Cooperative Electronic Structure Modulator of Fe Single-Atom Electrocatalyst for High Energy and Long Cycle Li-S Pouch Cell.

High-energy and long cycle lithium-sulfur (Li-S) pouch cells are limited by the insufficient capacities and stabilities of their cathodes under practical electrolyte/sulfur (E/S), electrolyte/capacity (E/C), and negative/positive (N/P) ratios. Herein, an advanced cathode comprising highly active Fe single-atom catalysts (SACs) is reported to form 320.2 W h kg-1 multistacked Li-S pouch cells with total capacity of ≈1 A h level, satisfying low E/S (3.0), E/C (2.8), and N/P (2.3) ratios and high sulfur loadings (8.4 mg cm-2 ). The high-activity Fe SAC is designed by manipulating its local environments using electron-exchangeable binding (EEB) sites. Introducing EEB sites comprising two different types of S species, namely, thiophene-like-S (-S) and oxidized-S (-SO2 ), adjacent to Fe SACs promotes the kinetics of the Li2 S redox reaction by providing additional binding sites and modulating the Fe d-orbital levels via electron exchange with Fe. The -S donates the electrons to the Fe SACs, whereas -SO2 withdraws electrons from the Fe SACs. Thus, the Fe d-orbital energy level can be modulated by the different -SO2 /-S ratios of the EEB site, controlling the electron donating/withdrawing characteristics. This desirable electrocatalysis is maximized by the intimate contact of the Fe SACs with the S species, which are confined together in porous carbon.

Cationic polymer-grafted graphene oxide/CNT cathode-coating material for lithium-sulfur batteries.

A cathode-coating material composed of cationic polymer-grafted graphene oxide (CPGO) and carbon nanotube (CNT) was prepared, where the CPGO was synthesized by grafting quaternized 2-(dimethylamino)ethyl methacrylate (QDMAEMA) onto graphene oxide (GO) via atom transfer radical polymerization (ATRP). GO has good compatibility with carbon black, the main component of the cathode in lithium-sulfur (Li-S) batteries. Here, the cationic polymer having the QDMAEMA unit was intentionally grafted onto GO to decrease the shuttle effect by increasing the chemical adsorption of polysulfide (PS). In addition, when CNT was mixed with CPGO, the compatibility with carbon black was found to be further increased. The lithium-sulfur (Li-S) battery with a sulfur-deposited Super P® carbon black (S/C) cathode coated with a mixture of CPGO and CNT was found to have much improved cell performance compared to those coated without any coating material, with only CPGO, with the mixture of GO and CNT, and with the mixture of PQDMAEMA and CNT. For example, the Li-S battery with the cathode coated using the mixture of CPGO and CNT retained a discharge capacity of 744 mA h g-1 after 50 cycles at 0.2C-rate, while those of the Li-S batteries with bare S/C and CPGO-S/C cathodes were found to be much smaller, i.e., 488 mA h g-1 and 641 mA h g-1, respectively, under the same conditions. Therefore, the mixture of CPGO with CNT as the cathode-coating material showed a synergetic effect to enhance the cell performance of the Li-S battery system.

Spherical Macroporous Carbon Nanotube Particles with Ultrahigh Sulfur Loading for Lithium-Sulfur Battery Cathodes.

A carbon host capable of effective and uniform sulfur loading is the key for lithium-sulfur batteries (LSBs). Despite the application of porous carbon materials of various morphologies, the carbon hosts capable of uniformly impregnating highly active sulfur is still challenging. To address this issue, we demonstrate a hierarchical pore-structured CNT particle host containing spherical macropores of several hundred nanometers. The macropore CNT particles (M-CNTPs) are prepared by drying the aerosol droplets in which CNTs and polymer particles are dispersed. The spherical macropore greatly improves the penetration of sulfur into the carbon host in the melt diffusion of sulfur. In addition, the formation of macropores greatly develops the volume of the micropore between CNT strands. As a result, we uniformly impregnate 70 wt % sulfur without sulfur residue. The S-M-CNTP cathode shows a highly reversible capacity of 1343 mA h g-1 at a current density of 0.2 C even at a high sulfur content of 70 wt %. Upon a 10-fold current density increase, a high capacity retention of 74% is observed. These cathodes have a higher sulfur content than those of conventional CNT hosts but nevertheless exhibit excellent performance. Our CNTPs and pore control technology will advance the commercialization of CNT hosts for LSBs.

Intrinsic Bauschinger effect and recoverable plasticity in pentatwinned silver nanowires tested in tension.

Silver nanowires are promising components of flexible electronics such as interconnects and touch displays. Despite the expected cyclic loading in these applications, characterization of the cyclic mechanical behavior of chemically synthesized high-quality nanowires has not been reported. Here, we combine in situ TEM tensile tests and atomistic simulations to characterize the cyclic stress-strain behavior and plasticity mechanisms of pentatwinned silver nanowires with diameters thinner than 120 nm. The experimental measurements were enabled by a novel system allowing displacement-controlled tensile testing of nanowires, which also affords higher resolution for capturing stress-strain curves. We observe the Bauschinger effect, that is, asymmetric plastic flow, and partial recovery of the plastic deformation upon unloading. TEM observations and atomistic simulations reveal that these processes occur due to the pentatwinned structure and emerge from reversible dislocation activity. While the incipient plastic mechanism through the nucleation of stacking fault decahedrons (SFDs) is fully reversible, plasticity becomes only partially reversible as intersecting SFDs lead to dislocation reactions and entanglements. The observed plastic recovery is expected to have implications to the fatigue life and the application of silver nanowires to flexible electronics.

