Design And Synthesis Of Graphene-Alloy Composites And Their Thermoelectric Properties And Li-Storage Properties

Graphene, a two-dimensional sheet composed of single-layer carbon atoms with the honeycomb lattice structure, has high electrical conductivity, high mobility, large specific surface area, good mechanical strength and thermal stability. Therefore, graphene is an ideal building block of composites. Because of the unique physical and chemical properties of graphene, graphene-based composites are considered promising for energy conversion and storage. Extensive attention has been paid to graphene-based composites for thermoelectric materials and Li ion batteries. This work focused on the design, synthesis and thermoelectric properties or Li-storage properties of graphene-alloy composites. Thermoelectric properties of graphene-CoSb3(CoSb3/G) and graphene-PbTe (PbTe/G) composites and Li-storage properties of graphene-Sn based alloy(CoSn2/G and FeSn2/G) composites were studied. The role of graphene in the composites is also discussed. The main results are summarized as follows:A strategy which combines nanostructuring and second phase incorporation for improving the thermoelectric performance of p-type CoSb3 is proposed. CoSb3/G, in which the crystal size of CoSb3 is around 5-10 nm, was prepared via solvothermal route. Due to the existence of graphene, grain size of CoSb3 maintained around 100 nm after hot pressing at 600℃ for 2 h, much smaller than that of graphene-free CoSb3,and the electrical conductivity at room temperature increased from 11000 to 51000 S m-1. The thermal conductivity at room temperature decreased from 1.05 to 0.9 W m-1 K-1. A dimensionless figure of merit zT= 0.61 at 800 K has been obtained for the CoSb3/G. while it is 0.26 for graphene-free CoSb3. The introducing of graphene benefits the composites in both carrier concentration and mobility. Therefore the electrical conductivity increases. In addition, the well dispersed graphene in the matrix also contributes to the low lattice thermal conductivity due to the increased interface scattering.The precursors of CoSb3/G were fabricated via the co-precipitation method. CoSb3/G composites were synthesized by sintering the precursors in different sintering atmosphere(H2 and H2/N2). Similar to the solvothermal route/hot-pressed CoSb3/G, the room temperature electrical conductivity of CoSb3/G prepared this way increased and thermal conductivity decreased. This means introducing graphene to the system may help increasing the electrical conductivity as well as decreasing the thermal conductivity, which has nothing to do with the synthetic process. PbTe/G powders have been prepared by solvothermal route. The cubic shaped PbTe particles with 100-200 nm are wrapped by graphene nanosheets. Due to the presence of graphene, the nanostructure of PbTe was maintained after hot pressing at 600℃ for 2 h. Similar to the solvothermal route/hot-pressed CoSb3/G, the electrical conductivity and thermal conductivity decreased. The electrical conductivity of PbTe/G at room temperature increased from 1300 to 22000 S m-1, and the thermal conductivity at room temperature decreased from 1.15 to 0.85 W m-1 K-1. This means introducing graphene to the system may help increasing the electrical conductivity as well as decreasing the thermal conductivity, which has nothing to do with the matrixs. Importantly, it was found that the electrical conductivity of graphene-based composites followed percolation theory. Below the percolation threshold, the more graphene was added, the electrical conductivity of graphene-based composites are better. And according to the model analysis, the thermal conductivity of the system depends on how the graphene dispersed in the matrix.CoSn2/G and FeSn2/G were synthesized by an in situ solvothermal route. CoSn2 (or FeSn2) nanoparticles with a size of 2-4 nm (10-30 nm) are uniformly dispersed and confined by graphene, forming a sandwich structure. The composites exhibit improved electrochemical cycle stability and rate capability compared to the bare alloy. The enhancement in the electrochemical properties could be attributed to the introduction of graphene that not only constructs two-dimensional conductive networks but also disperses and confines the nanoparticles, in addition to the buffering effect for the large volume changes.