An innovative approach for efficient synthesis of petal-like molybdenum disulfide nanosheets inside hollow mesoporous carbon spheres (HMCSs), the yolk–shell structured MoS2@C, has been developed. HMCSs effectively control and confine in situ growth of MoS2 nanosheets and significantly improve the conductivity and structural stability of the hybrid material. The yolk–shell structured MoS2@C is proven to achieve high reversible capacity (993 mA h g–1 at 1 A g–1 after 200 cycles), superior rate capability (595 mA h g–1 at a current density of 10 A g–1), and excellent cycle performance (962 mA h g–1 at 1 A g–1 after 1000 cycles and 624 mA h g–1 at 5 A g–1 after 400 cycles) when evaluated as an anode material for lithium-ion batteries. This superior performance is attributed to the yolk–shell structure with conductive mesoporous carbon as the shell and the stack of two-dimensional MoS2 nanosheets as the yolk.Keywords: enhanced electrochemistry performance; hollow mesoporous carbon spheres; in situ confined growth; lithium-ion batteries; yolk−shell structured MoS2@C;

•Fe3O4/C NFs with internal voids were synthesized without using any template.•The binder-free electrode membrane has good flexibility.•The internal voids can buffer the volume expansion of Fe3O4 during cycling.•Fe3O4/C NFs show high capacity and good cycling stability.Freestanding binder-free electrodes, as a new generation of electrode material, can effectively improve the energy density of lithium-ion batteries (LIBs). In this paper, novel structured Fe3O4/C composite nanofibers are successful synthesized by a simple electrospinning method followed by a thermal treatment process. The composite nanofibers have the unique internal voids between Fe3O4 nanoparticles and carbon matrix. The Fe3O4/C nanofibers film with good flexibility and excellent electrical conductivity can be directly used to fabricate half-cell without any current collector, binder and additional conductive agent. As anode material for LIBs, the Fe3O4/C composite nanofibers deliver high reversible capacity (762 mA h g−1 at 0.5 A g−1 after 300 cycles). The results show that the internal voids in flexible Fe3O4/C composite nanofibers effectively buffer volume expansion of Fe3O4 in lithium ion intercalation/deintercalation process and avoid the fracture of the nanofibers, which retain the structural integrity and improve the cycling stability of electrode. Therefore, the design and synthesis strategy of flexible nanofibers film are prospective for applications in next-generation flexible LIBs.Download high-res image (197KB)Download full-size image

The applicability of a novel macrocyclic multi-carbonyl compound, pillar[4]quinone (P4Q), as the cathode active material for lithium-ion batteries (LIBs) was assessed theoretically. The molecular geometry, electronic structure, Li-binding thermodynamic properties, and the redox potential of P4Q were obtained using density functional theory (DFT) at the M06-2X/6-31G(d,p) level of theory. The results of the calculations indicated that P4Q interacts with Li atoms via three binding modes: Li–O ionic bonding, O–Li···O bridge bonding, and Li···phenyl noncovalent interactions. Calculations also indicated that, during the LIB discharging process, P4Q could yield a specific capacity of 446 mAh g−1 through the utilization of its many carbonyl groups. Compared with pillar[5]quinone and pillar[6]quinone, the redox potential of P4Q is enhanced by its high structural stability as well as the effect of the solvent. These results should provide the theoretical foundations for the design, synthesis, and application of novel macrocyclic carbonyl compounds as electrode materials in LIBs in the future.

•The electronic structures of Pillar[5]quinone (P5Q) accepting electrons and binding lithium atoms.•The microstructural evolution of P5Q as cathode active materials for LIBs during charging and discharging processes.•The relationship between structural stability and electrochemical performance of Pillar[n]quinones.Multi-carbonyl macrocyclic compounds have recently attracted much attention due to their high performance relative to some short chain carbonyl compounds as the cathode active constituents for lithium-ion batteries (LIBs). However, little is known about the evolution mechanism of their electrochemical properties during charging and discharging processes. In this paper, the application of density functional calculations at the M06-2X/6-31G(d,p) level of theory is presented to study systematically the electrochemical properties of pillar[5]quinone (P5Q) as a cathode active material for LIBs. The optimized structures of P5Q accepting different number of electrons and binding different number of lithium atoms are obtained, respectively. The geometry structure, thermodynamics property, electronic structural property, solvent effect and redox potential are discussed in detail. The uneven-distribution of extra electrons in several P5Qn− anions can minimize the repulsive interactions as far as possible. The macrocyclic skeletons in P5QLin structures are distorted to different extents by the binding interactions between Li atoms and P5Q. More than eight intercalated lithium atoms into per P5Q molecule are confirmed in this work, indicating a high utilization ratio of carbonyl groups of P5Q as a cathode material. Compared with pillar[4]quinone and pillar[6]quinone, P5Q is predicted to have better cycling performance due to its higher structural stability.Download high-res image (123KB)Download full-size image

•N-doped urchin-like Fe3O4@C are fabricated via a facile hydrothermal process.•Fe3O4@C-N has unique urchin-like structure and N-doped mesoporous carbon shell.•Fe3O4@C-N exhibits low initial capacity loss and high reversible capacity.•Property improvement is attributed to unique nanostructure and N-doped carbon shell.Nitrogen-doped urchin-like Fe3O4@C composites are successfully fabricated via a facile hydrothermal process and subsequent carbonization. Urchin-like hydroxyferric oxide (α-FeOOH) is used as a template and coated by polydopamine (PDA) to form α-FeOOH@PDA. Under high temperature and argon atmosphere, urchin-like Fe3O4 with N-doped carbon shell (Fe3O4@C-N) are finally formed. In the electrochemical test, N-doped carbon shell can form stable solid electrolyte interface (SEI) layer, reduce initial capacity loss and improve the reversibility of Fe3O4 for Li ion storage. Compared with naked Fe3O4 nanospheres, Fe3O4@C nanospheres and urchin-like Fe3O4@C composites, the urchin-like Fe3O4@C-N composites display excellent electrochemical performance. The as-prepared Fe3O4@C-N-20-600 shows a reversible specific capacity of 800 mA h g−1 after 100 cycles at 500 mA g−1. The significant electrochemical property improvements of urchin-like Fe3O4@C-N composites are attributed to the unique urchin-like structure and the N-doped mesoporous carbon shell. Such a simple and scalable route to construct N-doped carbon encapsulated core-shell structure may be further extended to other high capacity anode and cathode materials.

