A Comparative Study of O3 and P2 Phases in Some Oxides for Na Batteries

Na-ion batteries have attracted growing attention due to the high abundance and low cost of Na compared with Li.

As a promising category of cathode materials for Na-ion batteries, layered oxides have been widely studied.  Among the rich diversity of structure, emphasis has been mainly focused on O3 and P2-phases (in Delmas’ denotion¹. ) Several layered oxides have been studied recently, NaMnO₂², NaNiO₂³, Na_xCoO₂⁴, Na_2/3Mn_1/2Fe_1/2O₂⁵, etc. For most O3-type oxides, the reversible capacity is limited to 120 mAh/g, corresponding to 0.5 Na per formula unit, while for most P2-type oxides, the reversible capacity is about 160 mAh/g which is equivalent to ≈ 0.67Na extracted and intercalated during cycling.

We synthesized a series of O3- and P2-type compounds with the same transition metal components via solid-state reaction and carried out a comparative study between the O3 and P2 structure.  Besides performance, we evaluated the structure/valence transition and Na⁺ mobility.

The structural transitions during Na⁺ intercalation/deintercalation are similar for most reported O3-type oxides with an O3-P3 transition, while the P2 phases usually experience more reversible phase transitions, accompanied by stacking faults. We carried out in-situ XRD observation to study the structure evolution of our new O3- and P2-type materials.

The valence evolution of the transition metals in O3- and P2-type materials was evaluated. A good understanding of the valence evolution of transition metals in layered oxides will benefit the design and tailoring of this category of materials.