Researchers from Nanyang Technical University (NTU) in Singapore have shown high-capacity, high-rate, and durable lithium- and sodium-ion battery (LIB and NIB) performance using single-crystalline long-range-ordered bilayered VO2 nanoarray electrodes. The VO2 nanoarrays are supported on graphene foam (GF) and coated with a thin (∼2 nm) layer of graphene quantum dots (GQDs) to enhance the electrochemical performance both in LIBs and NIBs.
In lithiation (for LIBs), the electrode delivers a capacity more than 420 mAh/g and a capacity retention of 94% after 1500 cycles at 18 A/g. During sodiation (for NIBs), it can also exhibit a high capacity of 306 mAh/g and superior rate tolerance and good capacity retention (88% after 1500 cycles at 18 A/g) with a power density of 42 kW/kg at an energy density more than 100 Wh/kg.
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.
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