Tuti Mariana Lim

Olivine-Type Nanosheets for Lithium Ion Battery Cathodes

Olivine-type LiMPO4 (M = Fe, Mn, Co, Ni) has become of great interest as cathodes for next-generation high-power lithium-ion batteries. Nevertheless, this family of compounds suffers from poor electronic conductivities and sluggish lithium diffusion in the [010] direction. Here, we develop a liquid-phase exfoliation approach combined with a solvothermal lithiation process in high-pressure high-temperature (HPHT) supercritical fluids for the fabrication of ultrathin LiMPO4 nanosheets (thickness: 3.7-4.6 nm) with exposed (010) surface facets. Importantly, the HPHT solvothermal lithiation could produce monodisperse nanosheets while the traditional high-temperature calcination, which is necessary for cathode materials based on high-quality crystals, leads the formation of large grains and aggregation of the nanosheets. The as-synthesized nanosheets have features of high contact area with the electrolyte and fast lithium transport (time diffusion constant in at the microsecond level). The estimated diffusion time for Li(+) to diffuse over a [010]-thickness of <5 nm (L) was calculated to be less than 25, 2.5, and 250 μs for LiFePO4, LiMnPO4, and LiCoPO4 nanosheets, respectively, via the equation of t = L(2)/D. These values are about 5 orders of magnitude lower than the corresponding bulk materials. This results in high energy densities and excellent rate capabilities (e.g., 18 kW kg(-1) and 90 Wh kg(-1) at a 80 C rate for LiFePO4 nanosheets).

Vanadium pentoxide cathode materials for high-performance lithium-ion batteries enabled by a hierarchical nanoflower structure via an electrochemical process

Hierarchical vanadium oxide nanoflowers (V10O24·nH2O) were synthesized via a simple, high throughput method employing a fast electrochemical reaction of vanadium foil in NaCl aqueous solution, followed by an aging treatment at room temperature. During the electrochemical process, the anodic vanadium foil is dissolved in the form of multi-valence vanadium ions into the solution, driven by the applied electrical field. After being oxidized, the VO2+ and VO2+ ions instantly react with the OH− in the electrolyte to form uniformly distributed vanadium oxide nanoparticles at a high solution temperature due to the exothermic nature of the reaction. Finally, nucleation and growth of one dimensional nanoribbons takes place on the surface of the nanoparticles during the aging process to form unique hierarchical V10O24·nH2O nanoflowers. Upon heat treatment, the hierarchical architecture of the vanadium pentoxide nanoflower morphology is maintained. Such a material provides porous channels, which facilitate fast ion diffusion and effective strain relaxation upon Li ion charge–discharge cycling. The electrochemical tests reveal that the V2O5 nanoflowers cathode could deliver high reversible specific capacities with 100% coulombic efficiency, especially at high C rates (e.g., 140 mAh g−1 at 10 C).

Facile preparation of hydrated vanadium pentoxide nanobelts based bulky paper as flexible binder-free cathodes for high-performance lithium ion batteries

Hydrated vanadium pentoxide (V2O5·0.44H2O, HVO) nanobelts were synthesized by a simply high-yield (e.g. up to ∼99%) hydrothermal approach. The length of these nanobelts was up to several hundred micrometers while the diameter was only ∼20 nm and the thickness was ∼10 nm. Binder-free bulky papers were prepared by using these HVO nanobelts and were tested as Li ion battery cathodes. The unique architecture of the HVO bulky paper provides hierarchical porous channels and large specific surface area, which facilitate fast ion diffusion and effectively strain relaxation upon charge-discharge cycling. The electrochemical tests revealed that the flexible HVO cathode could deliver high reversible specific capacities with ∼100% Coulombic efficiency, especially at high C rates. For example, it achieved a reversible capacity of 163 mAh g−1 at 6.8 C.

Direct growth of FeVO4 nanosheet arrays on stainless steel foil as high-performance binder-free Li ion battery anode

Amorphous FeVO4 nanosheet arrays have been grown directly from a flexible stainless steel (SS) substrate by a facile template-free and catalyst-free chemical vapour deposition (CVD) method. These FeVO4 nanosheets showed superior Li storage properties, especially at high current densities.

