Qi-Hui Wu

Germanium–graphene composite anode for high-energy lithium batteries with long cycle life

The high-energy lithium ion battery is an ideal power source for electric vehicles and grid-scale energy storage applications. Germanium is a promising anode material for lithium ion batteries due to its high specific capacity, but still suffers from poor cyclability. Here, we report a facile preparation of a germanium–graphene nanocomposite using a low-pressure thermal evaporation approach, by which crystalline germanium particles are uniformly deposited on graphene surfaces or embedded into graphene sheets. The nanocomposite exhibits a high Coulombic efficiency of 80.4% in the first cycle and a capacity retention of 84.9% after 400 full cycles in a half cell, along with high utilization of germanium in the composite and high rate capability. These outstanding properties are attributed to the monodisperse distribution of high-quality germanium particles in a flexible graphene framework. This preparation approach can be extended to other active elements that can be easily evaporated (e.g., sulfur, phosphorus) for the preparation of graphene-based composites for lithium ion battery applications.

Three-dimensional Sn–graphene anode for high-performance lithium-ion batteries

Tin (Sn) has been considered as one of the most promising anode materials for high-capacity lithium-ion batteries (LIBs) due to its high energy density, abundance, and environmentally benign nature. However, the problems of fast capacity fading at prolonged cycling and poor rate capacity hinder its practical use. Herein, we report the development of a novel architecture of Sn nanoparticle-decorated three-dimensional (3D) foothill-like graphene as an anode in LIBs. Electrochemical measurements demonstrated that the 3D Sn-graphene anode delivered a reversible capacity of 466 mA h g(-1) at a current density of 879 mA g(-1) (1 C) after over 4000 cycles and 794 mA h g(-1) at 293 mA g(-1) (1/3 C) after 400 cycles. The capacity at 1/3 C is over 200% that of conventional graphite anodes, suggesting that the 3D Sn-graphene structure enables a significant improvement in the overall performance of a LIB in aspects of capacity, cycle life, and rate capacity.

Facile synthesis of laminate-structured graphene sheet–Fe3O4 nanocomposites with superior high reversible specific capacity and cyclic stability for lithium-ion batteries

A facile one pot hydrothermal method has been developed to synthesize laminate-structured graphene sheet–Fe3O4 nanocomposites (GNS–Fe3O4). Fe3O4 nanoparticles were decorated densely and homogeneously in the graphene matrix. Galvanostatic charge–discharge cycling of the GNS–Fe3O4 nanocomposites exhibited a reversible specific capacity over 1200 mAh g−1 at 100 mA g−1 without palpable fading for 50 cycles in the voltage range 0.01–3.0 V. A cell for the rate capacity test indicated a high current density of 946 mAh g−1 at a cycling rate of 1000 mA g−1, which could be fully recovered to 1359 mAh g−1 at 100 mA g−1 after 50 cycles. The superior electrochemical performance of the nanocomposites can be attributed to the following factors: (i) the thermal expanded graphene oxide (TEGO) under atmosphere could attach more oxygen functional groups than the hydrogen reduced graphene, which benefited the adsorption and fastness of the nano-sized Fe3O4; (ii) annealing of TEGO–Fe3O4 nanocomposites further improved the conductivity of the graphene matrix, providing a high electron transport rate at the electrode–electrolyte interface; (iii) the laminated structure of nanocomposites could prevent the agglomeration of Fe3O4 nanoparticles and the restacking of graphene sheets, and effectively release the strain caused by the volume expansion of the Fe3O4 nanoparticles, facilitating ion/electron transportation within the electrode and at the electrode–electrolyte interface.

Substrate effect on the electronic structures of CuPc/graphene interfaces

The interfacial electronic structures of copper phthalocyanine (CuPc) deposited on a single-layer graphene (SLG) film prepared on Cu and SiO2 substrates (SLG/Cu and SLG/SiO2) were investigated using ultraviolet photoelectron spectroscopy. The ionization energy of CuPc on SLG/Cu and SLG/SiO2 substrate is, respectively, 5.62u2009eV and 4.97u2009eV. The energy level alignments at the two interfaces were estimated. The results revealed that the height of the electron (hole) injection barriers are 1.20 (1.10) and 1.38 (0.92) eV at CuPc/SLG/Cu and CuPc/SLG/SiO2 interfaces, respectively.

Mechanism of non-metal catalytic growth of graphene on silicon

Compared to preparation on metal substrates, graphene synthesis on non-metal surfaces is highly desirable to avoid the deleterious metallic effects in fabrication of electronic devices. However, study of graphene growth mechanism on non-metal surfaces is rare and little understood. Here, we report that few-layers graphene films can be grown directly on silicon-on-insulator surface. Furthermore, the graphene growth mechanism on non-metal surfaces is proposed as a surface reaction, adsorption, decomposition, and accumulation process.

