Juchen Guo

Sulfur-Impregnated Disordered Carbon Nanotubes Cathode for Lithium–Sulfur Batteries

The commercialization of lithium-sulfur batteries is hindered by low cycle stability and low efficiency, which are induced by sulfur active material loss and polysulfide shuttle reaction through dissolution into electrolyte. In this study, sulfur-impregnated disordered carbon nanotubes are synthesized as cathode material for the lithium-sulfur battery. The obtained sulfur-carbon tube cathodes demonstrate superior cyclability and Coulombic efficiency. More importantly, the electrochemical characterization indicates a new stabilization mechanism of sulfur in carbon induced by heat treatment.

Lithium–Sulfur Battery Cathode Enabled by Lithium–Nitrile Interaction

Lithium sulfide is a promising cathode material for high-energy lithium ion batteries because, unlike elemental sulfur, it obviates the need for metallic lithium anodes. Like elemental sulfur, however, a successful lithium sulfide cathode requires an inherent mechanism for preventing lithium polysulfide dissolution and shuttling during electrochemical cycling. A new scheme is proposed to create composites based on lithium sulfide uniformly dispersed in a carbon host, which serve to sequester polysulfides. The synthesis methodology makes use of interactions between lithium ions in solution and nitrile groups uniformly distributed along the chain backbone of a polymer precursor (e.g., polyacrylonitrile), to control the distribution of lithium sulfide in the host material. The Li(2)S-carbon composites obtained by carbonizing the precursor are evaluated as cathode materials in a half-cell lithium battery, and are shown to yield high galvanic charge/discharge capacities and excellent Coulombic efficiency, demonstrating the effectiveness of the architecture in homogeneously distributing Li(2)S and in sequestering lithium polysulfides.

Virus-Enabled Silicon Anode for Lithium- Ion Batteries

A novel three-dimensional Tobacco mosaic virus assembled silicon anode is reported. This electrode combines genetically modified virus templates for the production of high aspect ratio nanofeatured surfaces with electroless deposition to produce an integrated nickel current collector followed by physical vapor deposition of a silicon layer to form a high capacity silicon anode. This composite silicon anode produced high capacities (3300 mAh/g), excellent charge-discharge cycling stability (0.20% loss per cycle at 1C), and consistent rate capabilities (46.4% at 4C) between 0 and 1.5 V. The biological templated nanocomposite electrode architecture displays a nearly 10-fold increase in capacity over currently available graphite anodes with remarkable cycling stability.

Sponge-like porous carbon/tin composite anode materials for lithium ion batteries

A novel sponge-like porous C/Sn composite is synthesized by dispersing SnO2 nanoparticles into a soft-template polymer matrix followed by carbonization. The mesoporous C/Sn anodes can deliver a capacity as high as 1300 mAh g−1 after 450 charge/discharge cycles, and provide a capacity of 180 mAh g−1 even at 4000 mA g−1 charge/discharge current density. An extra reversible capacity over the theoretical value of the porous C/Sn anode is observed, which is attributed to the reversible formation/decomposition of the gel-like polymers formed on the mesoporous C/Sn composite due to the catalytic effect of the Sn nanoparticles. The high capacity, long cycle life, high power, ∼100% Coulombic efficiency, and inexpensive production method make the sponge-like porous C/Sn composite an attractive anode material in Li-ion batteries for electric vehicles and renewable energy storage.

Carbon scaffold structured silicon anodes for lithium-ion batteries

A unique methodology of fabricating Si anodes for lithium-ion batteries with porous carbon scaffold structure is reported. Such carbon scaffold Si anodes are fabricated via carbonization of porous Si-PVdF precursors which are directly deposited on the current collector. Unlike the conventional slurry casting method, binder and conductive additives are not used in the preparation of the carbon scaffold Si anodes. The carbon scaffold Si anode has a close-knit porous carbon structure that can not only accommodate the Si volume change, but also facilitate the charge transfer reaction. These advantages are demonstrated by the superior capacity, cycle stability and rate performance of the carbon scaffold Si anodes.

