Introduction of High‐Valent Metal in Transition Metal Layer as a Structural Reinforcement for a O3‐Type NaCrO 2 Sodium‐Ion Battery Cathode

O3‐type layered sodium chromium oxide (O3‐NaCrO 2 ) is a promising cathode material for cost‐effective and practical sodium‐ion batteries (SIBs). However, achieving a high energy density in SIBs with NaCrO 2 as the cathode remains challenging owing to chromium migration and irreversible phase transition at voltages above 3.6 V (vs Na/Na + ). Herein, a substantially improved high‐voltage stability of O3‐NaCrO 2 (O3‐NCO) as a cathode through the implementation of high‐valent Nb 5+ substitution strategies is reported. The strong interaction between Nb 5+ and O 2− ions alleviates structural stress during repeated charge and discharge processes. The introduced Nb 5+ enhances the disorder in the transition metal layer and structural stability, increasing the reversibility by suppressing Cr disproportionation reactions and irreversible Cr migration. Additionally, Nb 5+ creates Na + /vacancies in the Na layer owing to charge compensation, thereby facilitating the Na‐ion diffusion kinetics. Consequently, 3 mol% of Nb‐substituted O3‐Na 0.94 Cr 0.97 Nb 0.03 O 2 cathode demonstrates superior reversible capacity, cycle life, and rate capability. Furthermore, the high‐valent Nb substitution strategy improves water stability and shows good compatibility with all‐solid‐state battery systems, highlighting its excellent practical applicability.