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.

Researchers predicted where lithium ions (green spheres) would pack and move in an open framework of epitaxially strained vanadium dioxide, depicted here by a stick model (oxygen-connecting bonds are red and vanadium-connecting bonds, turquoise). Guided by theory and computation, they designed, synthesized and tested the material—proving it indeed had excellent storage capacity, ion conduction and structural stability. Image by Panchapakesan Ganesh, Oak Ridge National Laboratory/Dept. of Energy. Click to enlarge.

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.

Electrochemical performance of VO2(B) electrodes. (a) Galvanostatic discharge (Li intercalation)−charge (Li deintercalation) profile of an epitaxial VO2(B) film in a voltage range of 1.5−3.2 V. At various current densities, a plateau related to the Li reduction into VO2(B) appears near 2.6 eV. (b) Capacity as a function of the discharging rate. The capacity of the epitaxial VO2(B) films (solid symbols) is close to the theoretical value (323 mAh/g). The capacity values of pure V2O5, nanobelt VO2(B), nanoscroll VO2(B), mesocrystal VO2(B), carbon, or graphene quantum-dots coated VO2(B) are included for comparison. (c) Cyclic capacity test up to 200 cycles reveals a Coulombic efficiency close to 100%. Credit: ACS, Lee et al. Click to enlarge.

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

Resources

• 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