Interfacial and structural transformations in Ni-rich cathodes: a roadmap toward chemical stability

Abstract

This review highlights Ni-rich layered oxide cathodes, such as LiNi_xMn_γCo_zO₂ (NMC) and LiNi_xCo_yAl_zO₂ (NCA), where x ≥ 0.6, y + z ≤ 0.4, and x + y + z = 1, which have become the cornerstone of high-energy lithium-ion batteries due to their high specific capacities (>200 mA h g⁻¹), reduced cobalt dependence, and compatibility with both cylindrical and pouch-cell formats. However, as Ni content exceeds 80%, these materials suffer from coupled chemical and mechanical degradation—cation disorder, oxygen loss, and interfacial instability—that limits lifetime and safety. This feature article presents a comprehensive roadmap linking the mechanistic origins of degradation to scalable mitigation strategies, bridging fundamental insights and technology readiness level (TRL) 9 implementation. At the lattice level, antisite defects (Ni²⁺/Li⁺ mixing) and anisotropic H2–H3 phase transitions generate microstrain and intergranular cracking, which are effectively mitigated through bulk doping (e.g., W⁶⁺, Ti⁴⁺, Zr⁴⁺, Sc³⁺), co-doping, and single-crystal or columnar morphologies that distribute internal stress. At the electronic level, excessive delithiation triggers oxygen redox and lattice-oxygen release, initiating chemomechanical collapse and surface rock-salt reconstruction. Countermeasures include oxygen-constraining coatings, Li₂NiO₂ prelithiation, and redox-buffering additives (e.g., LiFePO₄ blending). At the interface, parasitic reactions with carbonate electrolytes produce resistive cathode–electrolyte interphases (CEIs) and gas evolution. Stabilization is achieved via fluorine-rich electrolytes, hybrid compartmentalized systems, and MOF-functionalized separators, which suppress HF formation and transition-metal dissolution. The article further highlights emerging manufacturing-compatible solutions—including solvent-free mechanofusion coatings, spatial atomic layer deposition, facing-target sputtering, and wet-chemical nanoshell growth—that integrate surface and bulk stabilization. These approaches not only improve high-voltage cycling (>4.5 V) but also meet industrial scalability and sustainability goals through direct regeneration and closed-loop cathode recycling. By unifying lattice, oxygen, and interfacial stabilization into a coherent framework, this roadmap provides actionable guidance for designing next-generation Ni-rich cathodes that achieve long-term durability, high safety, and industrial manufacturability for the global electrification era.