal dissolution. Furthermore, a comprehensive analysis of the causes, associated hazards, and evolution of these issues is provided. Subsequently, the main modification strategies for Ni-rich ternary cathode materials, including ion doping (such as of anions/cations), surface and interface modification (such as using electrochemically inert materials, ionic/electronic conductive materials, and a lithium residue compound), single-crystal structural design, concentration-gradient application, and core–shell structure design, which all aim to improve the overall performance of the materials, are summarized systematically. Finally, the paper reviews future development directions for Ni-rich ternary cathode materials, including (1) the precise design and regulation of material structures at the molecular level to address material challenges from a molecular design perspective; (2) considering green and controllable synthesis and closed-loop recycling to achieve the high-value utilization of resources; (3) adopting non-destructive testing technologies to accurately analyze battery behaviors such as thermal runaway, structural degradation, and life cycle decay, to ensure the efficient operation and safety of the battery; and (4) utilizing artificial intelligence (AI) and big data analysis techniques to develop more accurate, comprehensive, and effective SOC/SOH (state of health) prediction models, which will enable the real-time assessment of the state and health of a battery. This paper provides a comprehensive summary and scientific analysis of the research conducted on Ni-rich ternary cathode materials. Furthermore, it provides a reference for their design, development, and practical application to attain a high specific energy, long cycle time, and high level of safety.