In-depth study on diffusion of oxygen vacancies in Li(NixCoyMnz)O2 cathode materials under thermal induction

• The thermal stability of NCM cathode material is investigated from oxygen vacancy perspective at the microscopic level. • The relation between oxygen vacancy concentration and lattice structure transition is explored. • The origin of the formation and evolution of intergranular cracks in secondary particles under thermal induction is revealed. • Breaking the traditional view that the transformation of the lattice structure is the main factor affecting the thermal stability, which was considered in previous studies. It is generally believed that an increase in Ni content will severely reduce the lattice structure stability of the cathode material, causing it to release more oxygen during thermal induction. However, the thermal degradation mechanism at the microscopic level remains unclear, which hinders the safety design of cathodes. In this work, we focus on the secondary particles of different Li(Ni x Co y Mn z )O 2 cathodes, report the formation and condensation of oxygen vacancies on the surface in detail, and quantitatively analyze the oxygen vacancy concentration distribution inside the particles after thermal failure. The results reveal that the increase of Ni content promotes the oxygen vacancies to diffuse deeply from the surface of the secondary particles to the bulk under thermal induction, resulting in an increase of oxygen vacancy concentration in the bulk of the secondary particles and the release of more oxygen. When the Ni content x in Li(Ni x Co y Mn z )O 2 increases to 0.8, both the surface layer and bulk exhibit high oxygen vacancy concentrations, leading to an overall failure of the secondary particles. Furthermore, we suggest that the formation and evolution of intergranular cracks inside the secondary particles depend on the oxygen vacancy concentration gradient. Our work provides a new idea for future research on the thermal stability of cathode and the thermal safety design of Ni-rich cathode materials.