Yan Ling Cheah

Constructing Hierarchical Spheres from Large Ultrathin Anatase TiO2 Nanosheets with Nearly 100% Exposed (001) Facets for Fast Reversible Lithium Storage

Synthesis of nanocrystals with exposed high-energy facets is a well-known challenge in many fields of science and technology. The higher reactivity of these facets simultaneously makes them desirable catalysts for sluggish chemical reactions and leads to their small populations in an equilibrated crystal. Using anatase TiO(2) as an example, we demonstrate a facile approach for creating high-surface-area stable nanosheets comprising nearly 100% exposed (001) facets. Our approach relies on spontaneous assembly of the nanosheets into three-dimensional hierarchical spheres, which stabilizes them from collapse. We show that the high surface density of exposed TiO(2) (001) facets leads to fast lithium insertion/deinsertion processes in batteries that mimic features seen in high-power electrochemical capacitors.

Synthesis and electrochemical properties of electrospun V2O5 nanofibers as supercapacitor electrodes

Vanadium pentoxide (V2O5) nanofibers (VNF) were synthesized through a simple electrospinning method, and their application as supercapacitor electrodes demonstrated. The effect of annealing temperature on the microstructure and morphology of VNF was investigated systematically through scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) surface area measurements. Electrochemical properties of the synthesized products as electrodes in a supercapacitor device were studied using cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy in aqueous electrolyte of different pH and also in an organic electrolyte. The highest specific capacitance was achieved for VNF annealed at 400 °C, which yielded 190 F g−1 in aqueous electrolyte (2 M KCl) and 250 F g−1 in organic electrolyte (1 M LiClO4 in PC) with promising energy density of 5 Wh kg−1 and 78 Wh kg−1 respectively.

Electrospun Porous NiCo2O4 Nanotubes as Advanced Electrodes for Electrochemical Capacitors

Novel, porous NiCo2O4 nanotubes (NCO-NTs) are prepared by a single-spinneret electrospinning technique followed by calcination in air. The obtained NCO-NTs display a one-dimensional architecture with a porous structure and hollow interiors. The effect of precursor concentration on the morphologies of the products is investigated. Due to their unique structure, the prepared NCO-NT electrode exhibits a high specific capacitance (1647 F g(-1) at 1 A g(-1)), excellent rate capability (77.3 % capacity retention at 25 A g(-1)), and outstanding cycling stability (6.4 % loss after 3000 cycles), which indicates it has great potential for high-performance electrochemical capacitors. The desirable enhanced capacitive performance of NCO-NTs can be attributed to the relatively large specific surface area of these porous and hollow one-dimensional nanostructures.

Improved Elevated Temperature Performance of Al-Intercalated V2O5 Electrospun Nanofibers for Lithium-Ion Batteries

Al-inserted vanadium pentoxide (V2O5) nanofibers (Al-VNF) are synthesized by simple electrospinning technique. Powder X-ray diffraction (XRD) patterns confirm the formation of phase-pure structure. Elemental mapping and XPS studies are used to confirm chemical insertion of Al in VNF. Surface morphological features of as-spun and sintered fibers with Al-insertion are investigated by field-emission scanning electron microscopy (FE-SEM). Electrochemical Li-insertion behavior of Al-VNFs are explored as cathode in half-cell configuration (vs. Li) using cyclic voltammetry and galvanostatic charge-discharge studies. Al-VNF (Al0.5V2O5) shows an initial discharge capacity of ∼250 mA h g(-1) and improved capacity retention of >60% after 50 cycles at 0.1 C rate, whereas native VNF showed only ∼40% capacity retention at room temperature. Enhanced high current rate and elevated temperature performance of Al-VNF (Al1.0V2O5) is observed with improved capacity retention (∼70%) characteristics. Improved performance of Al-inserted VNF is mainly attributed to the retention of fibrous morphology, apart from structural stabilization during electrochemical cycling.

