Yefeng Yao

Tunable and Switchable Dielectric Constant in an Amphidynamic Crystal

The inclusion compound [(CH3)2NH2]2[KCo(CN)6] exhibits a marked temperature-dependent dielectric constant and can be considered as a model of tunable and switchable dielectric materials. Crystal structure and solid-state NMR studies reveal a switchable property between low and high dielectric states around 245 K. This originates from an order-disorder phase transition of the system, changing the dynamics of the polar dimethylammonium (DMA) cation. Furthermore, the tuning of the dielectric constant at temperatures below the phase transition point is related to increasing angular pretransitional fluctuations of the dipole moment of DMA.

Electrospun carbon nanofibers as anode materials for sodium ion batteries with excellent cycle performance

A simple and scalable electrospinning process followed by thermal treatment was used to fabricate carbon nanofibers (CFs). The as-prepared CFs were investigated as anode materials for sodium ion batteries (SIBs). Remarkably, due to their weakly ordered turbostratic structure and a large interlayer spacing between graphene sheets, the CFs exhibit a dominant adsorption/insertion sodium storage mechanism that shows high reversibility. As a result, the CFs show excellent electrochemical performance, especially cycle stability (97.7% capacity retention ratio over 200 cycles). Reversible capacities of 233 and 82 mA h g−1 are obtained for the CFs at a current density of 0.05 A g−1 and even a high current density of 2 A g−1, respectively. The excellent cycle performance, high capacity and good rate capability make the CFs promising candidates for practical SIBs.

Ring-Expansion Metathesis Polymerization: Catalyst-Dependent Polymerization Profiles

Ring-expansion metathesis polymerization (REMP) mediated by recently developed cyclic Ru catalysts has been studied in detail with a focus on the polymer products obtained under varied reaction conditions and catalyst architectures. Depending upon the nature of the catalyst structure, two distinct molecular weight evolutions were observed. Polymerization conducted with catalysts bearing six-carbon tethers displayed rapid polymer molecular weight growth which reached a maximum value at ca. 70% monomer conversion, resembling a chain-growth polymerization mechanism. In contrast, five-carbon-tethered catalysts led to molecular weight growth that resembled a step-growth mechanism with a steep increase occurring only after 95% monomer conversion. The underlying reason for these mechanistic differences appeared to be ready release of five-carbon-tethered catalysts from growing polymer rings, which competed significantly with propagation. Owing to reversible chain transfer and the lack of end groups in REMP, the final molecular weights of cyclic polymers was controlled by thermodynamic equilibria. Large ring sizes in the range of 60-120 kDa were observed at equilibrium for polycyclooctene and polycyclododecatriene, which were found to be independent of catalyst structure and initial monomer/catalyst ratio. While six-carbon-tethered catalysts were slowly incorporated into the formed cyclic polymer, the incorporation of five-carbon-tethered catalysts was minimal, as revealed by ICP-MS. Further polymer analysis was conducted using melt-state magic-angle spinning (13)C NMR spectroscopy of both linear and cyclic polymers, which revealed little or no chain ends for the latter topology.

A Chemically Triggered and Thermally Switched Dielectric Constant Transition in a Metal Cyanide Based Crystal

A dielectric constant transition is chemically triggered and thermally switched in (HPy)2[Na(H2O)Co(CN)6] (2, HPy=pyridinium cation) by single-crystal-to-single-crystal transformation and structural phase transition, respectively. Upon dehydration, (HPy)2[Na(H2O)2Co(CN)6] (1) transforms to its semi-hydrated form 2, accompanying a transition from a low-dielectric state to a high-dielectric state, and vice versa. This dielectric switch is also realized by a structural phase transition in 2 that occurs between room- and low-temperature phases, and which corresponds to high- and low-dielectric states, respectively. The switching property is due to the variation in the environment surrounding the HPy cation, that is, the hydrogen-bonding interactions and the crystal packing, which exert predominant influences on the dynamics of the cations that transit between the static and motional states.

Transferring Lithium Ions in Nanochannels: A PEO/Li+ Solid Polymer Electrolyte Design

A new category of crystalline polymer electrolyte prepared by the supramolecular self-assembly of polyethylene oxide (PEO), α-cyclodextrin (α-CD), and LiAsF6 is reported. The polymer electrolyte consists of the nanochannels formed by α-CDs in which the PEO/Li(+) complexes are confined. The nanochannels formed by α-CD provide the pathway for the directional motion of Li(+) ions and at the same time prevent the access of the anions by size exclusion, resulting in good separation of the Li(+) ions and the anions. The conductivity of the reported material is 30 times higher than that of the comparable PEO/Li(+) complex crystal at room temperature. By using state-of-art solid-state NMR spectroscopy, the structure and dynamics of the material were investigated in detail. The dynamics of the Li(+) ions was studied and correlated to the ionic conductivity of the material.

