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 are observed for both Na1-xMnO2 and Na1-x[Mn1/2Ni1/2]O2 (NMO and NMNO); contraction occurs on the ab plane whereas expansion (0.25 ≤ x ≤ 0.75) and contraction occur (0.75 ≤ x ≤ 1.0) on the c lattice 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.