Bulk and surface degradation in layered Ni-rich cathode for Li ions batteries: Defect proliferation via chain reaction mechanism

Ni-rich cathode materials have emerged as a class of low-cost and high-energy-density Li ions batteries cathodes. However, the bulk and surface degradation of Ni-rich cathode remains controversial. By transmission electron microscopy (TEM), the bulk and surface degradation of nickel-rich cathode are distinguished by the causes and development time. Structure deterioration with rock-salt phase is observed in the bulk and surface region. The triggered phase transition by defect chain reaction (DCR) mechanism is unveiled by the first-principle-calculation simulations. The critical factor in DCR mechanism is the migration barrier of Ni. The decrease in migration barrier of Ni due to the oxygen vacancy and the lowering of the system energy by the Li ion compensation causes extra cation mixing and lattice distortion. Hence, Ni ion migration introduces strain perpendicular to the Ni layers with oxygen defects and Li vacancies, which leads to dislocations and defects with lattice distortion. Based on the model, a manganese oxide coating method was proposed to adjust the valence state of nickel and avoid the oxygen defects. The treated cathode presented an excellent long-term cycling performance, about 80.6% of the initial capacity after 200 cycles.