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

Layered transition metal oxides Nax MO2 (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 Na0.66 Mg0.34 Ti0.66 O2 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 Nax MO2 . 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.