Na-Vacancy and Charge Ordering in Na approximate to 2/3FePO4

NaFePO4 is known as a promising intercalation cathode material for sodium-ion batteries. During the electrochemical reaction, an intermediate phase forms with a composition close to Na approximate to 2/3FePO4, whose crystal structure is defined with a supercell that results from both Na/vacancy and charge ordering. In this work, we present a detailed study of this superstructure through synchrotron powder X-ray diffraction and electron microscopy studies.