Libao Chen

Ultrathin Porous NiCo2O4 Nanosheet Arrays on Flexible Carbon Fabric for High-Performance Supercapacitors

NiCo2O4 with higher specific capacitance is an excellent pseudocapacitive material. However, the bulk NiCo2O4 material prevents the achievement of high energy desity and great rate performance due to the limited electroactive surface area. In this work, NiCo2O4 nanosheet arrays were deposited on flexible carbon fabric (CF) as a high-performance electrode for supercapacitors. The NiCo2O4 arrays were constructed by interconnected ultrathin nanosheets (10 nm) with many interparticle pores. The porous feature of NiCo2O4 nanosheets increases the amount of electroactive sites and facilitates the electrolyte penetration. Hence, the NiCo2O4/CF composites exhibited a high specific capacitance of 2658 F g(-1) (2 A g(-1)), good rate performance, and superior cycling life, suggesting the NiCo2O4/CF is a promising electrode material for flexible electrochemical capacitors.

A Li-rich Layered@Spinel@Carbon heterostructured cathode material for high capacity and high rate lithium-ion batteries fabricated via an in situ synchronous carbonization-reduction method

A novel Layered@Spinel@Carbon heterostructure is successfully fabricated via an in situ synchronous carbonization-reduction process based on a bio-inspired coating method, which comprises a core of Li-rich layered (Rm) oxide, a spinel phase (Fdm) interlayer and a carbon nano-coating. This unique structure, which combines the advantages of the high capacity Li-rich layered structure, 3D fast Li+ diffusion channels of the spinel structure and the high conductivity of the carbon coating, shows extremely high discharge capacity (as high as 334.5 mA h g−1) and superior rate capability. This strategy may provide some new insights into the design and synthesis of various electrode materials for high performance energy storage devices.

Hierarchical Nanocomposite of Hollow N-Doped Carbon Spheres Decorated with Ultrathin WS2 Nanosheets for High-Performance Lithium-Ion Battery Anode

Hierarchical nanocomposite of ultrathin WS2 nanosheets uniformly attached on the surface of hollow nitrogen-doped carbon spheres (WS2@HNCSs) were successfully fabricated via a facile synthesis strategy. When evaluated as an anode material for LIBs, the hierarchical WS2@HNCSs exhibit a high specific capacity of 801.4 mA h g(-1) at 0.1 A g(-1), excellent rate capability (545.6 mA h g(-1) at a high current density of 2 A g(-1)), and great cycling stability with a capacity retention of 95.8% after 150 cycles at 0.5 A g(-1). The Li-ion storage properties of our WS2@HNCSs nanocomposite are much better than those of the previously most reported WS2-based anode materials. The impressive electrochemical performance is attributed to the robust nanostructure and the favorable synergistic effect between the ultrathin (3-5 layers) WS2 nanosheets and the highly conductive hollow N-doped carbon spheres. The hierarchical hybrid can simultaneously facilitate fast electron/ion transfer, effectively accommodate mechanical stress from cycling, restrain agglomeration, and enable full utilization of the active materials. These characteristics make WS2@HNCSs a promising anode material for high-performance LIBs.

Solvent-Controlled Synthesis of NiO-CoO/Carbon Fiber Nanobrushes with Different Densities and Their Excellent Properties for Lithium Ion Storage.

NiO-CoO nanoneedles are grown on carbon fibers by a solvothermal strategy to form nanobrushes. The density of nanobrushes can be easily controlled by altering the solvents. The synthesis mechanism of NiO-CoO/carbon fiber nanobrushes is investigated by the time-dependent experiments in detail. As anodes for lithium ion batteries, the NiO-CoO/carbon fiber nanobrushes synthesized in ethanol show excellent properties with a discharge capacity of 801 mA h g(-1) after 200 cycles at a current density of 200 mA g(-1). The improvement can be ascribed to the carbon fibers as the highway for electrons and the interspace between NiO-CoO nanoneedles to accommodate the volume change and maintain the structural stability.

Stannous ions reducing graphene oxide at room temperature to produce SnOx-porous, carbon-nanofiber flexible mats as binder-free anodes for lithium-ion batteries

Tin oxides with high theoretical capacities as anodes for lithium-ion batteries always suffer from the electrical disconnect issue of active materials owing to huge volume changes. In this work, flexible mats composed of ultra-small SnOx nanoparticles, graphene, and carbon fibers are synthesized by reducing graphene oxide with stannous ions at room temperature following treatments. SnOx nanoparticles, including SnO and SnO2, with diameters of approximately 4 nm are embedded in the matrix composed of carbon fiber and graphene to form composite fibers that are woven into flexible mats without any binders. As binder-free anodes for lithium-ion batteries, SnOx–graphene–carbon fiber mats can deliver a high reversible capacity of 545 mA h g−1 after 1000 cycles at a current density of 200 mA g−1, which is much better than those of SnOx–carbon fiber mats. The improved performance of SnOx–graphene–carbon fiber mats can be attributed to the ultra-small size of SnOx nanoparticles and the double protection of both graphene and carbon fibers.

