Dahong Chen

Holey two-dimensional transition metal oxide nanosheets for efficient energy storage

Transition metal oxide nanomaterials are promising electrodes for alkali-ion batteries owing to their distinct reaction mechanism, abundant active sites and shortened ion diffusion distance. However, detailed conversion reaction processes in terms of the oxidation state evolution and chemical/mechanical stability of the electrodes are still poorly understood. Herein we explore a general synthetic strategy for versatile synthesis of various holey transition metal oxide nanosheets with adjustable hole sizes that enable greatly enhanced alkali-ion storage properties. We employ in-situ transmission electron microscopy and operando X-ray absorption structures to study the mechanical properties, morphology evolution and oxidation state changes during electrochemical processes. We find that these holey oxide nanosheets exhibit strong mechanical stability inherited from graphene oxide, displaying minimal structural changes during lithiation/delithiation processes. These holey oxide nanosheets represent a promising material platform for in-situ probing the electrochemical processes, and could open up opportunities in many energy storage and conversion systems.

Layered nickel metal–organic framework for high performance alkaline battery-supercapacitor hybrid devices

Alkaline battery-supercapacitor hybrid devices (ABSHDs) are attracting considerable attention because they combine the advantages of both alkaline batteries and supercapacitors. Herein, a nickel metal–organic framework (Ni-MOF) was demonstrated to have a high specific capacity and could further improve energy storage through a novel strategy: “synergistic effect between Ni-MOF and Fe(CN)64−/Fe(CN)63−”. The unique 2D-layered crystal structure of the Ni-MOF could provide enough space for Fe(CN)64−/Fe(CN)63− storage and diffusion and Fe(CN)64−/Fe(CN)63− could act as an electron relay during charge–discharge processes by coupling Ni(II)/Ni(III) in the Ni-MOF electrode. Moreover, we assembled an Ni-MOF//CNTs-COOH ABSHD in 3 M KOH and 0.1 M K4Fe(CN)6 mixed electrolyte with an extended voltage window of 1.4 V, which resulted in a high energy density (55.8 W h kg−1) and power density (7000 W kg−1) simultaneously. Hence, the results in this study could broaden the applications of MOFs in energy storage devices and provide insightful guidelines for developing other redox additives.

Mixed-metallic MOF based electrode materials for high performance hybrid supercapacitors

Metal–organic frameworks (MOFs) have obtained increasing attention as a kind of novel electrode material for energy storage devices. Yet low capacity in most MOFs largely thwarts their application. In this study, an effective strategy was developed to improve the conductivity of MOFs by partially substituting Ni2+ in the Ni-MOF with Co2+ or Zn2+. The mixed-metal organic frameworks (M-MOFs) showed excellent electrochemical performance, which is attributed not only to the favorable paths for charge transport due to the presence of free pores, but also to the raised electrochemical double-layer capacitance (EDLC) at the enlarged specific surface area of the material. Meanwhile, the cycling stability of the assembled hybrid supercapacitors (M-MOFs//CNTs–COOH) is enhanced due to the alleviation of phase transformation during electrochemical cycling tests. More interestingly, the Co/Ni-MOF//CNTs–COOH also exhibited an excellent energy density (49.5 W h kg−1) and power density (1450 W kg−1) simultaneously. These values demonstrated the better performance of all the MOF materials in supercapacitors at present. In addition to broadening the application of MOFs, our study may open a new avenue for bridging the performance gap between batteries and supercapacitors.

Two-Dimensional Holey Co3O4 Nanosheets for High-Rate Alkali-Ion Batteries: From Rational Synthesis to in Situ Probing

A general template-directed strategy is developed for the controlled synthesis of two-dimensional (2D) assembly of Co3O4 nanoparticles (ACN) with unique holey architecture and tunable hole sizes that enable greatly improved alkali-ion storage properties (demonstrated for both Li and Na ion storage). The as-synthesized holey ACN with 10 nm holes exhibit excellent reversible capacities of 1324 mAh/g at 0.4 A/g and 566 mAh/g at 0.1 A/g for Li and Na ion storage, respectively. The improved alkali-ion storage properties are attributed to the unique interconnected holey framework that enables efficient charge/mass transport as well as accommodates volume expansion. In situ TEM characterization is employed to depict the structural evolution and further understand the structural stability of 2D holey ACN during the sodiation process. The results show that 2D holey ACN maintained the holey morphology at different sodiation stages because Co3O4 are converted to extremely small interconnected Co nanoparticles and these Co nanoparticles could be well dispersed in a Na2O matrix. These extremely small Co nanoparticles are interconnected to provide good electron pathway. In addition, 2D holey Co3O4 exhibits small volume expansion (∼6%) compared to the conventional Co3O4 particles. The 2D holey nanoarchitecture represents a promising structural platform to address the restacking and accommodate the volume expansion of 2D nanosheets for superior alkali-ion storage.

Hierarchical Bi2Se3 microrods: microwave-assisted synthesis, growth mechanism and their related properties

Bi2Se3 microrods composed of nanoparticles have been successfully fabricated through a self-sacrificial template microwave-assisted method in the presence of a ethylene glycol (EG) solution, applying ascorbic acid (AA) as a reducing agent and soluble starch (SS) as a surfactant. The structure and morphology of the obtained products were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) techniques. Based on the time-dependent experiments, a possible formation mechanism was proposed. The electrical transport properties were investigated by measuring the electrical conductivity and the Seebeck coefficient at a temperature range of 298 to 523 K and the maximum power factor can be as high as 92 μW m−1 K−2 at 523 K. The intercalation behavior of Li ions into the obtained Bi2Se3 microrods was also investigated. The discharge capacity of the sample at room temperature can reach up to 870 mA h g−1, indicating potential applications in electrochemical lithium intercalation and high-energy batteries.

