Guangjie Shao

Construction of Hierarchical α-MnO2 Nanowires@Ultrathin δ-MnO2 Nanosheets Core–Shell Nanostructure with Excellent Cycling Stability for High-Power Asymmetric Supercapacitor Electrodes

Poor electrical conductivity and mechanical instability are two major obstacles to realizing high performance of MnO2 as pseudocapacitor material. The construction of unique hierarchical core-shell nanostructures, therefore, plays an important role in the efficient enhancement of the rate capacity and the stability of this material. We herein report the fabrication of a hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure by adopting a facile and practical solution-phase technique. The novel hierarchical nanostructures are composed of ultrathin δ-MnO2 nanosheets with a few atomic layers growing well on the surface of the ultralong α-MnO2 nanowires. The first specific capacitance of hierarchical core-shell nanostructure reached 153.8 F g(-1) at the discharge current density of as high as 20 A g(-1), and the cycling stability is retained at 98.1% after 10,000 charge-discharge cycles, higher than those in the literature. The excellent rate capacity and stability of the hierarchical core-shell nanostructures can be attributed to the structural features of the two MnO2 crystals, in which a 1D α-MnO2 nanowire core provides a stable structural backbone and the ultrathin 2D δ-MnO2 nanosheet shell creates more reactive active sites. The synergistic effects of different dimensions also contribute to the superior rate capability.

Tunable morphology synthesis of LiFePO4 nanoparticles as cathode materials for lithium ion batteries.

Olivine LiFePO4 with nanoplate, rectangular prism nanorod and hexagonal prism nanorod morphologies with a short b-axis were successfully synthesized by a solvothermal in glycerol and water system. The influences of solvent composition on the morphological transformation and electrochemical performances of olivine LiFePO4 are systematically investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and galvanostatic charge-discharge tests. It is found that with increasing water content in solvent, the LiFePO4 nanoplates gradually transform into hexagonal prism nanorods that are similar to the thermodynamic equilibrium shape of the LiFePO4 crystal. This indicates that water plays an important role in the morphology transformation of the olivine LiFePO4. The electrochemical performances vary significantly with the particle morphology. The LiFePO4 rectangular prism nanorods (formed in a glycerol-to-water ratio of 1:1) exhibit superior electrochemical properties compared with the other morphological particles because of their moderate size and shorter Li(+) ion diffusion length along the [010] direction. The initial discharge capacity of the LiFePO4@C with a rectangular prism nanorod morphology reaches to 163.8 mAh g(-1) at 0.2 C and over 75 mAh g(-1) at the high discharging rate of 20 C, maintaining good stability at each discharging rate.

In situ catalytic synthesis of high-graphitized carbon-coated LiFePO4 nanoplates for superior Li-ion battery cathodes.

The low electronic conductivity and one-dimensional diffusion channel along the b axis for Li ions are two major obstacles to achieving high power density of LiFePO4 material. Coating carbon with excellent conductivity on the tailored LiFePO4 nanoparticles therefore plays an important role for efficient charge and mass transport within this material. We report here the in situ catalytic synthesis of high-graphitized carbon-coated LiFePO4 nanoplates with highly oriented (010) facets by introducing ferrocene as a catalyst during thermal treatment. The as-obtained material exhibits superior performances for Li-ion batteries at high rate (100 C) and low temperature (-20 °C), mainly because of fast electron transport through the graphitic carbon layer and efficient Li(+)-ion diffusion through the thin nanoplates.

Enhancement of the Rate Capability of LiFePO4 by a New Highly Graphitic Carbon-Coating Method

Low lithium ion diffusivity and poor electronic conductivity are two major drawbacks for the wide application of LiFePO4 in high-power lithium ion batteries. In this work, we report a facile and efficient carbon-coating method to prepare LiFePO4/graphitic carbon composites by in situ carbonization of perylene-3,4,9,10-tetracarboxylic dianhydride during calcination. Perylene-3,4,9,10-tetracarboxylic dianhydride containing naphthalene rings can be easily converted to highly graphitic carbon during thermal treatment. The ultrathin layer of highly graphitic carbon coating drastically increased the electronic conductivity of LiFePO4. The short pathway along the [010] direction of LiFePO4 nanoplates could decrease the Li(+) ion diffusion path. In favor of the high electronic conductivity and short lithium ion diffusion distance, the LiFePO4/graphitic carbon composites exhibit an excellent cycling stability at high current rates at room temperature and superior performance at low temperature (-20 °C).

A novel approach for the preparation of Ni–CeO2 composite cathodes with enhanced electrocatalytic activity

In this work, supergravity fields were utilized to prepare Ni–CeO2 composite cathodes from a nickel sulphamate bath containing suspended nano-sized CeO2 particles. The prepared Ni–CeO2 composite coatings exhibit a significant enhancement in electrocatalytic activity for the hydrogen evolution reaction (HER) in alkaline solutions. The crystal structure, morphology and chemical compositions of the composite coatings were characterized by XRD, SEM and EDS measurements. It was shown that the prepared Ni–CeO2 composite coatings displayed a fine grain size and high contents of CeO2 incorporated into the Ni matrix. The electrochemical activity of the composite cathodes for HER was determined by polarization measurements and electrochemical impedance spectroscopy in 1.0 M NaOH solution. The results indicate that the catalytic activity of the Ni–CeO2 composite coatings is enhanced significantly, and the highest value of the exchange current density reaches 338.4 μA cm−2, which is obviously higher than the values previously reported in the literature. Meanwhile, the effects of both the CeO2 concentration and the intensities of the supergravity fields on the properties of the Ni–CeO2 coatings are investigated.