One-pot synthesis of carbon-coated VO2(B) nanobelts for high-rate lithium storage

Uniform carbon-coated single crystalline vanadium dioxide (VO2(B)@C) nanobelts were successfully prepared by using a facile one-pot hydrothermal approach. Sucrose plays a dual role in this hydrothermal process, namely as a carbon precursor for the carbon shell and, as a reductant to reduce V2O5 to VO2(B). The thickness of the carbon coating layer is tunable from 3.0 to 6.9 nm by changing the ratio of the precursors. Although a high carbon content can improve the electrical conductivity of VO2(B)@C nanobelts, a thick carbon coating layer would block the lithium ion diffusion. The optimal thickness is found to be 4.3 nm (carbon content: 6.6 wt%), where the cathode displays superior performance with highly reversible specific capacities, good cycling stabilities and excellent rate capabilities (e.g. 100 mA h g−1 at 12.4 C).

Synthesis of hexagonal-symmetry α-iron oxyhydroxide crystals using reduced graphene oxide as a surfactant and their Li storage properties

Microcrystalline α-iron oxyhydroxide/reduced graphene oxide (α-FeOOH/rGO) samples have been successfully synthesized by a facile hydrothermal process. The α-FeOOH/rGO samples are either hexagonal disks with a diameter of ∼1 μm and a thickness of 300 nm or hexapods with a diameter of ∼2 μm and a thickness of 700 nm, while only bulk and aggregated FeOOH is observed without the addition of graphene oxide sheets. The size and shape of the α-FeOOH depend on the reaction time, concentration of Fe3+, and the addition of graphene oxide. The growth of the hexagonal disks and hexapods is mainly due to a series of phase and structural transformations. The α-FeOOH/rGO displays superior anode performance with a high reversible specific capacity of 569 mA h g−1 at the 50th cycle.

Synthesis of Porous Amorphous FePO4 Nanotubes and Their Lithium Storage Properties

The controlled synthesis of functional materials with designed nanostructures has led to many unique applications, including optoelectronic devices, sensors, battery electrodes, and drug delivery. To prepare advanced electrode materials for energy storage devices (for example, lithium ion batteries (LIBs) or supercapacitors), it is desired to develop nanostructures with features such as: 1) a high surface area for the effective interaction between the electrolyte and active materials; 2) a fine feature size to shorten the charge carrier diffusion distance; 3) hollow channels to allow the effective penetration throughout the electrodes; and 4) some void space to buffer mechanical strain that may be generated during the cycling process. For examples, double-shelled nanocapsules of V2O5-based composites has a reversible capacity of 947 mAh g 1 at a rate of 250 mA g 1 and they retain a high capacity of 673 mAh g 1 after 50 cycles as high-performance anode materials for LIBs; Co3O4 porous nanocages show capacities of up to 1465 mAh g 1 that are attained after 50 cycles at a current density of 300 mA g 1 for LIBs. Porous SnO2 nanotubes delivered high specific capacity (540-600 mAh g ) and good cyclability (0.0375 % capacity loss per cycle) in rechargeable LIBs as anode electrode materials. Based on the above discussion, it is attractive to build nanostructures with a hollow interior and porous nature, which has been rarely demonstrated for cathode materials of lithium ion batteries. Herein, we report an oil-phase synthesis of porous aFePO4 nanotubes with a pore size of 2–10 nm. We first prepared Fe2O3 nanotubes and then reacted them in the oil phase with alkyl phosphonic/alkyl phosphinic acids (RP(O)(OH)2/R HP(O)(OH), R and R are alkyl groups ACHTUNGTRENNUNG(CH2)nCH3, n= 5–17), which exist as the impurities in technical-grade trioctylphosphine oxide (TOPO). The second step converted the Fe2O3 into iron alkyl phosphate (Fe2ACHTUNGTRENNUNG(RPO3)3/Fe ACHTUNGTRENNUNG(R1HPO2)3) and simultaneously generated pores in the tubes owing to etchingby the weak acid. After annealing at 350 8C in air, the samples were transferred to a-FePO4 porous nanotubes. Furthermore, the cathode performance of these porous a-FePO4 nanotubes was examined in a LIB half-cell. The results showed that these porous a-FePO4 nanotubes exhibited a reversible specific capacity of 130.4 mAh g 1 during the 35th cycle at a current density of 35.6 mA g 1 (C/5), which was considered better than that of bulk a-FePO4. To prepare the FePO4 samples, we first synthesized Fe2O3 nanotubes according to reported work (Supporting Information, Figures S1,S2). These Fe2O3 nanotubes were dispersed into TOPO (90 %, technical grade) and heating the mixture at about 308 8C under a degassing condition with a rotary pump. After 50 min, the solution was cooled to room temperature and we collected the precipitate by repeated centrifuging and washing with acetone and methanol. We then annealed the precipitates at 350 8C for 60 min in air to further promote the reaction and remove possibly attached surface ligands. The field emission scanning electron microscopy (FESEM) image (Figure 1 a) reveals that the obtained samples are nanotubes, which are highly uniform with length of about 250 nm and width of about 100 nm. The structures of these nanotubes were further studied by means of transmission electron microscopy (TEM) and selectedarea electron diffraction (SAED). The low-magnification TEM image (Figure 1 b) shows that these nanotubes are [a] R. Cai, Dr. Y. Du, W. Zhang, H. Tan, X. Huang, H. Liu, J. Zhu, S. Peng, J. Chen, Prof. Y. Huang, Prof. Q. Zhang, Prof. H. Zhang, Prof. Q. Yan School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore) E-mail : alexyan@ntu.edu.sg [b] C. Chen, Prof. R. Xu School of Chemistry and Biochemistry Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 (Singapore) [c] Prof. T. M. Lim School of Life Science and Chemical Technology Ngee Ann Polytechnic, Singapore 599489 (Singapore) [d] Prof. Q. Yan Energy research Institute @ NTU, Nanyang Technological University, TUM CREATE Centre for Electromobility@NTU, Singapore 637459(Singapore) [e] T. Zeng, H. Yang, Prof. Y. Zhao, Prof. H. Wu National Center for Nanoscience and Technology of China, Beijing 100190 (P. R. China), Key Laboratory for Biomedical Effects of Nanomaterials & Nanosafety, Institute of High Energy Physics Chinese Academy of Sciences, Beijing 100049 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201203935.

