Operando powder X-ray diffraction study of P2-Na_xNi_0.3Mn_0.7O₂ cathode material during electrochemical cycling

Layered transition metal oxides are of significant interest for applications in sodium-ion batteries. This article reports an operando powder X-ray diffraction study of the cathode material P2-Na_xNi_0.3Mn_0.7O₂ during electrochemical cycling. The structural changes are shown to be reversible over two full cycles, and refinement of sodium occupancies provides insight into the very complex ion movement during battery operation. The sodium loading progresses through a set of metastable compositions showing that the working battery is out of equilibrium on a structural level. Peak broadening caused by stacking faults is observed in the P2 structure at ∼4.0 V, prior to the ∼4.2 V phase transformation from P2 to the `Z' phase