Strain-Retardant Sodium Layered Cathodes Enabled by Conformal Octahedral Matching.

P2-type layered transition metal (TM) oxides have emerged as promising cathode candidates for sodium-ion batteries (SIBs) owing to their intrinsic air stability and open prismatic diffusion pathways. However, at moderate (de)sodiation, severe structural breakdown and performance failure associated with the intra-particle anisotropic lattice strain and stress are habitually neglected. In this study, we reveal the intrinsic correlation between strain evolution and capacity diving in P2-type layered cathodes, where strain expansion, rather than conventional phase transitions or oxygen loss, is identified as the center of cathodic degradation. Upon the build-up of lattice strain beyond a structural threshold during cycling, lattice fracture and amorphization are triggered to block the reversible Na-ion transport, as confirmed by ex situ XAS, EBSD analysis, and intuitive structural observations. By integrating highly matched conformal octahedra into the host lattice as a structural 'buffer', the pernicious strain expansion is significantly mitigated by nearly 30% per cycle relative to the bare counterpart, as evidenced by in situ XRD and mechanical simulations. The resulting cathode exhibits exceptional cycling durability with improved 81.0% capacity retention over 1000 cycles at 3 C. These findings highlight the mechanistic role of strain expansion on cathode degradation and rationalize the design of long-lived layered oxide cathodes.