Single-Crystal Nickel-Rich Cathode Materials: Challenges and Strategies

Abstract: Over the past three decades, significant advancements in lithium-ion battery technology have greatly improved human convenience, particularly in today's thriving electric vehicle industry. Further enhancements in the energy density, cycle life, and safety of lithium-ion batteries are crucial for the widespread adoption of electric vehicles. In recent years, transition metal layered oxides have garnered significant attention in the industrial power battery sector due to their advantages, including high specific capacity, commendable low-temperature performance, and cost-effectiveness. Increasing the nickel content and adjusting the charging cut-off voltage are recognized as effective means to enhance the energy density of transition metal layered oxides. However, these strategies tend to degrade cycling stability and thermal safety in conventional polycrystalline layered cathode materials. Benefiting from the mechanical stability of intact primary particles, the single-crystal structure of layered cathode materials can effectively mitigate intergranular cracking issues associated with high charging voltages. Nevertheless, due to the intrinsic structural properties of layered materials, singlecrystal structures still face challenges related to sluggish Li+ transport kinetics, heterogeneous state of charge, anisotropic changes in lattice parameters, cation mixing, and chemo-mechanical degradation. The temporal and spatial evolution of the physicochemical properties within the internal microstructure of materials still requires comprehensive analysis using advanced operando characterization techniques. Currently, there is limited understanding of the intricate interplay between thermodynamics and kinetics in the synthesis process of single-crystal cathode materials. A more profound exploration of the structural degradation and synthesis mechanisms of single-crystal materials will serve as a fundamental basis for targeted modification strategies. Regrettably, existing single-crystal synthesis processes and modification approaches still fall short of market expectations. This shortfall is especially noticeable in future applications in solid-state batteries, where interface issues related to solid-state-electrolyte and cathode material are serious. Addressing these challenges necessitates the precise regulation of the microstructure of composite cathodes. Therefore, this review systematically analyzes and summarizes common issues related to the failure of both polycrystal and single-crystal structures, taking into account the intrinsic structural evolution at various temporal and spatial scales. We also outline strategies for regulating the synthesis process, element doping, and surface-interface modification of single-crystal nickel-rich layered cathode materials from the perspective of coherent structural design. We also intent to elucidate the essential connection between structural design and electrochemical performance. The microstructural design of single-crystal nickel-rich cathode materials should emphasize the alignment of lattice parameters between heterostructures and layered oxides, as well as the modulation of their spatial distribution, thereby ensuring the long-term efficacy of element doping and surface-interface modification. Finally, we offer a perspective on the future development of single-crystal nickel-rich cathode materials, highlighting their potential success in the realm of power batteries.