•LTO NSs/CNTs composites are synthesized by a facile and scalable strategy.•The incorporation of CNTs into LTO NSs forms a delicate conductive network.•LTO NSs/CNTs composites display excellent rate and cycling performances.•LTO NSs/CNTs show low polarization and large diffusion coefficient of Li+.Li4Ti5O12 nanosheets (LTO NSs)/carbon nanotubes (CNTs) composites are synthesized using a facile, reproducible, and scalable strategy. In the hydrothermal process, the introduction of CNTs significantly improves the rate performance of LTO NSs. The incorporation of CNTs into the LTO NSs forms a delicate conductive network for rapid electron and lithium ions transport, resulting in excellent rate performance and superior cycling performance. LTO NSs/7.5%-CNTs composites show the highest reversible capacity and high-rate capability (a reversible capability of 157, 145, 132, 118, and 105 mA h g−1 at 1, 2, 3, 4, 5 A g−1, respectively) with good cycling performance (approximate 6.9% capacity loss after 1000 cycles at 2 A g−1 with a capacity retention of 135 mA h g−1), which is apparently larger than pristine LTO NSs. The significantly improved rate capability and cycling performance of the LTO NSs/CNTs composites are mainly attributed to their the lower polarization of potential difference, the larger diffusion coefficient of lithium ion and smaller charge-transfer resistance than pure LTO NSs.

In this paper, ferroferric oxide (Fe3O4) nanoparticles/porous carbon nanofiber (Fe3O4/PCNFs) composites were successfully fabricated by electrospinning and subsequent calcination. The composites were characterized by X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy to analyze the structure, composition and morphology. The electrochemical performance was evaluated by coin-type cells vs. metallic lithium. The results indicated that Fe3O4/PCNFs composites exhibited high reversible capacity and good capacity retention. The discharge capacity was maintained at 717.2 mA h g−1 at 0.5 A g−1 after 100 cycles. The excellent performances of Fe3O4/PCNFs composites are attributed to good crystallinity and uniformly dispersive Fe3O4 nanoparticles, and a porous carbon shell with high conductivity. The carbon coating buffered the tremendous volumetric changes between Fe3O4 nanoparticles and Fe atoms in the charge/discharge processes and kept the structure integrity of Fe3O4 nanoparticles. Porous carbon nanofibers prepared by the unique calcination process improved the conductivity of composites and provided free space for migration of lithium ions. The preparation strategy is expected to be applicable to the preparation of other transition metal oxide materials as superior anode materials for lithium-ion batteries.

The inclusion complexation behavior of the dye guest molecule neutral red with three kinds of water-soluble p-sulfonated calix[n]arene sodium (n=4,6,8) was investigated. p-Sulfonated calix[4,6,8]arene sodium (pSC4, pSC6, pSC8) can react with neutral red to form inclusion complexes, which were confirmed by UV-vis absorption and fluorescence spectroscopy. Fluorescence spectroscopy experiments were performed in pH=4.6 acetic buffer solutions at 25 °C to calculate the stability constants (KS) for the stoichiometric 1:1 inclusion complexes of pSC4, pSC6 and pSC8 with neutral red. The thermodynamic parameters for the inclusion complexes were determined through a van’t Hoff analysis. Formation of the inclusion complexes of pSC4, pSC6 and pSC8 with neutral red was driven by favorable enthalpic changes with their accompanying negative entropy changes. The complex stability constants monotonically increased with the number of phenolic units in the calixarene ring, which was attributed mainly to electrostatic interactions and hydrogen bonding, rather than to the cavity size.

•Fe3O4@C-N yolk−shell nanocapsules were synthesized without sacrificial template.•Yolk−shell structure allows Fe3O4 to expand freely without breaking carbon shell.•The volume expansion of Fe3O4 results in the in-depth nanocrystallization.•In-depth nanocrystallization of Fe3O4 enhances the capability.•Fe3O4@C-N-700 delivers a high capacity and excellent cycling stability.In this paper nitrogen-doped carbon-encapsulation Fe3O4 yolk−shell magnetic nanocapsules (Fe3O4@C-N nanocapsules) have been successfully constructed though a facile hydrothermal method and subsequent annealing process. Fe3O4 nanoparticles are completely enclosed in nitrogen-doped carbon shells with void space between the nanoparticle and the shell. The yolk−shell structure allows Fe3O4 nanoparticles to expand freely without breaking the outer carbon shell during the lithiation/delithiation processes. The volume expansion of Fe3O4 results in the in-depth nanocrystallization. Fortunately, the new generated small nanoparticles can increase the capability with the cycle increase due to the unique confinement effect and excellent electronic conductivity of the nitrogen-doped carbon shells. Hence, after 150 cycles, the discharge capacity of Fe3O4@C-N-700 nanocapsules still remained 832 mA h g−1 at 500 mA g−1, which corresponds to 116.7% of the lowest capacity (713 mA h g−1) at the 16th cycle. We believe that the yolk−shell structure is conducive to enhance the capacity of easy pulverization metal oxidation during the charge/discharge processes.