Bulk/Surface Coengineering in Layered Oxides via Selective Niobium Doping to Enable Durable High‐Energy Sodium‐Ion Pouch Cells

ABSTRACT Layered cathode oxides for sodium‐ion batteries exhibit promising application prospects owing to their high theoretical capacity and high working voltage, yet they suffer from transition metal (TM) dissolution and rapid capacity losses at high voltages. Here, we present a selective Nb‐doping approach to achieve concurrent bulk and surface phase engineering in a Na 0.85 Mn 0.58 Ni 0.29 Zn 0.04 Li 0.08 Nb 0.01 O 2 cathode, imparting exceptional durability under high‐voltage cycling. We discover that steric hindrance in the O3 phase forces Nb to selectively occupy interstitial sites on the P2 surface, where strong Nb─O bonding significantly elevates the P2‐phase fraction from 25% to 54% under same sodium content, while simultaneously forming a surface protective layer that mitigates TM dissolution. Meanwhile, bulk Li/Zn co‐doping suppresses Na vacancy ordering and further stabilizes the intergrowth framework. Such a unique structure suppresses detrimental phase evolution and transition metal dissolution during high‐voltage deep desodiation, allowing stable cycling stability of 600 cycles at an elevated voltage of 4.3 V. The as‐prepared Ah‐level pouch cells deliver a high energy density of 154 Wh kg −1 and exhibit a significant reduction on gas release. Meanwhile, this pouch cell passed the nail penetration tests without temperature rise or thermal runaway. This work provides a robust paradigm for safe, high‐voltage sodium‐ion batteries.