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

Abstract

Herein, the substitution strategy of Sr2+ into Na sites is proposed and its important role in improving the high-voltage stability of O3-type NaCrO2 cathode (O3-NCO) for high-energy sodium-ion batteries (SIBs) is systematically analyzed. Sr2+ possesses similar physicochemical characteristics to that of Na+; hence, Sr2+ can preferentially occupy the NaO6 octahedral sites in O3-NCO, with one Sr2+ ion replacing two Na+ ions. The introduction ofSr(2+) generates sodium vacancies in the Na+ layer to compensate for charge neutrality, which facilitates the Na(+ )diffusion kinetics. Additionally, Sr2+ 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 Na0.92Sr0.04CrO2 (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.