Synthesis And Electrochemical Properties Of Transition Metal Oxides/carbon Composites By In Situ Pyrolysis Of Organic-inorganic Layered Hybrid

Recently, transition metal oxides have been extensively exploited as new anode materials for high-performance LIBs due to their higher capacities than that of the current commercial anode material (graphite). However, the intrinsic disadvantages associated with severe volume variation and low electrical conductivity make transition metal oxides based anodes suffer from poor cycling performance and low capacity. In order to circumvent the above intractable problems, to construct hybrid electrodes composed of transition metal oxides and carbon on the nanoscale has received more interest. In most of these studies, the carbonaceous materials are used not only as effective buffer zones to accommodate the volume changes but also as conductive material to alleviate the electrical conductivity of transition metal oxides anode materials, which result in enhanced cycle performance and rate capability. In spite of this, some challenges such as heterogeneous dispersion of transition metal oxides nanoparticles in carbon matrices and inadequate buffer space to relieve the large volume change during Li+ insertion/extraction are still hampering the application of these composites. In this themes, organic-inorganic layered precursors are used as precursors to produce Co3O4/C and Fe3O4/C hybrids by in situ pyrolysis approach. The main research results is as follows:(1) The novel flower-like Co/carbon nanosheets composites are prepared by in situ pyrolysis of the organic-inorganic layered precursor Co2(OH)2BDC. Detailed microstructural analyses review that the flower-like carbon nanosheets embedded uniformly with Co nanoparticles with the average value of about 5 nm. This composite exhibits superior capacity retention, excellent cycling performance, and high rate capability as LIBs anode materials. The capacity can reach 1109 mAh g-1 at the first cycle and keeps 767 mAh g-1 after 70 cycles. At a current density of 2000 mA g-1, the reversible capacity is 347 mAh g-1, and then back to 697 mAh g-1 at 200 mAh g-1(2) The flower-like Co3O4/carbon nanoplates composites are prepared by heating the as-obtained flower-like Co/carbon nanoplates composites at 300 ℃ for 3 h in air. Detailed microstructural analyses review that the Co3O4 nanoparticles with the average value of about 5 nm embed in the graphitic carbon sheets. Such Co3O4/C composites exhibits high surface areas of 47.64 m2 g-1 and the reversible capacity of 1170 mAh g-1 beyond 30 cycles at a current density of 100 mA g-1. Even at a current density of 2000 mA g-1, the reversible capacity is still 395 mAh g-1, and then back to 950 mAh g-1 at 100 mA g-1.(3) The novel porous Fe3O4 nanoplates/carbon (Fe3O4-NPs@C) composites are prepared by in situ pyrolysis of the organic-inorganic layered precursor [N2C6H18][Fe1.5(SO4)F3] at 500 ℃ for 36 h in Ar atmosphere. Micro structure analysis reviews that the 3D carbon network with amounts of irregular voids is made of graphene-like carbon with a thickness of less than 10 nm, and Fe3O4 nanoplates in polygon shape are coated with carbon shells and homogeneously embedded in such carbon network. In this architecture, the Fe3O4 nanoplates shorten the channels for fast Li+ supply and the porous carbon network accommodates the mechanical stress induced by the volume change of Fe3O4 as well as facilitates good contact of the internal active materials with electrolyte, thus leading to a fast transportation of Li+ ions. As a result, the as-prepared Fe3O4 nanoplates@C composites exhibit high surface areas of 65.2 m2 g-1 and the reversible capacity of 1181.9 mAh g-1 (97.35% of initial capacity) beyond 100 cycles at a current density of 100 mA g-1, and even at a current density of 1000 mA g-1, the reversible capacity of 580 mAh g-1.