Fabrication processes of GF-supported GQDs-coated VO2 nanobelt arrays. (a−c) Schematics of the fabrication process. The yellow basis represent the GF substrate. The green arrays represent VO2 nanoarrays, and the blue covering represents the GQDs. (d−f) The corresponding SEM images (fine structure in inset). Credit: ACS, Chao et al. Click to enlarge.

The team suggests, in a paper published in the ACS journal Nano Letters, that the results, showing rechargeable sodium-ion batteries with a comparable performance to current Li-ion batteries, could push NIBs as a cost-effective alternative for next-generation post-lithium batteries.

Vanadium oxide has been long regarded as a promising electrode material for LIBs owing to its high capacity, low cost, and abundant sources. In particular, vanadium dioxide stands out because of its unique VO₂ (B) bilayers formed from edge-sharing VO₆ octahedra, rapid lithium ion diffusion rate and higher capacity than other types of vanadium oxides. So far, various forms of VO₂ nanostructures for LIBs cathode materials have been prepared, such as VO₂ nanoparticles, VO₂ nanowires, VO₂ nanobelts, starlike VO₂ mesocrystals, and VO nanowires assembled hollow microspheres.

While VO₂-related nanomaterials are being extensively studied for LIBs, their application in NIB has not been explored. It is noted that V₂O₅ nanoparticles and V₂O₅ nanobelts with larger lattice spacing have been recently synthesized and demonstrated good sodium ion storage performance. Despite these efforts, the major problem is that these materials suffer from fast capacity fading and poor high-rate performance, probably due to reasons such as self-aggregation, dissolution, and the fast increased charge transfer resistance during cycles. Therefore, tailored nanoarchitecture design and additional surface engineering of active materials are desirable, which can both secure high surface conductivity and sustain the structure integrity for long-time and high-rate cycling.

In this Letter, we demonstrate our nanostructure tailored VO₂ array as the cathode for both LIBs and NIBs with significantly improved electrochemical properties (high-rate capacity tolerance and long-term stability).

—Chao et al.

Benefits of the nanostructured electrode include:

• A binder-free electrode rendered by biface graphene foam. The graphene foam (GF) acts as both scaffold for the bottom-up growth of VO₂ and an efficient current collector. Compared to other common electrode substrates such as Ni foam, carbon cloth, and metal foils, GF is super light, more porous, and still highly conductive.

  The use of GF eliminates the necessity of binder, conductive additive and current collectors, decreasing the weight of the full cell. In the particular case of NIB, most of current electrode materials for NIBs are in powder form and require conductive additives and binders. Although some conductive additives contribute to capacity during sodiation (50−150 mAh/g), the widely used binder PVDF accelerates the deterioration of electrode during sodiation.

• VO₂ nanobelts are beneficial to fast ion diffusion. As the diffusion time of ions is proportional to the square of the diffusion length, an effective strategy to enhance rate performance is to reduce the dimensions and thickness of the active material. The VO₂ nanobelt nanoarray effectively reduces the Li⁺ and Na⁺ diffusion length and also enhances the electrolyte mobility. This is important in boosting the high-rate performance in both Li and Na ion storage.

• Graphene quantum dots as an effective sensitizer and stabilizer. Graphene quantum dots (GQDs, small graphene flakes usually smaller than 100 nm in size and less than 10 layers in thickness) have interesting optical properties due to tunable size and surface chemistry. In the new electrodes, functionalized GQDs were coated onto individual VO₂ nanobelt by electrophoresis deposition. The homogeneous GQD covering effectively separates the VO₂ nanobelts from each other, thus avoiding agglomeration as well as minimizing the dissolution of active materials, especially during the long-term cycling.

  Moreover, the “stacking” feature of the GQDs layer can provide extra Na⁺ storage venues between the graphene flakes (nanocavities).

The electrodes provide bicontinuous electron and lithium/sodium ion transfer channels through the GQD-GF network without the necessity of extra conductive additives. During lithiation, electrolyte can enter the spaces between nanoarrays on both the outer and inner surfaces of GF, so that the Li/Na ions and electrons can react with the VO₂ nanoarrays directly.

The GQD coating provides both sensitization and protection. Sensitization improves the ion diffusion and charge transport kinetics; protection suppresses the dissolution of VO₂ and agglomeration of the array, which is particularly important for long-term cycles.

Resources

• Dongliang Chao, Changrong Zhu, Xinhui Xia, Jilei Liu, Xiao Zhang, Jin Wang, Pei Liang, Jianyi Lin, Hua Zhang, Ze Xiang Shen, and Hong Jin Fan (2014) “Graphene Quantum Dots Coated VO₂ Arrays for Highly Durable Electrodes for Li and Na Ion Batteries” Nano Letters doi: 10.1021/nl504038s