In Situ Intergrowth Tunnel/P3 Cathode Enhancing High Voltage Stability Toward High Energy Sodium‐Ion Batteries

Abstract Improving the upper voltage limit in layered oxide cathodes is an effective strategy to achieve high‐energy sodium‐ion batteries (SIBs). However, in the high‐voltage region with deep Na + extraction, the phase structure becomes severely unstable, and interfacial side reactions result in rapid capacity decay, hindering their commercial application. This issue is particularly pronounced in low cost P3 Mn‐based cathode materials, where the Jahn–Teller effect exacerbates the instability. Here, the robust tunnel phase is introduced into P3 particles to form intergrowth Na₀.₅Ni₀.₁₅Mn₀.₆₅Al₀.₂O₂ (T/P‐NNMA) materials. The tightly integrated tunnel phase enables pseudo‐solid solution reaction, suppresses irreversible oxygen release, and maintains stable interface stability even when the charge cut‐off voltage increases to 4.5 V. Consequently, the T/P‐NNMA cathode delivers an outstanding reversible capacity of 187.8 mAh g −1 at 0.1 C, yielding an energy density of 544.8 Wh kg −1 (based on the cathode) and displays excellent capacity retention of 83.08% over 200 cycles at 1.0 C. Embedding the tunnel phase into P3‐type material to achieve high structure and interficial stability offers a promising strategy for the development of high energy and low cost SIBs.