The Effect of Boron Doping on Structure and Electrochemical Performance of Lithium-Rich Layered Oxide Materials

Polyanion doping shows great potential to improve electrochemical performance of Li-rich layered oxide (LLO) materials. Here, by optimizing the doping content and annealing temperature, we obtained boron-doped LLO materials Li1.2Mn0.54Ni0.13Co0.13BxO2 (x = 0.04 and 0.06) with comprehensively improved performance (94% capacity retention after 100 cycles at 60 mA/g current density and a rate capability much higher compared to that of the pristine sample) at annealing temperatures of 750 and 650 °C, respectively, which are much lower than the traditional annealing temperature of similar material systems without boron. The scenario of the complex crystallization process was captured using Cs-corrected high-angle annular dark field scanning transmission electron microscopic (HAADF-STEM) imaging techniques. The existence of layered, NiO-type, and spinel-like structures in a single particle induced by boron doping and optimization of annealing temperature is believed to contribute to the remarkable improvement of cycling stability and rate capability.

Phase transition and mechanical properties of tungsten nanomaterials from molecular dynamic simulation

Molecular dynamic simulation is used to systematically find out the effects of the size and shape of nanoparticles on phase transition and mechanical properties of W nanomaterials. It is revealed that the body-centered cubic (BCC) to face-centered cubic (FCC) phase transition could only happen in cubic nanoparticles of W, instead of the shapes of sphere, octahedron, and rhombic dodecahedron, and that the critical number to trigger the phase transition is 5374 atoms. Simulation also shows that the FCC nanocrystalline W should be prevented due to its much lower tensile strength than its BCC counterpart and that the octahedral and rhombic dodecahedral nanoparticles of W, rather than the cubic nanoparticles, should be preferred in terms of phase transition and mechanical properties. The derived results are discussed extensively through comparing with available observations in the literature to provide a deep understanding of W nanomaterials.

Understanding the Enhanced Kinetics of Gradient-Chemical-Doped Lithium-Rich Cathode Material

Although chemical doping has been extensively employed to improve the electrochemical performance of Li-rich layered oxide (LLO) cathodes for Li ion batteries, the correlation between the electrochemical kinetics and local structure and chemistry of these materials after chemical doping is still not fully understood. Herein, gradient surface Si/Sn-doped LLOs with improved kinetics are demonstrated. The atomic local structure and surface chemistry are determined using electron microscopy and spectroscopy techniques, and remarkably, the correlation of local structure-enhanced kinetics is clearly described in this work. The experimental results suggest that Si/Sn substitution decreases the TMO2 slab thickness and enlarges the interslab spacing, and the concentration gradient of Si/Sn affects the magnitude of these structural changes. The expanded interslab spacing accounts for the enhanced Li+ diffusivity and rate performance observed in Si/Sn-doped materials. The improved understanding of the local structure-enhanced kinetic relationship for doped LLOs demonstrates the potential for the design and development of other high-rate intercalated electrode materials.

Unravelling the reaction chemistry and degradation mechanism in aqueous Zn/MnO2 rechargeable batteries

Aqueous Zn/MnO2 rechargeable batteries utilizing a near neutral electrolyte have demonstrated great potential for large-scale energy storage applications, due to their safe and sustainable nature. Nevertheless, the reaction chemistry and degradation process associated with the MnO2-based cathode is not yet fully understood. Herein, a novel reversible Zn/MnO2 battery with zinc hydroxide sulfate (Zn4(OH)6SO4·5H2O, ZHS) as the cathode has been designed, where active MnO2 is formed in situ during the initial charge process from the Mn(II)-containing ZnSO4 electrolyte. A combination of electrochemical and material characterizations reveal two-step redox reactions (Mn(II) ions ⇌ ZnMn2O4 spinel ⇌ layered Zn-birnessite) during the charge–discharge process. Excellent cycling stability with a capacity retention of 100% after 1500 cycles is achieved at 500 mA g−1. The mechanism for long-term capacity fading is also studied. Cycling reversibility is destroyed by the irreversible consumption of Mn(II) to form woodruffite with a tunnel structure and poor electrochemical activity.

Intrinsic conductivity optimization of bi-metallic nickel cobalt selenides toward superior-rate Na-ion storage

Enhancing the conductivity of electrode materials is critically important for improving the high-rate performance of Na-ion batteries (NIBs). Herein, we report a multifaceted strategy for optimizing the conductivity and electrochemical properties of nickel cobalt selenides via the combination of fine component regulation and C coating. The electrical conductivity of C@Ni0.33Co0.67Se2/C nanofiber (CNF) (Co0.67) hybrids achieved in this study was 0.3733 S mm−1, a conductivity five-fold higher than that of selenides with a Ni/Co ratio of 2u2006:u20061. Coupled with desirable three-dimensional (3D) nanobrush morphology and the 1D conducting path of CNFs, the Co0.67 electrode achieved a superior rate performance of 413.1 mA h g−1, even at 2 A g−1. Furthermore, the Co0.67 electrode exhibited an impressive cycling performance of 499 mA h g−1 after 100 cycles (exhibiting an 89.5% capacity retention of the second cycle). Finally, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry analysis at different sweep rates were conducted to demonstrate the Co0.67 electrodes fast charge/ion transport ability and increased electrode kinetics.