Controlled Synthesis of Double-Wall a-FePO4 Nanotubes and their LIB Cathode Properties

Double-wall amorphous FePO4 nanotubes are prepared by an oil-phase chemical route. The inward diffusion of vacancies and outward diffusion of ions through passivation layers result in double-wall nanotubes with thin walls. Such a process can be extended to prepare hollow polydedral nanocrystals and hollow ellipsoids. The double-wall FePO4 nanotubes show interesting cathode performance in Li ion batteries.

Synthesis of Porous, Hollow Metal MCO3 (M=Mn, Co, Ca) Microstructures and Adsorption Properties Thereof

Porous, hollow metal carbonate microstructures show many unique properties, and are attractive for various applications. Herein, we report the first demonstration of a general strategy to synthesize hollow metal carbonate structures, including porous MnCO3 hollow cubics, porous CoCO3 hollow rhombuses and porous CaCO3 hollow capsules. For example, the porous, hollow MnCO3 microcubes show larger Brunauer-Emmett-Teller (BET) surface areas of 359.5 m(2)  g(-1) , which is much larger than that of solid MnCO3 microcubics (i.e., 12.03 m(2)  g(-1) ). As a proof of concept, these porous MnCO3 hollow microcubes were applied to water treatment and exhibited an excellent ability to remove organic pollutants in waste water owing to their hollow structure and large specific surface area.

Solvothermal-induced conversion of one-dimensional multilayer nanotubes to two-dimensional hydrophilic VOx nanosheets: Synthesis and water treatment application

Ultrathin 2D nanostructures have shown many unique properties and are attractive for various potential applications. Here, we demonstrated a strategy to synthesize ultrathin VOx nanosheets. The as-obtained ultrathin VOx nanosheets showed a large Brunauer-Emmett-Teller (BET) surface area of 136.3 m2 g(-1), which is much larger than that of 1D multilayer VOx nanotubes. As a proof of concept, these hydrophilic ultrathin nanosheets were applied in water treatment and exhibited excellent absorption capability to remove Rhodamine B (RhB) in wastewater owing to their large specific surface area, good hydrophilic property, and more negative zeta potential. In addition, this method could be generalized to prepare other 2D nanostructures with great potential for various attractive applications.