Yufeng Tang

Scotch-tape-like exfoliation of graphite assisted with elemental sulfur and graphene–sulfur composites for high-performance lithium-sulfur batteries

A new composite structure of graphene–sulfur with a high electrochemical performance is proposed. Scotch-tape-like sulfur-assisted exfoliation of graphite is developed to produce the graphene–sulfur composites and freestanding low-defect graphene sheets. The intimate interaction between sulfur and graphene, attributed to the similar electronegativities of the two elements, is stronger than the van der Waals forces between adjacent π–π stacked graphene layers. This causes cleavage of the graphene layers when the sulfur molecules stick to the surface and edges of the graphite, similar to Scotch tape in micromechanical exfoliation processes. This approach enables us to obtain graphene with an electrical conductivity as high as 1820 S cm−1 and a Hall mobility as high as 200 cm2 V−1 s−1, superior to most reported graphene. Furthermore, the graphene sheets which uniformly anchor sulfur molecules provide a superior confinement ability for polysulfides, sufficient space to accommodate sulfur volumetric expansion, a large contact area with the sulfur and a short transport pathway for both electrons and Li+. The unique structure containing 73 wt.% sulfur exhibits excellent overall electrochemical properties of 615 mA h g−1 at the 1 C (1 C = 1675 mA g−1) rate after 100 cycles (corresponding average Coulombic efficiency of over 96%) and 570 mA h g−1 at 2 C. These encouraging results represent that sulfur molecules bound onto graphene sheets could be a promising cathode material for lithium batteries with a high energy density.

Synthesis of graphene-supported Li4Ti5O12 nanosheets for high rate battery application

The composite structure of Li4Ti5O12 (LTO) nanosheets rooted on two-dimensional graphene (GR) was proposed to achieve an enhanced rate performance for high rate lithium ion batteries. Such a nanostructured material of graphene-supported Li4Ti5O12 nanosheets (GR-LTOs) was fabricated by using TiO2 colloid-containing graphene oxide (GO) sheets as precursor in a hydrothermal reaction. TiO2 colloids serve as the seeds to realize the growth of LTO on GR and ensure the tight bonding between GR and LTO to benefit the charge transfer from the two types of sheets. The GR-LTOs sample possesses excellent electrochemical properties with good cycle stability and a high specific capacity of 140 mA h g−1 at 20 C. It demonstrates that LTO nanosheets, graphene sheets, and their tightly-bonded interfaces provide short ion diffusion distance and good electron conduction in such a composite structure.

Phase-Controlled Synthesis of Cobalt Sulfides for Lithium Ion Batteries

The polyhedral CoS(2) with a narrow size distribution was synthesized by a facile solid-state assembly process in a sealed silica tube. The flux of potassium halide (KX; X = Cl, Br, I) plays a crucial role in the formation of polyhedrons and the size distribution. The S(2)(2-) groups in CoS(2) can be controllably withdrawn during heat treatment in air. The obtained phases and microstructures of CoS(2), Co(3)S(4), CoS, Co(9)S(8), and CoO depended on heating temperature and time. These cobalt materials, successfully used as the electrodes of lithium ion batteries, possessed good cycling stability in lithium ion batteries. The discharge capacities of 929.1 and 835.2 mAh g(-1) were obtained for CoS(2) and CoS respectively, and 76% and 71% of the capacities remained after 10 cycles. High capacities and good cycle performance make them promising candidates for lithium ion batteries. The approach combining solid-state assembly and heat treatment provides a simple and versatile way to prepare various metal chalcogeides for energy storage applications.

Large-scale preparation of highly conductive three dimensional graphene and its applications in CdTe solar cells

High-yield three dimensional (3D) graphene networks were prepared on Ni foams by ambient pressure chemical vapour deposition (APCVD). The layer number of graphene can be tuned by changing the gas flow ratio and growth time. The assembled films from the vacuum filtration of the 3D graphene possessed excellent electrical transport properties (Rs ∼ 0.45 Ω/sq, σ ∼ 600 S cm−1), superior to the reported graphene and carbon nanotube films. Highly conductive films as back electrodes of CdTe solar cells significantly improved the photovoltaic efficiency (9.1%).

Low-Temperature Aluminum Reduction of Graphene Oxide, Electrical Properties, Surface Wettability, and Energy Storage Applications

Low-temperature aluminum (Al) reduction is first introduced to reduce graphene oxide (GO) at 100-200 °C in a two-zone furnace. The melted Al metal exhibits an excellent deoxygen ability to produce well-crystallized reduced graphene oxide (RGO) papers with a low O/C ratio of 0.058 (Al-RGO), compared with 0.201 in the thermally reduced one (T-RGO). The Al-RGO papers possess outstanding mechanical flexibility and extremely high electrical conductivities (sheet resistance R(s) ~ 1.75 Ω/sq), compared with 20.12 Ω/sq of T-RGO. More interestingly, very nice hydrophobic nature (90.5°) was observed, significantly superior to the reported chemically or thermally reduced papers. These enhanced properties are attributed to the low oxygen content in the RGO papers. During the aluminum reduction, highly active H atoms from H(2)O reacted with melted Al promise an efficient oxygen removal. This method was also applicable to reduce graphene oxide foams, which were used in the GO/SA (stearic acid) composite as a highly thermally conductive reservoir to hold the phase change material for thermal energy storage. The Al-reduced RGO/SnS(2) composites were further used in an anode material of lithium ion batteries possessing a higher specific capacity. Overall, low-temperature Al reduction is an effective method to prepare highly conductive RGO papers and related composites for flexible energy conversion and storage device applications.

