Twin Polymerization Enabled Fabrication Of S-Doped Porous Carbon And Its Mental Oxide Composites Electrode Materials

With the continuous consumption of traditional fossil resource,the resulted environmental pollution and energy shortage have become increasingly serious.There is an urgent need to develop new clean and renewable energy storage and conversion technologies.Currently,electrochemical energy storage devices represented by lithium-ion batteries and supercapacitors are the most promising energy storage technologies.The electrochemical energy storage technology is strongly dependent on the design and synthesis of key electrode materials.Designing nanostructures and constructing nanocomposite electrodes are the most effective ways to improve the electrochemical properties of electrode materials.In this dissertation,based on the low specific capacity of conventional carbon materials,poor rate performance and cycling stability of metal oxide electrodes,a universal twin polymerization method was developed to synthesize sulfur-doped micro/mesoporous carbon materials,ultrafine anatase TiO2 and SnO2 nanocrystalline embedded in the three-dimensional continuous carbon network with large capacity,high rate capability and long-term cycling performance.(1)Firstly,the twin monomer was synthesized by using 2-thiophenemethanol and tetraethyl orthosilicate as precursors,then the sulfur-doped micro/mesoporous carbon material(S-MC-TP)was fabricated through the polymerization of the twin monomer with the followed carbonization and etching.Its specific surface area is as high as 792 m2/g,and the average pore size is only about 3 nm.When used as the supercapacitor electrode,the S-MC-TP electrode exhibited a high specific capacitance of 455 F/g at 1.0 A/g and kept a capacitance of 200 F/g even at a large current density of 100.0 A/g.When used as the lithium-ion battery anode,the S-MC-TP electrode demonstrated a large reversible capacity of 590 mAh/g,far higher than that of the traditional graphite anode.There was almost no capacity fade after 350 cycles at 200 mA/g.The high capacity,excellent rate capability and long-term cycling life can be ascribed to the easier ions adsorption/desorption process facilitated by the large specific area and the mesoporous hybrid nanostructure.Also,sulfur doping can improve the electrode/electrolyte interface compatibility and contribute the extra capacity.(2)The twin monomer was firstly synthesized by using 2-thiophenemethanol and tetraethanol titanium as precursors,then the ultrafine anatase TiO2 nanocrystalline embedded in the three-dimensional continuous carbon network was fabricated through the polymerization of the twin monomer with the followed carbonization process.The average particle size of TiO2 nanocrystals is only 5 nm,and they are evenly embedded in a continuous carbon network structure.When used as the lithium-ion battery anode,the TiO2 composite electrode exhibited a highly reversible capacity(667 mAh/g)and excellent cycle stability with the capacity retention rate of 97.8%after 500 cycles at 5.0 A/g.The excellent electrochemical performance should be attributed to the shortened ions diffusion path for the ultrafine TiO2 nanocrystalline,the enhanced electronic conductivity and structural stability of TiO2 nanocrystalline provided by the three-dimensional carbon network structure.(3)Using the same method as described above,the ultrafine SnO2 nanocrystalline embedded in the three-dimensional continuous carbon network was synthesized by simply replacing the titanium source with Tin source.The as-prepared SnO2 particles are evenly embedded in the continuous carbon network structure with the particle size of 1?5 nm.When used as the lithium-ion battery anode,the SnO2 composite electrode demonstrated a reversible capacity of 792 mAh/g at 50 mA/g.Even at a large current density of 5.0 A/g,the capacity can be still kept at 110 mAh/g and kept no obvious decay after 300 cycles,indicating the high rate capability and excellent cycling stability.The improved electrochemical performance is mainly ascribed to the shortened ions diffusion path for the ultrafine SnO2 nanocrystalline,the enhanced electronic conductivity and the suppressed volume change of SnO2 upon lithium insertion endowed by the three-dimensional carbon network structure.