Significantly Improved Sodium-Ion Storage Performance of CuS Nanosheets Anchored into Reduced Graphene Oxide with Ether-Based Electrolyte

Currently sodium-ion batteries (SIBs) as energy storage technology have attracted lots of interest due to their safe, cost-effective, and nonpoisonous advantages. However, many challenges remain for development of SIBs with high specific capacity, high rate capability, and long cycle life. Therefore, CuS as an important earth-abundant, low-cost semiconductor was applied as anode of SIBs with ether-based electrolyte instead of conventional ester-based electrolyte. By incorporating reduced graphene oxide (RGO) into CuS nanosheets and optimizing the cutoff voltage, it is found that the sodium-ion storage performance can be greatly enhanced using ether-based electrolyte. The CuS-RGO composites deliver an initial Coulombic efficiency of 94% and a maximum specific capacity of 392.9 mAh g-1 after 50 cycles at a current density of 100 mA g-1. And a specific capacity of 345 mAh g-1 is kept after 450 cycles at a current density of 1 A g-1. Such an excellent electrochemical performance is ascribed to the conductive network construction of CuS-RGO composites, the suppression of dissolved polysulfide intermediates by using ether-based electrolyte, and the avoidance of conversion-type reaction by optimizing the cutoff voltage.

Controlling the Particle Size of Interpolymer Complexes through Host−Guest Interaction for Drug Delivery

A new method to adjust the particle size of interpolymer complexes has been developed by introduction of host-guest interaction into the dilute aqueous solution of poly(acrylic acid) (PAA) and poly(ethylene glycol) (PEG). Because of the cooperative hydrogen-bonding interaction, PAA can form the interpolymer complexes with PEG. Putting beta-cyclodextrin (beta-CD) into dilute PAA/PEG aqueous solution, the competition between host-guest and hydrogen-bonding interactions happens. The beta-CD/PAA/PEG ternary systems have been well characterized by ultraviolet-visible absorption spectroscopy (UV-vis), dynamic light scattering (DLS), transmission electron microscopy (TEM), diffusion NMR spectroscopy, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), and solid-state (13)C NMR spectroscopy. The results indicate that the hydrophobic cavity of beta-CD is threaded by linear polymers so that the hydrophilicity of PAA/PEG interpolymer complexes is improved greatly. Adjusting the amounts of beta-CD, the particle size of the interpolymer complexes can be readily controlled. The low cytotoxicity of various beta-CD/PAA/PEG ternary complexes has been confirmed using the MTT assay in COS-7 cell line. Doxorubicin (DOX), an anticancer drug, has been encapsulated into the beta-CD/PAA/PEG ternary complexes. The DOX-loaded beta-CD/PAA/PEG ternary complexes have been analyzed by confocal laser scanning microscopy (CLSM), flow cytometry analysis, and the MTT assay against human cervical carcinoma cell (Hela). The results indicate that beta-CD/PAA/PEG ternary complexes with controlled particle size could be used as safe and promising drug carriers.

Switching Dielectric Constant Near Room Temperature in a Molecular Crystal

The organic salt bis(2‐chloroethyl)amine hydrochloride shows a sharp switching of its dielectric constant at 320 K. The switching property originates from the dynamic changes of the (2‐chloroethyl)ammonium cation between frozen and motional states, corresponding to a structural phase transition.

Influence of Crystal Thickness and Topological Constraints on Chain Diffusion in Linear Polyethylene

(13) C solid-state exchange NMR is applied to study the influence of morphology on chain diffusion between crystalline and noncrystalline regions in ultrahigh molecular weight linear polyethylene (PE). Lamellar-doubling reduces the exchange rate by a factor of two indicating that the chain diffusion coefficient is largely independent of the lamellar thickness. This is discussed in terms of molecular processes in the crystallites leading to chain diffusion, confirming that the role of defects is minor compared to helical jumps of extended stems. Hindrance of the chain diffusion resulting from chain entanglements was only observed after the chains diffuse over long distances. Moreover, the role of the interphase between the noncrystalline and the crystalline regions on chain diffusion is discussed.

ZnS nanoparticles decorated on nitrogen-doped porous carbon polyhedra: a promising anode material for lithium-ion and sodium-ion batteries

Rational fabrication and structure design of anode materials with high specific capacity and excellent cycling stability are of significant importance for the development of high-performance lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). In this paper, a zeolitic imidazolate framework-8 (ZIF-8) with a unique polyhedral morphology and large size (about 2 μm) was successfully synthesized through a facile co-precipitation method. After successive carbonization and sulfidation, ZnS nanoparticles decorated on nitrogen-doped porous carbon polyhedra (ZnS/NPC) were obtained. When applied as the anode material for LIBs, the ZnS/NPC hybrid displays the highest reversible specific capacity for ZnS-based electrodes reported so far (1067.4 mA h g−1 at 0.1 A g−1 after 200 cycles), excellent rate capability (364.6 mA h g−1 at 4 A g−1), and robust long-term cycling performance (856.8 mA h g−1 at 1 A g−1 after 1000 cycles). As for SIBs, the resultant ZnS/NPC also exhibits a desirable capacity of 370.6 mA h g−1 after 100 cycles at 0.1 A g−1 and 289.2 mA h g−1 after 1000 cycles at 1 A g−1. Such superior lithium and sodium storage performances should be attributed to the distinctive structure advantages inherited from ZIF-8, where the Zn ions were in situ converted to ZnS with high reactivity upon electrochemical cycling and the organic linkers were pyrolyzed to nitrogen-doped porous carbon polyhedra to enhance the conductivity of the hybrid and keep the structure stability during cycling.