An O3‐type Oxide with Low Sodium Content as the Phase‐Transition‐Free Anode for Sodium‐Ion Batteries

Abstract Layered transition metal oxides Na x MO 2 (M=transition metal) with P2 or O3 structure have attracted attention in sodium‐ion batteries (NIBs). A universal law is found to distinguish structural competition between P2 and O3 types based on the ratio of interlayer distances of the alkali metal layer d (O‐Na‐O) and transition‐metal layer d (O‐M‐O) . The ratio of about 1.62 can be used as an indicator. O3‐type Na 0.66 Mg 0.34 Ti 0.66 O 2 oxide is prepared as a stable anode for NIBs, in which the low Na‐content (ca. 0.66) usually undergoes a P2‐type structure with respect to Na x MO 2 . This material delivers an available capacity of about 98 mAh g −1 within a voltage range of 0.4–2.0 V and exhibits a better cycling stability (ca. 94.2 % of capacity retention after 128 cycles). In situ X‐ray diffraction reveals a single‐phase reaction in the discharge–charge process, which is different from the common phase transitions reported in O3‐type electrodes, ensuring long‐term cycling stability.