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

Abstract

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−xMnO₂ and Na_1−x[Mn_1/2Ni_1/2]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.