Sn and SnO2-graphene composites as anode materials for lithium-ion batteries

Sn and SnO2-graphene composites were synthesized using hydrothermal process, followed by annealing in Ar/H2 atmosphere, and characterized using x-ray diffraction, scanning electron microscopy, and transition electron microscopy. The results indicated that the polycrystalline metallic Sn forms nanospheres with a diameter of 100u2009∼u2009300xa0nm, while the SnO2 nanoparticles are much smaller with a size below 15xa0nm, which adsorb tightly on the surface of graphene sheets. The Sn and SnO2-gaphene composites showed good electrochemical performance. After 55 charging/discharging cycles, the capacity remains above 440xa0mAh/g at a cycling rate of 400xa0mA/g and the coulombic efficiency is 99.1xa0%. The good electrochemical properties of the composites are partially contributed to the graphene component with good mechanical flexibility and electrical conductivity, which is an excellent carbon matrix for dispersing the Sn and SnO2 nanostructures and provides the electron transport pathways as well.

MnO modified carbon nanotubes as a sulfur host with enhanced performance in Li/S batteries

Lithium/sulfur (Li/S) batteries have become promising future power sources owing to the high energy density. Carbon materials are the most used sulfur hosts, but their ability to adsorb polysulfide intermediates has been unreliable, thus recently many researchers have turned their interest to metal oxide materials. Here, we manufactured a composite of carbon nanotubes modified with manganese oxide nanoparticles (CNTs/MnO) as a sulfur host material. In Li/S cells, the CNTs/MnO–S cathode showed a rather better cycling stability over 100 cycles than a CNTs–S cathode with the same carbon/sulfur weight ratio of about 1u2006:u20068. In addition, the CNTs/MnO–S cathode presented an initial discharge capacity of 716 mA h g−1 at a high current density of 5.0C, in contrast to the result of only 415 mA h g−1 with the CNTs–S cathode. Physical and electrochemical characterization proved that the MnO modification does not vary the surface area of the CNTs but lowers their electrical conductivity. By carefully comparing the differences in the 1st discharge capacities of the two cathodes, the MnO modification could obviously improve the initial utilization of S especially at high current densities. The improved electrochemical characteristics of the CNTs/MnO–S electrode can be attributed to its properties of a stronger adsorption capability for polysulfides.

Surface doping of nitrogen atoms on graphene via molecular precursor

Surface doping can be a powerful way to modify the electronic properties of graphene with the unique potential to retain the excellent pristine properties of graphene. Here, we report an atomic surface doping method for graphene via dissociation of adsorbed precursor molecules of tetrafluorotetracyanoquinodimethane (F4-TCNQ) induced by hydrogen plasma treatment. Significantly, the location of the dopant N atoms can be pre-determined by the location and orientation of the F4-TCNQ molecule precursor on graphene, leading in principle to site-selective doping. Furthermore, the molecular precursor is stable under ambient conditions, satisfying an important consideration for patterning processes.

Ruthenium@mesoporous graphene-like carbon: a novel three-dimensional cathode catalyst for lithium–oxygen batteries

Nonaqueous lithium–oxygen (Li–O2) batteries are considered as the most promising energy storage systems, because of their very high energy densities, which are significantly greater than those of lithium-ion batteries. Recently, carbon materials have been widely used as cathode catalysts for Li–O2 batteries due to their outstanding conductivity. However, side reactions are inevitable when the carbon materials are exposed to the Li2O2 product and the organic electrolytes, leading to degradation in their cycling performances. Herein, we propose a simplified strategy, which combines the template method and in situ growth approach in order to establish a three-dimensional (3D) mesoporous graphene-like carbon structure, and growth of a uniform layer of ruthenium particles on it. The as-prepared 3D ruthenium@mesoporous graphene-like carbon material delivers a reversible capacity of about 6433 mA h g−1 (based on the total mass of the composite) at a current density of 200 mA g−1, when it is used as a cathode catalyst for Li–O2 batteries. Under a curtaining capacity of 500 mA h g−1, it exhibits an extremely low charge voltage of 3.20 V (only 240 mV higher than the thermodynamic potential) and a high discharge voltage of 2.84 V at a current density of 100 mA g−1.

Synthesis of hierarchical nanospheres Fe2O3/graphene composite and its application in lithium-ion battery as a high-performance anode material

A hierarchically nanospherical α-Fe2O3/graphene composite with a homogeneous mono-pore size of 4xa0nm has been prepared using a hydrothermal method. The composite showed an extremely high rate performance and good cycling stability when applied as an anode material for lithium-ion batteries owing to its unique three dimensional architecture. A specific capacity of 110xa0mAh/g was obtained at an extremely high current rate of 40xa0A/g and recover to 830xa0mAh/g at 0.5xa0A/g after 60xa0cycles. After 250xa0cycles at 2xa0A/g, the composite electrode exhibited a capacity of 630xa0mAh/g with a columbic efficiency of 99.5xa0%.