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.