Feng-Yu Wu

Graphene-modified LiFePO4 cathode for lithium ion battery beyond theoretical capacity

The specific capacity of commercially available cathode carbon-coated lithium iron phosphate is typically 120-160 mAh g(-1), which is lower than the theoretical value 170 mAh g(-1). Here we report that the carbon-coated lithium iron phosphate, surface-modified with 2 wt% of the electrochemically exfoliated graphene layers, is able to reach 208 mAh g(-1) in specific capacity. The excess capacity is attributed to the reversible reduction-oxidation reaction between the lithium ions of the electrolyte and the exfoliated graphene flakes, where the graphene flakes exhibit a capacity higher than 2,000 mAh g(-1). The highly conductive graphene flakes wrapping around carbon-coated lithium iron phosphate also assist the electron migration during the charge/discharge processes, diminishing the irreversible capacity at the first cycle and leading to ~100% coulombic efficiency without fading at various C-rates. Such a simple and scalable approach may also be applied to other cathode systems, boosting up the capacity for various Li batteries.

Three‐Dimensional Molybdenum Sulfide Sponges for Electrocatalytic Water Splitting

Electroactive MoSx catalysts on porous 3D sponges synthezied by a simple and scalable thermolysis process are proposed. Although no conducting materials are used to host the MoSx catalysts, they still serve as efficient electrodes for hydrogen evolution. The high current density of the MoSx-coated sponges are attributed to the large electrochemical surface area and their S-rich chemical structure.

Flexible transparent electrodes made of electrochemically exfoliated graphene sheets from low-cost graphite pieces

Abstract Graphene is a highly promising candidate to replace oxide-based conducting materials for fabrication of transparent electrodes used in flexible displays and electronic devices. However, so far an appropriate method is still not found to provide high-quality, low-cost graphene in a large quantity. To overcome this issue, we demonstrate that massive graphene sheets can be synthesized by electrochemical exfoliation process using artificial graphite as a starting material. The product is mainly composed of bilayer thin graphene sheets. Their lateral size can be up to several to 20xa0μm, and the quality determined by Raman spectroscopy is better than the reduced graphene oxide derived from graphene oxide. The transparent conducting films made of electrochemically exfoliated graphene can be simply prepared by an airbrush spraying method, which is easy to scale up for large-area deposition, and compatible with flexible polymer substrates.

Direct conversion of multilayer molybdenum trioxide to nanorods as multifunctional electrodes in lithium-ion batteries†

In this study we prepared molybdenum trioxide (MoO3) nanorods having average lengths of 0.5-1.5 μm and widths of approximately 100-200 nm through a one-step mechanical break-down process involving favorable fracturing along the crystal direction. We controlled the dimensions of the as-prepared nanorods by applying various imposing times (15-90 min). The nanorods prepared over a reaction time of 90 min were, on average, much shorter and narrower relative to those obtained over 30 min. Evaluations of lithium-ion storage properties revealed that the electrochemical performance of these nanorods was much better than that of bulk materials. As cathodes, the nanorods could deliver a high specific capacity (>315 mA h g(-1)) with losses of less than 2% in the first cycle at a rate of 30 mA g(-1); as anodes, the specific capacity was 800 mA h g(-1) at a rate of 50 mA g(-1). Relative to α-MoO3 microparticles, these nanorods displayed significantly enhanced lithium-ion storage properties with higher reversible capacities and better rate performance, presumably because their much shorter diffusion lengths and higher specific surface areas allowed more-efficient insertion/deinsertion of lithium ions during the charge/discharge process. Accordingly, enhanced physical and/or chemical properties can be obtained through appropriate nanostructuring of materials.

