Deep External Engineering in Layered Cathode for High‐Performance Sodium‐Ion Batteries

O3‐type layered oxide cathodes have garnered considerable attention for sodium‐ion batteries (SIBs) by virtue of their high energy and low cost. But the harmful phase transitions and notorious interface side‐reactions seriously deteriorate the electrochemical performance of materials. Here, we propose a deep external engineering strategy by coupling sodium‐ion‐sites anchoring with in situ induced segregation layer to enable synergistic reinforcement of the overall framework from surface to bulk. Theoretical calculations and advanced in/ex situ characterizations demonstrate that the charge density around oxygen atoms is dramatically increased, promoting the electron localization, which widens the NaO 2 layer distance, thus accelerating the Na + transport kinetics. Meanwhile, the resulting higher‐energy bond with oxygen creates localized rigid structural units, which remarkably restrain the local lattice distortion. More importantly, the generated buffer layer effectively ameliorates the undesirable interface parasitic reactions, which assist the formation of a robust cathode‐electrolyte interface, ensuring the overall structural stability of developed materials. Therefore, the optimized sample delivers an extraordinary cycling durability with ultralow voltage attenuation (only 0.02% V per cycle) and outstanding rate performance (114 mAh g −1 at 10 C). This unique design paradigm opens a versatile avenue toward high‐performance layered cathode materials for SIBs.