In situ grown graphene-encapsulated germanium nanowires for superior lithium-ion storage properties

Alloying anode materials (Si, Ge, Sn etc.) in lithium-ion batteries usually suffer from a remarkable loss of capacity during the charge–discharge cycling. Herein, homogeneous in situ-grown graphene-encapsulated Ge nanowires are successfully achieved by a simple, completely catalyst-free route via arc-discharge. The Ge@G composite is composed of a graphene sheath and a metallic Ge nanowire core. This unique composite of graphene-encapsulated Ge nanowires is an ideal anode material for lithium ion storage. It can exhibit excellent electrochemical performance with a reversible specific capacity of 1400 mA h g−1 after 50 cycles at a current density of 1600 mA g−1 (the theoretical specific capacity of Ge is 1624 mA h g−1). These encouraging results demonstrate that arc-discharge synthesis provides efficient graphene encapsulation of Ge nanowires, and graphene encapsulation is a feasible solution to protect electrode materials for lithium-ion storage.

New Graphene Form of Nanoporous Monolith for Excellent Energy Storage

Extraordinary tubular graphene cellular material of a tetrahedrally connected covalent structure was very recently discovered as a new supermaterial with ultralight, ultrastiff, superelastic, and excellent conductive characteristics, but no high specific surface area will keep it from any next-generation energy storage applications. Herein, we prepare another new graphene monolith of mesoporous graphene-filled tubes instead of hollow tubes in the reported cellular structure. This graphene nanoporous monolith is also composed of covalently bonded carbon network possessing high specific surface area of ∼1590 m(2) g(-1) and electrical conductivity of ∼32 S cm(-1), superior to graphene aerogels and porous graphene forms self-assembled by graphene oxide. This 3D graphene monolith can support over 10 000 times its own weight, significantly superior to CNT and graphene cellular materials with a similar density. Furthermore, pseudocapacitance-active functional groups are introduced into the new nanoporous graphene monolith as an electrode material in electrochemical capacitors. Surprisingly, the electrode of 3D mesoporous graphene has a specific capacitance of 303 F g(-1) and maintains over 98% retention after 10 000 cycles, belonging to the list for the best carbon-based active materials. The macroscopic mesoporous graphene monolith suggests the great potential as an electrode for supercapacitors in energy storage areas.

Highly conductive, free-standing and flexible graphene papers for energy conversion and storage devices

A simple and scalable method was proposed to fabricate graphene papers, and the graphene sheets were prepared using conventional chemical vapor deposition (CVD) method. The CVD graphene papers possess much higher electrical conductivity of 1097 S cm−1, compared with other reported carbon-related papers (graphene, carbon nanotube, etc.). The graphene papers have good flexibility with only <5% loss of electrical conductivity after mechanically bending 500 times. Such free-standing graphene papers can replace expensive Pt/FTO counter electrodes of dye-sensitized solar cells with better energy conversion efficiency, and also be used as anodes of lithium ion batteries possessing a superior high-rate capacity and cycling performance. The highly conductive, free-standing and flexible graphene papers reveal potential in high-performance, flexible energy conversion and storage devices.

Niobium doped anatase TiO2 as an effective anode material for sodium-ion batteries

Sodium-ion batteries are considered to be a promising low-cost alternative to common lithium-ion batteries in the areas where specific energy is less critical. Among all the anode materials studied so far, TiO2 is very promising due to its low operating voltage, high capacity, nontoxicity, and low production cost. Herein, we present Nb-doped anatase TiO2 nanoparticles with high capacity, excellent cycling performance, and excellent rate capability. The optimized Nb-doped TiO2 anode delivers high reversible capacities of 177 mA h g−1 at 0.1C and 108.8 mA h g−1 at 5C, in contrast to 150.4 mA h g−1 at 0.1C and only 54.6 mA h g−1 at 5C for the pristine TiO2. The good performance is likely to be associated with enhanced conductivity and lattice expansion due to Nb doping. These results, in combination with its environmental friendliness and cost efficiency, render Nb-doped TiO2 a promising anode material for high-power sodium-ion batteries.

Superelastic Few-Layer Carbon Foam Made from Natural Cotton for All-Solid-State Electrochemical Capacitors.

Flexible/stretchable devices for energy storage are essential for future wearable and flexible electronics. Electrochemical capacitors (ECs) are an important technology for supplement batteries in the energy storage and harvesting field, but they are limited by relatively low energy density. Herein, we report a superelastic foam consisting of few-layer carbon nanowalls made from natural cotton as a good scaffold to growth conductive polymer polyaniline for stretchable, lightweight, and flexible all-solid-state ECs. As-prepared superelastic bulk tubular carbon foam (surface area ∼950 m(2)/g) can withstand >90% repeated compression cycling and support >45,000 times its own weight but no damage. The flexible device has a high specific capacitance of 510 F g(-1), a specific energy of 25.5 Wh kg(-1) and a power density of 28.5 kW kg(-1) in weight of the total electrode materials and withstands 5,000 charging/discharging cycles.