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.