Layered Cobalt (Co)-free Nickel (Ni)-rich cathode materials have attracted much attention due to their high energy density and low cost. Still, their further development is hampered by material instability caused by the chemical/mechanical degradation of the material. Although there are numerous doping and modification approaches to improve the stability of layered cathode materials, these approaches are still in the laboratory stage and require further research before commercial application. To fully exploit the potential of layered cathode materials, a more comprehensive theoretical understanding of the underlying issues is necessary, along with active exploration of previously unrevealed mechanisms. This paper presents the phase transition mechanism of Co-free Ni-rich cathode materials, the existing problems, and the state-of-the-art characterization tools employed to study the phase transition. The causes of crystal structure degradation, interfacial instability, and mechanical degradation are elaborated, from the material's crystal structure to its phase transition and atomic orbital splitting. By organizing and summarizing these mechanisms, this paper aims to establish connections among common research problems and to identify future research priorities, thereby facilitating the rapid development of Co-free Ni-rich materials.
Keywords: characterization methods; co-free Ni-rich; layered structures; lithium-ion batteries; phase transition mechanisms.
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