Facile synthesis of Li4Ti5O12 nanosheets stacked by ultrathin nanoflakes for high performance lithium ion batteries

Li4Ti5O12 nanosheets stacked by ultrathin nanoflakes derived from the interlayer splitting and exfoliation of the layered orthorhombic Li1.81H0.19Ti2O5·xH2O precursors are obtained by a facile method. The precursors are synthesized through a one-step, low-temperature hydrothermal method with a mixed solvent of ethanol and water. The surfactants and templates are free during the fabrication process. The ultrathin nanoflakes are interconnected and their thicknesses are only ∼3 nm. Possible morphology formation and crystal structure transition mechanisms are proposed through time-dependent experiments. As an anode material for rechargeable lithium-ion batteries, the Li4Ti5O12 nanosheets with a stacked structure delivered an initial discharge capacity of 175.9 mA h g−1, together with a discharge capacity of 166.8 mA h g−1 after 100 cycles at 0.5 C. The discharge capacity could reach up to 100.2 mA h g−1 even at 20 C. We infer that except for the self advantages of nanosheets as nanomaterials, the delicate structure consisted of stacks of interconnected ultrathin nanoflakes and can promote the kinetic property of lithium ions and electrons diffusion through offering more transporting channels, which is favorable for high-rate performance.

Self-decorated Cu2−xSe nanosheets as anode materials for Li ion batteries and electrochemical hydrogen storage

Hierarchical self-decorated Cu2−xSe nanosheets were synthesized through a facile solvothermal route in a binary solution of ethylene glycol and distilled water in the absence of a template. The X-ray diffraction (XRD), scanning electron microscope (SEM), and high-resolution transmission electron microscopy (HRTEM) analyses identified that the as-prepared Cu2−xSe nanosheets were single-crystalline decorated by Cu2−xSe nanodots. Based on the time-dependence experiment, the reaction and growth process was discussed in detail. Furthermore, the electrochemistry Li and hydrogen storage properties of the hierarchical self-decorated Cu2−xSe nanosheets were measured. This hierarchical self-decorated Cu2−xSe nanosheet showed a good cycle and rate performance, indicating its potential application as an anode material for lithium ion batteries. The good cycle and rate performance may be attributed to the unique hierarchical morphology which can buffer the volume change to some degree during the discharging/charging process.

Electrical transport properties of microwave-synthesized Bi2Se3−xTex nanosheet

Tellurium (Te) doped bismuth selenide (Bi2Se3−xTex) nanosheets have been successfully synthesized by the microwave-assisted method in the presence of ethylene glycol (EG). The obtained products were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected-area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy techniques. The electrical transport properties of the nanosheets are investigated by measuring the electrical conductivity and the Seebeck coefficient at temperatures ranging from 298 to 523 K. The power factor values of the Bi2Se3−xTex nanosheet vary with different doping concentrations of Te, and the maximum power factor can reach 178 μW m−1 K−2 at 523 K for Bi2Se2.7Te0.3, indicating the potential application in thermoelectric devices.

Microwave-assisted synthesis of Bi2Se3 ultrathin nanosheets and its electrical conductivities

Ultrathin Bi2Se3 nanosheets have been successfully fabricated through a microwave-assisted approach in the presence of ethylene glycol (EG) under 1 kW microwave power for 1 minute. The structure and morphology of the obtained products were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected-area electron diffraction (SAED) and Raman spectroscopy techniques. Based on the control experiments, a possible growth mechanism of Bi2Se3 ultrathin nanosheets was proposed. Furthermore, the thermoelectric transport properties of the nanosheets were investigated by measuring the electrical conductivity and the Seebeck coefficient at temperatures ranging from 298 to 523 K. The maximum power factor can reach up to 157 μW m−1 K−2 at 523 K due to the ultrathin nature of the as-prepared sample, indicating that this promising approach can be extended to the synthesis of other thermoelectrical materials.

Hierarchical Zn3V3O8/C composite microspheres assembled from unique porous hollow nanoplates with superior lithium storage capability

Fabricating hollow/porous structures and coating carbon on their surfaces are two effective strategies to improve the electrochemical properties of electrode materials for lithium-ion batteries (LIBs). In this work, we report the successful synthesis of novel hierarchical Zn3V3O8/C microspheres assembled from unique porous hollow nanoplates via a facile reverse microemulsion method and a subsequent annealing process. Notably, the PVP assisted heterogeneous contraction effect plays an important role in the formation of the distinctive hollow structure in the constituent nanoplates during the annealing process. Impressively, the resulting Zn3V3O8/C composite shows excellent electrochemical performance with a large capacity (912 mA h g−1 at 0.4 A g−1), excellent rate behavior (410 mA h g−1 at 3.2 A g−1) and cycling stability (756 mA h g−1 at 1.6 A g−1 after 500 cycles) when evaluated as the anode material for LIBs. The superior electrochemical performance of the as-synthesized Zn3V3O8/C microspheres can be attributed to the existence of carbon on the surface and, in particular, the unique porous hollow nanoplate structure.