Gaoshao Cao

Preferential c-Axis Orientation of Ultrathin SnS2 Nanoplates on Graphene as High-Performance Anode for Li-Ion Batteries

A SnS2/graphene (SnS2/G) hybrid was synthesized by a facile one-step solvothermal route using graphite oxide, sodium sulfide, and SnCl4·5H2O as the starting materials. The formation of SnS2 and the reduction of graphite oxide occur simultaneously. Ultrathin SnS2 nanoplates with a lateral size of 5-10 nm are anchored on graphene nanosheets with a preferential (001) orientation, forming a unique plate-on-sheet structure. The electrochemical tests showed that the nanohybrid exhibits a remarkably enhanced cycling stability and rate capability compared with bare SnS2. The excellent electrochemical properties of SnS2/G could be ascribed to the in situ introduced graphene matrix which offers two-dimensional conductive networks, disperses and immobilizes SnS2 nanoplates, buffers the volume changes during cycling, and directs the growth of SnS2 nanoplates with a favorable orientation.

Self-assembly of a CoFe2O4/graphene sandwich by a controllable and general route: towards a high-performance anode for Li-ion batteries

A CoFe2O4-nanocrystal/graphene-nanosheet (CFO/GS) nanohybrid has been synthesized by a facile in situ solvothermal route and has been investigated as a promising high-performance anode material for Li-ion batteries. The crystal size of CoFe2O4 can be controlled to 10–20 nm by pre-treating the precursors before the solvothermal reactions. The method for synthesizing the CFO/GS hybrid can be extended to synthesize MFe2O4/graphene (MFe2O4/G, M = Mn and Ni) hybrids. The CFO/GS hybrid exhibits superior cycling stability and rate capability compared to bare CoFe2O4. The improved electrochemical performance can be attributed to a combination of the conducting, confining and dispersing effects of graphene.

Double-shelled hollow microspheres of LiMn2O4 for high-performance lithium ion batteries

Double-shelled hollow microspheres of LiMn2O4 were prepared by a facile self-template method. The inner and outer shells of the obtained microspheres are composed of nanoparticles with diameters ranging between 200–400 nm. Galvanostatic charge/discharge cycling shows that this material delivers a discharge capacity of 127 mAh g−1 at C/10 rate and a capacity retainability of 80% after 800 cycles at 5 C rate, revealing a high reversible capacity, superior rate capability and excellent cycling stability under high rates. The improved performance is attributed to the short Li+ ion diffusion lengths in the nanobuilding blocks and the void core and space between the inner and outer shells to accommodate the volume expansion/contraction during Li+ ions insertion/extraction processes.

Facile one-pot synthesis of ultrathin NiS nanosheets anchored on graphene and the improved electrochemical Li-storage properties

A NiS/graphene (NiS/G) nanohybrid has been synthesized by a facile in situ one-pot hydrothermal route using graphite oxide, thiourea, NiCl2·4H2O and sodium citrate as the raw materials. The growth of NiS nanosheets and the reduction of graphite oxide occur simultaneously during the hydrothermal reactions. The hybrid exhibits a unique sheet-on-sheet structure, where ultrathin NiS sheets (below 5 nm) are anchoring on few-layer (below 8 layers) graphene sheets. The electrochemical measurements indicate that the NiS/G hybrid exhibits remarkably improved Li-storage properties compared with bare NiS, due to the ultrathin feature of the NiS sheets, unique sheet-on-sheet structure of the hybrid, and the combined conducting, buffering and confining effects of the in situ introduced graphene nanosheets.

Self-assembly of a ZnFe2O4/graphene hybrid and its application as a high-performance anode material for Li-ion batteries

A nanohybrid based on ZnFe2O4 nanospheres and graphene nanosheets (ZnFe2O4/G) has been synthesized by a facile one-pot in situ solvothermal route. The results show that the formation of ZnFe2O4 and the reduction of graphite oxide take place simultaneously during the solvothermal reactions. Spheric ZnFe2O4 nanoparticles with a size of 100–200 nm are confined in between the graphene sheets, forming a unique hybrid nanostructure. The electrochemical measurements have shown that the ZnFe2O4/G hybrid exhibits improved electrochemical Li-storage properties compared with bare ZnFe2O4, due to the combined buffering, confining and conducting effects of the in situ introduced graphene nanosheets.

Nitrogen-doped reduced graphene oxide for high-performance flexible all-solid-state micro-supercapacitors

The rapid development of microelectronic devices has stimulated an increasing demand for micro-energy storage devices, typically, micro-supercapacitors (MSCs). Despite recent advances, the fabrication of MSCs using a facile, scalable and inexpensive method still remains challenging. In this work, we use a facile screen printing technique to fabricate flexible all-solid-state MSCs using N-doped reduced graphene oxide (rGO) as the electrode material. The effective area of MSCs and the thickness of the active material are only 0.396 cm2 and 10 μm, respectively. The MSCs can deliver a high specific areal capacitance of 3.4 mF cm−2, which is among the high values of graphene-based materials for all-solid-state MSCs reported so far. Easy fabrication and good performance make MSCs promising on-chip energy storage devices.

