Sodium-Vanadium Bronze Na(9)V(14)O(35): An Electrode Material for Na-Ion Batteries

Na(9)V(14)O(35) (η-Na(x)V(2)O(5)) 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, Na(9)V(14)O(35) adopts a monoclinic structure consisting of layers of corner- and edge-sharing VO(5) tetragonal pyramids and VO(4) tetrahedra with Na cations positioned between the layers, and can be considered as sodium vanadium(IV,V) oxovanadate Na(9)V(10)(4.1+)O(19)(V(5+)O(4))(4). Behavior of Na(9)V(14)O(35) 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 Na(9)V(14)O(35) formula, resulting in electrochemical capacity of ~60 mAh g(−1). Upon discharge below 1 V, Na(9)V(14)O(35) uptakes sodium up to Na:V = 1:1 ratio that is accompanied by drastic elongation of the separation between the layers of the VO(4) tetrahedra and VO(5) tetragonal pyramids and volume increase of about 31%. Below 0.25 V, the ordered layered Na(9)V(14)O(35) 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(−1) delivered at first cycle due to the conversion reaction fades with the number of charge-discharge cycles.