Chemomechanics in Ni-Mn binary cathode for advanced sodium-ion batteries

Spherical secondary particles for cathode active materials have been highlighted owing to their superior electrochemical performance compared to other types. However, they suffer from micro-cracking, which is a crucial factor of electrochemical performance degradation, owing to the highly anisotropic mechanical deformation of primary particles during cycling. In particular, anisotropy is significant for Ni-Mn binary layered oxides, which utilize oxygen redox reactions and suffer severe structural variations occurring in sodium-ion batteries (SIBs). To elucidate the intrinsic origins, we focused on the anisotropic structure distortion of Ni-Mn binary-layered oxides and their correspondence with the Ni redox picture using first-principles calculations. Analysis of the atomic-scale structure indicated that opposite deformation in the lattice parameters is observed for both Na_1−xMnO₂and Na_1−x[Mn_1/2Ni_1/2]O₂(NMO and NMNO); contraction occurs on theabplane whereas expansion (0.25 ≤x≤ 0.75) and contraction occur (0.75 ≤x≤ 1.0) on theclattice direction upon desodiation. Notably, the mechanical anisotropy of the Ni-Mn binary-layered oxide is accelerated attributable to the dual contraction of Ni ionic radii owing to Ni²⁺/Ni⁴⁺double redox and the suppression of contraction of the transition metal layer because of the Jahn-Teller distortion. Therefore, we established that the shape of the radially oriented secondary particle could alleviate the impact of the anisotropic distortion from primary particles, resulting in a stabilized cycle performance. Thus, adjusting the shape of the secondary particle is a suitable approach for alleviating the anisotropic features of the primary particles, thus enhancing cycle stability with oxygen redox and fast charging for further advances in lithium-ion batteries (LIBs) or SIBs.