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

Na₉V₁₄O₃₅ (η-Na_xV₂O₅) 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 Na₉V₁₀^4.1+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.