Substitution of Sr into the Na Layer Elevates the High Voltage Stability of O3‐Type NaCrO₂ as Sodium‐Ion Battery Cathode

Herein, the substitution strategy of Sr 2+ into Na sites is proposed and its important role in improving the high‐voltage stability of O3‐type NaCrO 2 cathode (O3‐NCO) for high‐energy sodium‐ion batteries (SIBs) is systematically analyzed. Sr 2+ possesses similar physicochemical characteristics to that of Na + ; hence, Sr 2+ can preferentially occupy the NaO 6 octahedral sites in O3‐NCO, with one Sr 2+ ion replacing two Na + ions. The introduction of Sr 2+ generates sodium vacancies in the Na + layer to compensate for charge neutrality, which facilitates the Na + diffusion kinetics. Additionally, Sr 2+ exhibits electrochemical inactivity and strongly interacts with O 2− ions, which triggers the smooth atomic rearrangement related to the sequential phase transformation of O3‐NCO at high charging potentials. For the charge–discharge process in a wide operating voltage window (1.5–3.8 V vs. Na/Na + ), the optimal substitution level of 4 mol% substantially suppresses the extent of irreversible phase transition of O3‐NCO; as a result, compared to O3‐NCO, the O3‐type Na 0.92 Sr 0.04 CrO 2 (O3‐NS4CO) cathode demonstrates the superior discharge capacity with stable Coulombic efficiency, long‐term cycling stability, and advanced power capability. Furthermore, O3‐NS4CO demonstrates excellent practical applicability in pouch‐type full cells constructed using a hard carbon anode.