Researchers at Oak Ridge National Laboratory (ORNL) have discovered a way to achieve the promise of bronze-phase vanadium dioxide [VO2(B)] as an electrode material for Li-ion batteries. In a paper in the ACS journal Nano Letters, the team reports that epitaxial VO2(B) films can accomplish the theoretical limit for capacity with persistent charging−discharging cyclability owing to the high structural stability and unique open pathways for Li-ion conduction.
Using atomic-scale characterization via scanning transmission electron microscopy and density functional theory calculations, the researchers determined that that the unique open pathways in VO2(B) provide the most stable sites for Li adsorption and diffusion. The work ultimately demonstrates that VO2(B) is a highly promising energy storage material and has no intrinsic hindrance in achieving superior cyclability with a very high power and capacity in a Li-ion conductor.
VO2(B) has been long regarded as a promising electrode material for LIBs after being first proposed in 1994 … owing to low cost, nontoxicity, and abundant sources. In addition, the theoretical capacity and energy density of VO2(B) (323 mAh/g at a redox voltage of 2.6 V, 0.84 Wh/g) are higher than those of state-of-the-art LIB electrodes, such as LiCoO2 (274 mAh/g at 4 V, 1.1 Wh/g), LiFePO4 (165 mAh/g at 3.5 V, 0.58 Wh/g), Nb2O5 (200 mAh/g at 1.2−2.4 V, 0.24−0.48 Wh/ g), and V2O5 (294 mAh/g at 2.7 V, 0.79 Wh/g). In particular, VO2(B) stands out because of its unique open framework with channels formed from edge-sharing VO6 octahedra. Thus, VO2(B) has been expected to exhibit both high capacity and rapid Li ion diffusion.
However, there has been no successful experimental confirmation of such intriguing properties despite many attempts with various VO2(B) nanostructures, including quantum-dots coated nano-structures, nanobelts, nanoscrolls, and mesocrystals. So far, the low capacity and rapid irreversible capacity loss have been observed in VO2(B) electrodes, hampering their further development. Furthermore, since VO2(B) is a metastable phase and easily forms a mixed phase with other VO2 polymorphs and/or other vanadium oxides with oxidation states other than V4+, it is still unclear whether such poor performance is intrinsic to VO2(B) or attributed to extrinsic factors, such as binder-induced losses, structural defects, dislocations, amorphous phases, and mixed orientations.
Here, we report the high capacity and excellent cycling performance with binder-free highly conductive VO2(B) films as LIB electrodes.—Lee et al.
ORNL’s Ho Nyung Lee and his team used an advanced synthesis technique using pulsed laser epitaxy (PLE) to fabricate thin-film crystals and demonstrated that they remained stable even after numerous electrochemical charge/discharge cycles.
The research provides a design strategy for more efficient, long-lived, miniaturized ionic conductors.—Panchapakesan Ganesh of ORNL, who predicted vanadium dioxide’s theoretical capacity and lithium ion pathways
Shinbuhm Lee, Xiao-Guang Sun, Andrew A. Lubimtsev, Xiang Gao, Panchapakesan Ganesh, Thomas Z. Ward, Gyula Eres, Matthew F. Chisholm, Sheng Dai, and Ho Nyung Lee (2017) “Persistent Electrochemical Performance in Epitaxial VO2(B)” Nano Letters 17 (4), 2229-2233 doi: 10.1021/acs.nanolett.6b04831
Howard T Evans and John M Hughes (1990) “Crystal Chemistry of the natural vanadium bronzes” American Mineralogist, Vol. 75, pages 508-521