O′3–NaxVO2 System: A Superstructure for Na1/2VO2

The electrochemical cycling in a sodium battery of the lamellar oxide NaVO2 is reversible in the NaxVO2 composition range 1/2 ≤ x ≤ 1. The complex electrochemical curve reveals the presence of several transitions taking place during deintercalation. With the help of in situ X-ray diffraction, we observed the structural transitions taking place between Na2/3VO2 and Na1/2VO2. The diffractograms show the presence of several monophasic domains separated by biphasic domains. All phases present a monoclinic distortion of the α-NaFeO2 structure in the composition range 1/2 ≤ x ≤ 2/3. Moreover the presence of a superstructure is evidenced for Na1/2VO2. It is the first time that an ordered structure is reported at the Na1/2MO2 composition with an O′3 oxygen stacking. A thorough investigation of electrochemically obtained O′3-Na1/2VO2 was performed. The structure refinement reveals the existence of a sodium/vacancy ordering, with a peculiar arrangement of the V–V distances hinting at a pairing of vanadium atoms. Our first measurements of the physical properties of O′3-Na1/2VO2 show a semiconductor behavior and a complex thermal dependence of the magnetic susceptibility related to the pairing of the vanadium atoms.