One-pot synthesis of ultrafine ZnFe2O4 nanocrystals anchored on graphene for high-performance Li and Li-ion batteries

A ZnFe2O4-nanocrystals/graphene-nanosheets (ZnFe2O4/G) nanohybrid has been prepared by a facile in situ hydrothermal route using Zn(NO3)2·6H2O, Fe(NO3)3·6H2O and graphite oxide (GO) as the precursors. Ultrafine ZnFe2O4 nanocrystals (below 10 nm) are confined by the few-layer graphene sheets reduced from GO, forming a unique sheet-like hybrid. In this structure, the direct restacking of the hydrophobic graphene sheets is refrained by loading ZnFe2O4 nanocrystals as the spacers and the aggregation of ZnFe2O4 nanocrystals is inhibited by the dispersing and confining effects of the graphene sheets. ZnFe2O4/G shows excellent rate capability and high-rate cycling stability for lithium storage. It also shows a high capacity when used as an anode for a ZnFe2O4/G–LiFePO4/C full cell.

Reduced graphene oxide-induced recrystallization of NiS nanorods to nanosheets and the improved Na-storage properties.

Preparation of two-dimensional (2D) graphene-like materials is currently an emerging field in materials science since the discovery of single-atom-thick graphene prepared by mechanical cleavage. In this work, we proposed a new method to prepare 2D NiS, where reduced graphene oxide (rGO) was found to induce the recrystallization of NiS from nanorods to nanosheets in a hydrothermal process. The process and mechanism of recrystallization have been clarified by various characterization techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) mapping, and X-ray photoelectron spectroscopy (XPS). The characterization of ex situ NiS/rGO products by SEM and EDS mapping indicates that the recrystallization of NiS from nanorods to nanosheets is realized actually through an exfoliation process, while the characterization of in situ NiS/rGO products by SEM, TEM, and EDS mapping reveals the exfoliation process. The XPS result demonstrates that hydrothermally assisted chemical bonding occurs between NiS and rGO, which induces the exfoliation of NiS nanorods into nanosheets. The obtained NiS/rGO composite shows promising Na-storage properties.

Controllable synthesis of high-performance LiMnPO4 nanocrystals by a facile one-spot solvothermal process

Olivine-type LiMnPO4 has been considered as a promising cathode material for the next-generation high-power lithium ion batteries. Preparing high-performance LiMnPO4 cathodes by a simple approach has been a subject of much scientific inquiry for several years. Here, we report a simple solvothermal synthesis of LiMnPO4 nanocrystals using LiOH·H2O, H3PO4 and MnSO4·H2O as the precursors and ethylene glycol as the reaction medium. We found that the ratio of the starting materials exerts a significant influence on the morphology, size and crystal orientation of LiMnPO4 nanocrystals. We confirmed the critical role of H+ concentration in altering the crystallization characteristics of LiMnPO4. The results showed that after carbon coating, the plate-like LiMnPO4, which was synthesized from the precursor with a LiOH/H3PO4/MnSO4 ratio of 3 : 1.1 : 1, exhibited the best electrochemical performance, yielding a discharge capacity of 108.2 mA h g−1 at 10 C and maintaining a discharge capacity of 133.5 mA h g−1 after 100 cycles at 0.5 C. This simple, one-spot solvothermal preparation method sheds light on the synthesis of high-performance LiMnPO4 cathode material.

Enhanced thermoelectric properties of p-type CoSb3/graphene nanocomposite

Nanostructuring and second phase incorporation are considered to be promising ways of enhancing the thermoelectric performance of bulk materials. Here, a design principle is proposed which combines these two methods for improving the thermoelectric performance of p-type CoSb3 by fabricating a CoSb3/graphene (CoSb3/G) nanocomposite, where a second phase, graphene, is introduced in the nanostructured CoSb3 matrix via an in situ one-pot solvothermal route. In addition, CoSb3/G bulk materials were prepared by hot pressing the solvothermally synthesized CoSb3/G powder. It was found that addition of a small amount of graphene can drastically enhance the electrical conductivity due to the increase in both carrier concentration and mobility. In addition, the well dispersed graphene in the nanostructured CoSb3 matrix also contributes to the low lattice thermal conductivity. A dimensionless figure of merit ZT = 0.61 at 800 K has been obtained for the CoSb3/G nanocomposite, which is about a 130% improvement over that of graphene-free CoSb3 (∼0.26).