Thermal dynamics of P2-Na_0.67Ni_0.33Mn_0.67O₂ cathode materials for sodium ion batteries studied by in situ analysis

Layered Na_0.67Ni_0.33Mn_0.67O₂ is an attractive cathode material for sodium ion batteries. The thermal stability of cathode materials is crucial to their practical applications. In this work, we investigate structural and morphological evolution in layered P2-type Na_0.67Ni_0.33Mn_0.67O₂ cathode materials during annealing via in situ synchrotron X-ray diffraction and transmission electron microscopy. Insights are obtained from two complementary in situ characterizations (at different length scales) in terms of the thermal stability of P2-Na_0.67Ni_0.33Mn_0.67O₂ cathode materials. The results indicate that the hexagonal P2 phase remains unchanged during the heat-treatment process, and thermally driven expansion/contraction of the lattice parameters exhibits an anisotropic change in the a and c directions. In addition, interfaces/grain boundaries play an important role in the structural stability, which leads to the distinct morphological evolution between the polycrystalline and single-crystal particles.