Facile Approach to Prepare Porous CaSnO3 Nanotubes via a Single Spinneret Electrospinning Technique as Anodes for Lithium Ion Batteries

CaSnO₃ nanotubes are successfully prepared by a single spinneret electrospinning technique. The characterized results indicate that the well-crystallized one-dimensional (1D) CaSnO₃ nanostructures consist of about 10 nm nanocrystals, which interconnect to form nanofibers, nanotubes, and ruptured nanobelts after calcination. The diameter and wall thickness of CaSnO₃ nanotubes are about 180 and 40 nm, respectively. It is demonstrated that CaSnO₃ nanofiber, nanotubes, and ruptured nanobelts can be obtained by adjusting the calcination temperature in the range of 600-800 °C. The effect of calcination temperature on the morphologies of electrospun 1D CaSnO₃ nanostructures and the formation mechanism leading to 1D CaSnO₃ nanostructures are investigated. As anodes for lithium ion batteries, CaSnO₃ nanotubes exhibit superior electrochemical performance and deliver 1168 mAh g⁻¹ of initial discharge capacity and 565 mAh g⁻¹ of discharge capacity up to the 50th cycle, which is ascribed to the hollow interior structure of 1D CaSnO₃ nanotubes. Such porous nanotubular structure provides both buffer spaces for volume change during charging/discharging and rapid lithium ion transport, resulting in excellent electrochemical performance.

Layered NaxMnO2+z in Sodium Ion Batteries–Influence of Morphology on Cycle Performance

Due to its potential cost advantage, sodium ion batteries could become a commercial alternative to lithium ion batteries. One promising cathode material for this type of battery is layered sodium manganese oxide. In this investigation we report on the influence of morphology on cycle performance for the layered NaxMnO2+z. Hollow spheres of NaxMnO2+z with a diameter of ∼5 μm were compared to flake-like NaxMnO2+z. It was found that the electrochemical behavior of both materials as measured by cyclic voltammetry is comparable. However, the cycle stability of the spheres is significantly higher, with 94 mA h g(-1) discharge capacity after 100 cycles, as opposed to 73 mA h g(-1) for the flakes (50 mA g(-1)). The better stability can potentially be attributed to better accommodation of volume changes of the material due to its spherical morphology, better contact with the added conductive carbon, and higher electrode/electrolyte interface owing to better wetting of the active material with the electrolyte.

Synthesis and Enhanced Lithium Storage Properties of Electrospun V2O5 Nanofibers in Full-Cell Assembly with a Spinel Li4Ti5O12 Anode

We have successfully demonstrated the reversible electrochemical Li-insertion properties of electrospun vanadium pentoxide nanofibers (VNF) in full-cell assembly with a Li4Ti5O12 anode. Li-insertion in to VNF is restricted for the intercalation of 1 mol of Li by adjusting lower cutoff potential (2.5-4 V vs Li). The half-cell (Li/VNF) delivered a reversible capacity of ~148 mA h g(-1) with excellent cycleability and capacity retention of over 85% after 30 cycles. Full-cell assembly is conducted for such VNF cathodes after the electrochemical lithiation (LiV2O5) with spinel Li4Ti5O12 anode under the optimized mass loadings. Full-cell (LiV2O5/Li4Ti5O12) delivered an excellent cycleability irrespective of applied current densities with good reversible capacity of ~119 mA h g(-1) (at 20 mA g(-1) current density). This work clearly demonstrates the possibility of using LiV2O5/Li4Ti5O12 configuration for high power applications such as hybrid electric vehicles and electric vehicles in the near future.

Electrospun hierarchical CaCo2O4 nanofibers with excellent lithium storage properties.

Hierarchical CaCo2O4 nanofibers (denoted as CCO-NFs) with a unique hierarchical structure have been prepared by a facile electrospinning method and subsequent calcination in air. The as-prepared CCO-NFs are composed of well-defined ultrathin nanoplates that arrange themselves in an oriented manner to form one-dimensional (1D) hierarchical structures. The controllable formation process and possible formation mechanism are also discussed. Moreover, as a demonstration of the functional properties of such hierarchical architecture, the 1D hierarchical CCO-NFs were investigated as materials for lithium-ion batteries (LIBs) anode; they not only delivers a high reversible capacity of 650 mAh g(-1) at a current of 100 mA g(-1) and with 99.6% capacity retention over 60 cycles, but they also show excellent rate capability with respect to counterpart nanoplates-in-nanofibers and nanoplates. The high specific surface areas as well as the unique feature of hierarchical structures are probably responsible for the enhanced electrochemical performance. Considering their facile preparation and good lithium storage properties, 1D hierarchical CCO-NFs will hold promise in practical LIBs.