In situ electron microscopy four-point electromechanical characterization of freestanding metallic and semiconducting nanowires.

Electromechanical coupling is a topic of current interest in nanostructures, such as metallic and semiconducting nanowires, for a variety of electronic and energy applications. As a result, the determination of structure-property relations that dictate the electromechanical coupling requires the development of experimental tools to perform accurate metrology. Here, a novel micro-electro-mechanical system (MEMS) that allows integrated four-point, uniaxial, electromechanical measurements of freestanding nanostructures in-situ electron microscopy, is reported. Coupled mechanical and electrical measurements are carried out for penta-twinned silver nanowires, their resistance is identified as a function of strain, and it is shown that resistance variations are the result of nanowire dimensional changes. Furthermore, in situ SEM piezoresistive measurements on n-type, [111]-oriented silicon nanowires up to unprecedented levels of ∼7% strain are demonstrated. The piezoresistance coefficients are found to be similar to bulk values. For both metallic and semiconducting nanowires, variations of the contact resistance as strain is applied are observed. These variations must be considered in the interpretation of future two-point electromechanical measurements.

Nucleation-controlled distributed plasticity in penta-twinned silver nanowires.

A unique size-dependent strain hardening mechanism, that achieves both high strength and ductility, is demonstrated for penta-twinned Ag nanowires (NWs) through a combined experimental-computational approach. Thin Ag NWs are found to deform via the surface nucleation of stacking fault decahedrons (SFDs) in multiple plastic zones distributed along the NW. Twin boundaries lead to the formation of SFD chains that locally harden the NW and promote subsequent nucleation of SFDs at other locations. Due to surface undulations, chain reactions of SFD arrays are activated at stress concentrations and terminated as local stress decreases, revealing insensitivity to defects imparted by the twin structures. Thick NWs exhibit lower flow stress and number of distributed plastic zones due to the onset of necking accompanied by more complex dislocation structures.

Oil absorbing graphene capsules by capillary molding.

Oil absorbing graphene capsules are synthesized by capillary molding of graphene oxide (GO) sheets against a polystyrene bead template in evaporating aerosol droplets, followed by simultaneous reduction of GO and decomposition of the polymer template during ultrasonic spray pyrolysis.

Plasmon Length: A Universal Parameter to Describe Size Effects in Gold Nanoparticles.

Localized surface plasmon resonances are central to many sensing and signal transmission applications. Tuning of the plasmon energy and line width through particle size and shape is critical to the design of such devices. To gain quantitative information on the size dependence of plasmonic properties, mainly due to retardation effects, we correlated optical spectra and structures for 500 individual gold particles of five different shapes. We show that the effects of size on the dipolar plasmon frequency and line width are shape-independent when size is described by the plasmon length, the length over which the oscillations take place. This result suggests that edge effects are rather unimportant for dipolar modes in a large size range between 50 and 350 nm. Therefore, in describing the size-dependent plasmonic properties of nanoparticles, one should focus on the distance along which the oscillation occurs rather than its intrinsic shape.

Construction of evolutionary tree for morphological engineering of nanoparticles.

In addition to chemical composition, the chemistry of nanocrystals involves an extra structural factor--morphology--since many of their properties are size- and shape-dependent. Although often described as artificial atoms or molecules, the morphological control of nanoparticles has not advanced to a level comparable to organic total synthesis, where complex molecular structures can be rationally designed and prepared through stepwise reactions. Here we report a morphological engineering approach for gold nanoparticles by constructing an evolutionary tree consisting of a few branches of independent growth pathways. Each branch yields a string of evolving, continuously tunable morphologies from one reaction, therefore collectively producing a library of nanoparticles with minimal changes of reaction parameters. In addition, the tree also provides ground rules for designing new morphologies through crossing over different pathways.

Chemical synthesis of gold nanowires in acidic solutions.

High aspect ratio gold nanowires with single crystalline surface have long been a missing piece in the toolbox of plasmonics metal nanostructures. Such wires are now made with a room temperature, surfactant assisted chemical synthesis in acidic aqueous solution. The diameters and lengths of the multiply twinned gold nanowires can be tuned by varying the amount of seed particles and acid in the growth solution. Nanowires with diameters around 35 nm and lengths up to 10 micron were made with a low seed concentration in pH approximately 1 solution.

Synthesis of carbon tubes with mesoporous wall structure using designed silica tubes as templates.

Hollow silica tubes with mesoporous wall structure were synthesized through the sol-gel reactions of tetraethoxysilane and n-octadecyltrimethoxysilane (TEOS/C18-TMS) on the surface of ammonium dl-tartrate crystals. Novel hollow carbon tubes with mesoporous walls and rectangular-shaped channels were fabricated using the silica tubes as templates.

Low-cost and facile synthesis of mesocellular carbon foams.

Mesocellular carbon foam composed of nanometer sized primary particles was synthesized using hydrothermally synthesized MSU-F silica as a template and poly(furfuryl alcohol) as a carbon source.