Sodium-Vanadium Bronze Na9V14O35: An Electrode Material for Na-Ion Batteries

Na9V14O35 (η-NaxV2O5) 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, Na9V14O35 adopts a monoclinic structure consisting of layers of corner- and edge-sharing VO5 tetragonal pyramids and VO4 tetrahedra with Na cations positioned between the layers, and can be considered as sodium vanadium(IV,V) oxovanadate Na9V104.1+O19(V5+O4)4. Behavior of Na9V14O35 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 Na9V14O35 formula, resulting in electrochemical capacity of ~60 mAh g−1. Upon discharge below 1 V, Na9V14O35 uptakes sodium up to Na:V = 1:1 ratio that is accompanied by drastic elongation of the separation between the layers of the VO4 tetrahedra and VO5 tetragonal pyramids and volume increase of about 31%. Below 0.25 V, the ordered layered Na9V14O35 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

Sodium-Vanadium Bronze Na9_9V14_{14}O35_{35}: An Electrode Material for Na-Ion Batteries

Na$_9$V$_{14}$O$_{35}$ ($η$-NaxV$_2$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, a$_9$V$_{14}$O$_{35}$ adopts a monoclinic structure consisting of layers of corner- and edge-sharing VO5 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$. Behavior of a$_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 a$_9$V$_{14}$O$_{35}$ formula, resulting in electrochemical capacity of ~60 mAh g$^{−1}$. Upon discharge below 1 V, Na9V14O35 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 a$_9$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