Scalable Approach To Construct Free-Standing and Flexible Carbon Networks for Lithium–Sulfur Battery

Reconstructing carbon nanomaterials (e.g., fullerene, carbon nanotubes (CNTs), and graphene) to multidimensional networks with hierarchical structure is a critical step in exploring their applications. Herein, a sacrificial template method by casting strategy is developed to prepare highly flexible and free-standing carbon film consisting of CNTs, graphene, or both. The scalable size, ultralight and binder-free characteristics, as well as the tunable process/property are promising for their large-scale applications, such as utilizing as interlayers in lithium-sulfur battery. The capability of holding polysulfides (i.e., suppressing the sulfur diffusion) for the networks made from CNTs, graphene, or their mixture is pronounced, among which CNTs are the best. The diffusion process of polysulfides can be visualized in a specially designed glass tube battery. X-ray photoelectron spectroscopy analysis of discharged electrodes was performed to characterize the species in electrodes. A detailed analysis of lithium diffusion constant, electrochemical impedance, and elementary distribution of sulfur in electrodes has been performed to further illustrate the differences of different carbon interlayers for Li-S batteries. The proposed simple and enlargeable production of carbon-based networks may facilitate their applications in battery industry even as a flexible cathode directly. The versatile and reconstructive strategy is extendable to prepare other flexible films and/or membranes for wider applications.

High-performance graphene/sulphur electrodes for flexible Li-ion batteries using the low-temperature spraying method.

Elementary sulphur (S) has been shown to be an excellent cathode material in energy storage devices such as Li-S batteries owing to its very high capacity. The major challenges associated with the sulphur cathodes are structural degradation, poor cycling performance and instability of the solid-electrolyte interphase caused by the dissolution of polysulfides during cycling. Tremendous efforts made by others have demonstrated that encapsulation of S materials improves their cycling performance. To make this approach practical for large scale applications, the use of low-cost technology and materials has become a crucial and new focus of S-based Li-ion batteries. Herein, we propose to use a low temperature spraying process to fabricate graphene/S electrode material, where the ink is composed of graphene flakes and the micron-sized S particles prepared by grinding of low-cost S powders. The S particles are found to be well hosted by highly conductive graphene flakes and consequently superior cyclability (∼70% capacity retention after 250 cycles), good coulombic efficiency (∼98%) and high capacity (∼1500 mA h g(-1)) are obtained. The proposed approach does not require high temperature annealing or baking; hence, another great advantage is to make flexible Li-ion batteries. We have also demonstrated two types of flexible batteries using sprayed graphene/S electrodes.

Few layer graphene paper from electrochemical process for heat conduction

Abstract The graphene paper fabricated through an electrochemical exfoliation and filtration process provides high quality and massive graphene sheets in an ink form synthesised using artificial graphite as starting material. The product is mainly composed of bilayer and few layer graphene. Their lateral size can be up to >10 μm, and the quality determined by Raman spectroscopy is better than reduced graphene oxide. The cross-plane and in-plane thermal conductivity of the graphene paper can reach 5·5 and 3300 W m−1 K−1 measured by the direct method and the thermoelectric method respectively.

New layered cuprates CaBa2R2Cu2Ti2O12 and (R,Ce)2Ba2(R,Ca)2Cu2Ti2O14 (R = Sm, Gd)

Abstract New layered cuprates CaBa 2 R 2 Cu 2 Ti 2 O 12 and (R,Ce) 2 Ba 2 (R,Ca) 2 Cu 2 Ti 2 O 14 (R = Sm,Gd) are reported based on the frameworks of CuO 5 square pyramids and TiO 6 octahedra with BaO, Ca or fluorite-type (R,Ce) 2 O 2 , and rocksalt-type (RO) 2 layers in between. Single phases were obtained for the compositions CaBa 2 R 2 Cu 2 Ti 2 O 12 and R 2.0 Ce 0.9 Ba 2 Ca 1.1 Cu 2 Ti 2 O 14 (R = Sm,Gd), respectively. For CaBa 2 R 2 Cu 2 Ti 2 O 12 , the atom array along the c -axis is Ca(CuO 2 )(BaO)(TiO 2 )(RO) 2 (TiO 2 )(BaO)(CuO 2 ) Ca, where (RO) 2 takes the rocksalt-type configuration. Replacement of Ca by the fluorite-type layer (R,Ce) 2 O 2 gives the latter system. X-ray Rietveld refinements for the single phases CaBa 2 Sm 2 Cu 2 Ti 2 O 12 and Sm 2.0 Ce 0.9 Ba 2 Ca 1.1 Cu 2 Ti 2 O 14 revealed the partial substitutions of R for Ca in the Ca sites and Ca for R in the R sites. Superconductivity has not been achieved in either system even after cation substitutions or annealing under various conditions.