Facile synthesis of nitrogen-doped hierarchical porous lamellar carbon for high-performance supercapacitors

Three-dimensional (3D) interconnected N-enriched hierarchical porous lamellar carbon (NPLC) with a multilevel pore structure has been fabricated by a wet impregnation method using waste nitrogen-containing mantis shrimp shell as a carbon precursor and KOH as an impregnation solution. The synthesized NPLC-2 shows a large surface area of 1222.961 m2 g−1 calculated by the BET method, a hierarchical porous structure analyzed by the density functional theory (DFT) model, and a high nitrogen content of 1.78% quantified by X-ray photoelectron spectroscopy (XPS). Moreover, the NPLC-2 sample exhibits an ultra-high specific capacitance of 312.62 F g−1 at 0.3 A g−1, excellent rate capability with a specific capacitance of 272.56 F g−1 at 20.0 A g−1 and outstanding cycling stability with around 96.26% capacitance retention after 10 000 cycles at a high current density of 20.0 A g−1. In addition, NPLC-2 presents a high energy density of 15.05 W h kg−1 at 270 W kg−1, and up to 10.12 W h kg−1 even at a large power density of 14 580 W kg−1. Therefore, the prepared material can be applied in high energy density and high power density demanding fields.

Preparation of size-selective Mn3O4 hexagonal nanoplates with superior electrochemical properties for pseudocapacitors

Porous Mn3O4 hexagonal nanoplates were synthesized through annealing the hydrohausmannite precursor obtained by a one-pot hydrothermal process and by precisely controlling the concentrations of potassium hydroxide and glucose. The effect of potassium hydroxide and glucose on the growth of hexagonal nanoplates was investigated, and a growth mechanism was also proposed. Due to its abundant pores, the pure Mn3O4-based electrode exhibits excellent cycling stability with 100% capacity retention after 5000 cycles. The asymmetric supercapacitor exhibited high performance with an energy density of 17.276 W h kg(-1) at a power density of 207.3 W kg(-1) in a wide potential window of 1.5 V.

Synthesis of nitrogen-doped carbon cellular foam with ultra-high rate capability for supercapacitors

Three-dimensional (3D) interconnected N-doped porous carbons (NPCs) with different levels of pore structure are synthesized by a template method using MnOx as template and N-enriched pyrrole as carbon source. The fabricated materials show favorable pore size distribution in the range about 3–4 nm and moderate nitrogen content changing from 0.94 to 1.63 at%. Used as electrode material, the NPC originated from the optimum pyrolysis temperature of 750 °C demonstrates the best capacitance performance with a high specific capacitance of about 239.30 F g−1 at 0.5 A g−1. Moreover, it reveals an outstanding rate capability and the specific capacitance reaches 212.90 F g−1 at 10.0 A g−1 (up to 88.97% capacitance retention), as well as excellent cycling stability (∼10% capacitance loss after 5000 cycles) tested in 6 M KOH aqueous solution. Such exquisite performance may be ascribed to a unique combination of high specific surface area, suitable pore size distribution and introduction of nitrogen atoms.

Biotemplated fabrication of a novel hierarchical porous C/LiFePO4/C composite for Li-ion batteries

Hierarchical porous composites are a potentially attractive material for high-rate cathode. This work presents a facile sol–gel process for the fabrication of a hierarchical porous C/LiFePO4/bio-C composite by using artemia cyst shells as natural biological carbon templates. The C/LiFePO4/bio-C composite exhibits a superior electrochemical performance with discharge capacities of 105 mA h g−1, 93 mA h g−1 and 80 mA h g−1 at 5 C, 10 C and 20 C, respectively. Remarkably, it produces a high discharge capacity of 69.1 mA h g−1 and no fading after 50 cycles even at a high current density of 6800 mA g−1.

Comparing the electrochemical performance of LiFePO 4 /C modified by Mg doping and MgO coating

Supervalent cation doping and metal oxide coating are the most efficacious and popular methods to optimize the property of LiFePO4 lithium battery material. Mg-doped and MgO-coated LiFePO4/C were synthesized to analyze their individual influence on the electrochemical performance of active material. The specific capacity and rate capability of LiFePO4/C are improved by both MgO coating and Mg doping, especially the Mg-doped sample--Li0.985Mg0.015FePO4/C, whose discharge capacity is up to 163mAh g-1, 145.5mAh g-1, 128.3m Ah g-1, and 103.7mAh g-1 at 1C, 2C, 5C, and 10C, respectively. The cyclic life of electrode is obviously increased by MgO surface modification, and the discharge capacity retention rate of sample LiFePO4/C-MgO2.5 is up to 104.2% after 100 cycles. Comparing samples modified by these two methods, Mg doping is more prominent on prompting the capacity and rate capability of LiFePO4, while MgO coating is superior in terms of improving cyclic performance.