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

Abstract

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.

Metal-Ion Intercalation Mechanisms in Vanadium Pentoxide and Its New Perspectives

“…The crystal structure of layered V 2 O 5 , which has an orthorhombic P mmn lattice [41], undergoes a phase transition when sodiated [42]. This structure is composed of edge-and vertex-sharing VO 5 square pyramids (Figure 4, left [43]) throughout the crystal structures of the bronzes, which are exceptionally flexible as exemplified with the layered (α-phase) and tunnel (β-phase) materials.…”

“…The compound η-Na x V 2 O 5 or Na 9 V 14 O 35 crystallizes in a monoclinic lattice with the space group P2/c [50]. Its crystal structure consists of layers oriented along the (010) direction, which are composed of VO 5 (V 4+ ) square pyramids and VO 4 (V 5+ ) tetrahedrons with the sodium atoms interspersed between these layers (Figure 4, right) [43]. If all sodium ions are extracted from the structure, the theoretical capacity of η-Na x V 2 O 5 can be estimated to be approximately 163 mAh g −1 .…”