Sodium-Vanadium Bronze NaVO: An Electrode Material for Na-Ion Batteries.

NaVO (η-NaVO) has been synthesized via solid-state reaction in an evacuated sealed silica ampoule and tested as electroactive material for Na-ion batteries. According to powder X-ray diffraction, electron diffraction and atomic resolution scanning transmission electron microscopy, NaVO adopts a monoclinic structure consisting of layers of corner- and edge-sharing VO tetragonal pyramids and VO tetrahedra with Na cations positioned between the layers, and can be considered as sodium vanadium(IV,V) oxovanadate NaVO(VO). Behavior of NaVO as a positive and negative electrode in Na half-cells was investigated by galvanostatic cycling against metallic Na, synchrotron powder X-ray diffraction and electron energy loss spectroscopy. Being charged to 4.6 V vs. Na/Na, almost 3 Na can be extracted per NaVO formula, resulting in electrochemical capacity of ~60 mAh g. Upon discharge below 1 V, NaVO uptakes sodium up to Na:V = 1:1 ratio that is accompanied by drastic elongation of the separation between the layers of the VO tetrahedra and VO tetragonal pyramids and volume increase of about 31%. Below 0.25 V, the ordered layered NaVO structure transforms into a rock-salt type disordered structure and ultimately into amorphous products of a conversion reaction at 0.1 V. The discharge capacity of 490 mAh g delivered at first cycle due to the conversion reaction fades with the number of charge-discharge cycles.