Hongwei Mi

In situ coating of nitrogen-doped graphene-like nanosheets on silicon as a stable anode for high-performance lithium-ion batteries

Carbon coating is an effective approach to improve the cycling stability of silicon (Si) anodes for lithium-ion batteries. In this research, we report a facile one-step carbon-thermal method to coat Si nanoparticles with nitrogen-doped (N-doped) graphene-like nanosheets derived from a liquid-polyacrylonitrile (LPAN) precursor. The coated Si anode displays an initial coulombic efficiency of 82%, which is about three times greater than its pristine counterpart, as well as superior cycling stability. The performance improvement is a result of the N-doped graphene-like nanosheet conformal coating, which not only creates an electrically conductive network for the electrode, but also provides a buffering matrix to accommodate the volume change of Si during charging and discharging processes.

Scalable 2D Hierarchical Porous Carbon Nanosheets for Flexible Supercapacitors with Ultrahigh Energy Density

2D carbon nanomaterials such as graphene and its derivatives, have gained tremendous research interests in energy storage because of their high capacitance and chemical stability. However, scalable synthesis of ultrathin carbon nanosheets with well-defined pore architectures remains a great challenge. Herein, the first synthesis of 2D hierarchical porous carbon nanosheets (2D-HPCs) with rich nitrogen dopants is reported, which is prepared with high scalability through a rapid polymerization of a nitrogen-containing thermoset and a subsequent one-step pyrolysis and activation into 2D porous nanosheets. 2D-HPCs, which are typically 1.5 nm thick and 1-3 µm wide, show a high surface area (2406 m2 g-1 ) and with hierarchical micro-, meso-, and macropores. This 2D and hierarchical porous structure leads to robust flexibility and good energy-storage capability, being 139 Wh kg-1 for a symmetric supercapacitor. Flexible supercapacitor devices fabricated by these 2D-HPCs also present an ultrahigh volumetric energy density of 8.4 mWh cm-3 at a power density of 24.9 mW cm-3 , which is retained at 80% even when the power density is increased by 20-fold. The devices show very high electrochemical life (96% retention after 10000 charge/discharge cycles) and excellent mechanical flexibility.

Air plasma etching towards rich active sites in Fe/N-porous carbon for the oxygen reduction reaction with superior catalytic performance

Herein, an electrocatalyst consisting of iron and nitrogen co-doped porous carbon (Fe–N/C) was prepared by catalytic carbonization of chitin with the assistance of FeCl3 and ZnCl2. The catalytic activity of Fe–N/C towards the oxygen reduction reaction (ORR) in both acidic and alkaline electrolytes was significantly enhanced by air-plasma etching for only 120 s, showing a four-electron ORR process with an onset potential and limiting current comparable to those of Pt catalysts. This performance enhancement originated from the removal of less stable sp3 and amorphous sp2 carbons which would expose more active catalytic FeN4 centers, as well as the transformation of a small fraction of iron-based nanoparticles into FeN4 species.

PdNi alloy decorated 3D hierarchically N, S co-doped macro–mesoporous carbon composites as efficient free-standing and binder-free catalysts for Li–O2 batteries

A novel free-standing and binder-free air electrode with excellent electrochemical performance was designed for highly reversible Li–O2 batteries. The 3D hierarchically N, S co-doped macro–mesoporous carbon (NSMmC) was deposited on carbon paper (CP) via a template method, and then uniformly decorated with PdNi nanoparticles. The macropores of the porous carbon can provide enough space to accommodate discharge products, while the interconnected pores and channels efficiently facilitate oxygen and electrolyte diffusion. The introduction of PdNi nanoparticles greatly reduce the charge transfer resistance, resulting in the improvement of electron mobility of the whole cathode. Compared with Pd–NSMmC/CP and NSMmC/CP cathodes, the PdNi–NSMmC/CP cathode shows considerable enhancement of Li–O2 battery performance. The ultrafine and evenly distributed PdNi nanoparticles can not only provide enough catalytic sites, but can also tailor the discharge products into a cage-like morphology, which provides enough channels for electron and lithium ion transport. Moreover, the smaller size of the cage-like Li2O2 makes it decompose more easily, resulting in lower charge overpotential. This study provides a promising strategy to design 3D structured air cathodes for Li–O2 batteries with high electrocatalytic performance.

Carbothermal Synthesis of Nitrogen-Doped Graphene Composites for Energy Conversion and Storage Devices

Metal oxides and carbonaceous composites are both promising materials for electrochemical energy conversion and storage devices, such as secondary rechargeable batteries, fuel cells and electrochemical capacitors. In this study, Fe3O4 nanoparticles wrapped in nitrogen-doped (N-doped) graphene nanosheets (Fe3O4@G) were fabricated by a facile one-step carbothermal reduction method derived from Fe2O3 and liquid-polyacrylonitrile (LPAN). The unique two-dimensional structure of N-doped graphene nanosheets, can not only accommodate the volume changes during lithium intercalation/extraction processes and suppress the particles aggregation but also act as an electronically conductive matrix to improve the electrochemical performance of Fe3O4 anode, especially the rate capability. Whats more, by etching Fe3O4@G to remove the iron-based oxide template, porous N-doped graphene composites (NGCs) were prepared and presented abundant pore structure with high specific surface area, delivering a specific capacitance of 172 F·g−1 at 0.5 A·g−1. In this way, Fe2O3 was both template and activator to adjust the pore size of graphene. And the effect of specific surface area and pore size tuned by the Fe2O3 activator were also revealed.

LiFePO4/RGO composites synthesized by a solid phase combined with carbothermal reduction method

ABSTRACT LiFePO4/reduced graphene oxide composites (LiFePO4/RGO) were synthesized via a simple solid phase combined with carbothermal reduction method. The samples were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). Furthermore, cyclic voltammetry (CV), electrochemicaal impendence spectroscopy (EIS) and galvanostatic charge/discharge tests were conducted to further study the electrochemical properties of LiFePO4/RGO composites. The initial discharge specific capacity of LiFePO4/RGO was 151.5 mAh g−1 at 0.1C, and after 50 cycles, it still remains 149.2 mAh g−1. The results reveal that LiFePO4/RGO composites improved the discharge specific capacity and rate charge-discharge performance.