In situ synthesis of lithium sulfide–carbon composites as cathode materials for rechargeable lithium batteries

Lithium–sulfur batteries are among the most promising candidates for next-generation rechargeable lithium batteries in view of recent progress on sulfur–carbon composite cathodes. However, further progress on such batteries is hampered by their concomitant need for a metallic lithium anode, which introduces new challenges associated with uneven electrodeposition and lithium dendrite formation. Here we report a method of creating lithium sulfide–carbon composites as cathode materials, which can be paired with high-capacity anodes other than metallic lithium. Lithium sulfide is dispersed in a porous carbon matrix, which serves to improve its electrical conductivity and provides a framework for sequestration of sulfur and lithium polysulfides. The in situ synthesis approach allows facile, scalable synthesis of lithium sulfide–carbon composite materials that exhibit improved electrochemical properties. We also investigate the effect of lithium polysulfides dissolved in the electrolyte on the stability and cycling behavior of Li2S–carbon composite cathodes.

Aerosol Assisted Synthesis of Hierarchical Tin-carbon Composites and their Application as Lithium Battery Anode Materials

We report a method for synthesizing hierarchically structured tin–carbon (Sn–C) composites via aerosol spray pyrolysis. In this method, an aqueous precursor solution containing tin(II) chloride and sucrose is atomized, and the resultant aerosol droplets carried by an inert gas are pyrolyzed in a high-temperature tubular furnace. Owing to the unique combination of high reaction temperature and short reaction time, this method is able to achieve a hetero-structure in which small Sn particles (15 nm) are uniformly embedded in a secondary carbon particle. This procedure allows the size and size distribution of the primary Sn particles to be tuned, as well as control over the size of the secondary carbon particles by addition of polymeric surfactant in the precursor solution. When evaluated as anode materials for lithium-ion batteries, the resultant Sn–C composites demonstrate attractive electrochemical performance in terms of overall capacity, electrochemical stability, and coulombic efficiency.

Synergistic gelation of silica nanoparticles and a sorbitol-based molecular gelator to yield highly-conductive free-standing gel electrolytes.

Lithium-ion batteries have emerged as the preferred type of rechargeable batteries, but there is a need to improve the performance of the electrolytes therein. Specifically, the challenge is to obtain electrolytes with the mechanical rigidity of solids but with liquid-like conductivities. In this study, we report a class of nanostructured gels that are able to offer this unique combination of properties. The gels are prepared by utilizing the synergistic interactions between a molecular gelator, 1,3:2,4-di-O-methyl-benzylidene-d-sorbitol (MDBS), and a nanoscale particulate material, fumed silica (FS). When MDBS and FS are combined in a liquid consisting of propylene carbonate with dissolved lithium perchlorate salt, the liquid electrolyte is converted into a free-standing gel due to the formation of a strong MDBS-FS network. The gels exhibit elastic shear moduli around 1000 kPa and yield stresses around 11 kPa-both values considerably exceed those obtainable by MDBS or FS alone in the same liquid. At the same time, the gel also exhibits electrochemical properties comparable to the parent liquid, including a high ionic conductivity (~5 × 10(-3) S/cm at room temperature) and a wide electrochemical stability window (up to 4.5 V).

Cation reduction and comproportionation as novel strategies to produce high voltage, halide free, carborane based electrolytes for rechargeable Mg batteries

Here we describe the cation reduction and comproportionation as novel routes to synthesize electrolytes for rechargeable Mg-ion batteries. Reduction of the ammonium cation in [HNMe31+][HCB11H111−] with metallic Mg affords the halide free carborane salt [Mg2+][HCB11H111−]2. Comproportionation of [Mg2+][HCB11H111−]2 with MgPh2 affords the novel monocationic electrolyte [MgPh1+][HCB11H111−], which reversibly deposits/strips Mg with a remarkable oxidative stability of 4.6 V vs. Mg0/+2.

An Acrylate‐Polymer‐Based Electrolyte Membrane for Alkaline Fuel Cell Applications

