Origin of elastic anisotropy in O3-type layered sodium cathode

O3-type Ni/Fe/Mn-based layered oxides are promising cathode materials for sodium-ion batteries (SIBs) owing to their high reversible capacity and cost-effectiveness. However, their practical application is severely hindered by chemomechanical degradation induced by complex phase transitions and large volume changes during electrochemical cycling. A fundamental understanding of the mechanical behavior of O3-type cathodes is therefore critical for improving their mechanical robustness. Here, we investigate the complete evolution of anisotropic mechanical properties in O3-type Na_xNi_0.4Fe_0.2Mn_0.4O₂ (NFM) during desodiation (x = 1.0 → 0.1) using first-principles calculations. We reveal that NFM undergoes a sequential of phase transitions (O3→O’3→P3→O3’), accompanied by transition-metal layer gliding and significant volume changes, which collectively induce pronounced anisotropic softening of elastic moduli. Elastic anisotropy exhibits an approximately exponential increase with Na extraction, driven by more rapidly softening of c-axis elastic moduli than those within the ab-plane. Chemical bonding analysis reveal that contrasting bonding interactions of intralayer (TM-O) and interlayer (Na-O) is the fundamental origin of this anisotropic mechanical response. Furthermore, elastic moduli degrade more rapidly in the O3 and O’3 phases than in the P3 and O3’ phases, a behavior predominantly attributed to the significantly increase in the population of Jahn-Teller active ions (e.g., Ni^3 +), which accelerates lattice distortion and structural instability. This work establishes a clear structure-mechanics relationship in layered sodium cathodes and provides mechanistic guidance for the rational design of mechanically robust cathode materials for SIBs.