Investigation On Three-dimensional Self-assembled Composite Materials For High Performance Lithium Ion Batteries

Rechargeable lithium ion batteries?LIBs?are regarded as one of the promising energy storage devices for electric vehicles and large energy integration.The performance of LIBs largely depends on both physical and chemical properties of electrode materials,and thus rational design of functional structures plays an important role in meeting ionic and electronic transport kinetics requirements.Lithium iron phosphate?LiFePO₄,LFP?,titanium dioxide?TiO₂?as well as silicon?Si?have been receiving increasing attention owing to high abundance,low toxicity and other respective advantages.However,low electronic conductivity and sluggish lithium ions diffusion greatly restrict their high-rate performance.It has been proved that carbon-based composites with hierarchical structures can improve their electrochemical performance.However,it is very difficult to use traditional solid-state route or sel-gel method to realize such structures.Chemical vapor deposition,spray-drying or hard-templated routes involve multiple steps and high-cost,and thus is not suitable for large-scale synthesis.In this regard,it is greatly desirable to explore new routes to develop hierarchical carbon/inorganic nanomaterial for the electrode materials in LIBs.In this research,different kinds of novel hierarchical carbon/inorganic nanomaterial composite have been well designed and successfully prepared through a facile self-assembly process,combining one or more of functional carbon materials?mesoporous carbon,carbon nanotubes and graphene?with one of the above-mentioned electrode materials.It is noted that phenolic resols with phenolic hydroxyl groups were especially chosen as basic carbon precursor in the self-assembly process.Then,the structural and morphological properties,formation mechanism and electrochemical performance of as-synthesized carbon/inorganic nanomaterial composite were detailedly investigated.In addition,the synergetic effect between functional carbon and active material on the electrochemical behaviors of the final product has been well discussed.The main research contents are summarized as follows:A novel and facile solvent evaporation induced self-assembly?EISA?route was proposed to prepare LFP/mesoporous carbon nanosheets?LFP/meso-CNSs?,and it has been proved that the synergistic effect between LFP precursor and phenolic resols plays a key role in controlling the structure and improving its electrochemical performance.LFP/meso-CNSs display an excellent high-rate capability(122.1 mAh g^-1at 5 C and 102.1 mAh g^-1 at 10 C)and stable cycling property as the cathode material of LIBs,benefitting from its high electronic conductivity,open mesoporosity,and the nano-size of its active material.Through a soft-templated self-assembly process followed by thermal treatment,LFP nanocrystals in-situ grown on interconnected carbon nanotubes/mesoporous carbon nanosheets?LFP@CNTs/CNSs?have been designed and synthesized.The as-obtained LFP@CNTs/CNSs samples exhibit high specific capacity with a discharge capacity of 167.9 mAh g^-1at 0.1 C,and superior long-term cycling stability with only 10%capacity decay at 10 C after 1000 cycles.Moreover,a concept of double carbonaceous?CNTs/CNSs?conductive network for fast transport of both electrons and lithium ions is proposed,which has been successfully confirmed by a systematically electrochemical study.A hydrothermal induced self-assembly process was proposed to synthesizehierarchicallymesoporousTiO₂-carboncomposite nanospheres?meso-ATCCNs?,and it has been proved that phenolic resols play a key role in controlling the morphology and microstructure of meso-ATCCNs.In an enlarged potential window of 0.01-3.0 V,meso-ATCCNs demonstrat excellent rate capabilities(413.7,289.7 and206.8 mAh g^-1 at 100,1000 and 3000 mA g^-1,respectively)as well as stable cyclability(90%capacity retention over 500 cycles at 1000 mA g^-1),which can be attributed to a synergetic effect induced by its hierarchical structure,including TiO₂ nanoparticles?⁵ nm?,in-situ deposited carbon,a high surface area and open mesoporosity.Based on a hydrothermal induced co-assembly process of phenolic resols and graphene,carbon-coated Si nanoparticles encapsulated in a three dimensional graphene network?nano-Si@C/rGO?were designed and fabricated.The discharge capacity of the nano-Si@C/rGO remains stable at an extremely high current density of 1000 mAh g^-1,and the corresponding capacity is approximately 1082.5 mAh g^-1,which is much higher than that of nano-Si@C electrode.In addition,it shows a capacity of 815.2 mAh g^-1 at 2000 mA g^-1.The synergistic effect of carbon coating and graphene framework create a hierarchical and effective conductive network on a macroscopic scale,thus resulting in enhancing its high-rate performance and cycling stability.In summary,three-dimensional composite materials?LFP,TiO₂ and Si based composite?for high performance LIBs have been well designed and successfully prepared by novel self-assembly processes,which provide effective conductive networks for both electrons and lithium ions,and thus remarkably improve the electrode kinetics.Considering the unique structure and promising performance of these carbon/inorganic nanomaterial,the self-assembly strategies demonstrated here can be potentially applied to fabricate other cathode and anode materials of LIBs.