Alkaline fuel cells (AFCs) recently attracted renewed attention because of their potential to surpass proton exchange membrane fuel cells (PEMFCs). The long-existing issues of PEMFCs, including expensive noble-metal catalysts and polymer electrolytes, as well as CO poisoning and inferior temperature endurance, prevented them from being used in a broad range of applications. Contrarily, advantages of AFCs include fast kinetics in the reduction of the oxidizing agent and the possibility to use base-metal catalysts. However, a critical challenge for conventional AFCs is the use of aqueous alkaline electrolytes, which can react with CO2 from air to form carbonate salts (e.g. , K2CO3). As a result, the performance of the fuel cell would quickly deteriorate. To solve this problem, recent investigations focused on intrinsically OH -conducting alkaline polymer electrolyte (APE) materials to replace the alkaline electrolytes. By using APEs, the formation of carbonate salts can be prevented, which is attributable to the absence of metal ions. However, carbonate ions might still be formed through a reaction with CO2, which would result in a reduced OH conductivity. Application of APEs can also enable a compact design and eliminate corrosion from alkaline solutions. These advantages confirm that APE fuel cells (APEFCs) present a very promising energy conversion technology. Because APEs are a key component determining the ultimate performance, they should exhibit a high OH conductivity and superior mechanical properties, and in addition be of low cost. To date, the most common synthesis route for APEs is chloromethylation of polymers having a phenyl structured backbone, which is followed by quaternization. Many polymers have been used as precursors to synthesize APEs, including polysulfone, poly(arylene ether sulfone), polyetherketone, poly(ether imide), polyethersulfone cardo, poly(phthalazinon ether sulfone ketone), poly(dimethyl phenylene oxide), and poly(phenylene). Also, a recent study by Lin and co-workers reported high conductivity and mechanical strength for an alkaline polymer electrolyte based on a crosslinked ionic liquid. The phenyl backbones of the polymers have in common that they are all excellent engineering polymers exhibiting good mechanical properties because of rigid ring structures. However, this advantage can be seriously weakened by the chloromethylation–quaternization process, which converts the polymer from an ionic insulator into an ionomer and thus, from hydrophobic to hydrophilic. As a result of the hydrophilicity, the mechanical properties of the APEs in the humid working environment of a fuel cell can be very different from that of the precursors. Because their backbones consist of aromatic groups, these precursor polymers can be modified to exhibit extreme hydrophilicity through the chloromethylation–quaternization process. The resulting APE may have a very high anionic conductivity, but with very poor mechanical properties in humid environment. Therefore, an obvious shortcoming of the chloromethylation–quaternization process is the difficulty to control the degree of chloromethylation and quaternization precisely, thus making it difficult to balance conductivity and mechanical properties. Cost is also a concern, since the aforementioned APE precursors are highcost polymers because of the sophisticated synthesis process. In a previous study, we reported a novel APE made from poly (methyl methacrylate-co-butyl acrylate-co-vinylbenzyl chloride) (PMBV). This copolymer was synthesized using solution-free radical polymerization. Xu and co-workers also reported an independent study on APE made from a copolymer using similar polymerization methods. Although this copolymer exhibits a promising performance, our previous study encountered two problems: The three monomers, methyl methacrylate (MMA), butyl acrylate (BA), and 4-vinylbenzyl chloride (VBC), have different reactivity ratios so that they polymerize at different reaction rates. Because of the slow diffusion of propagating copolymer chains and the diluted monomer concentration in the polymerization solution, the monomers with lower reactivity ratios have a smaller possibility for complete conversion. Therefore, the copolymer composition did not match the designed monomer ratio. The second concern is that the molecular weight of the copolymer in our previous study was not as high as expected, which could considerably weaken the mechanical strength. To address these problems, we demonstrate a novel bottom-up synthesis of PMBV by using mini-emulsion polymerization for the first time. Unlike chloromethylation of existing polymers, we synthesized PMBV by using various monomers selected to meet the specifications for conductivity and mechanical strength. Specifically, VBC (15 mol%) contained the chloromethyl functional group, which could be quaternized and then successively ion-exchanged to obtain OH conductivity. Polymerized MMA exhibits a high rigidity and toughness. As a result, the MMA monomer (80 mol.%) was chosen to provide mechanical strength. The brittleness inherent to MMA and VBC was overcome by adding a small portion of BA (5 mol%), which conferred flexibility to the resulting APE. [a] Y. Luo, Dr. J. Guo, Prof. Dr. C. Wang Chemical and Biomolecular Engineering 2113 Chemical and Nuclear Engineering University of Maryland, College Park, MD 20742 (USA) Fax: (+1)301-314-9126 E-mail : cswang@umd.edu [b] Dr. D. Chu Sensors and Electron Device Directorate US Army Research Laboratory Adelphi, MD 20783 (USA) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100287.