Silicon-Carbon Composite Structures:Controllable Preparation And Lithium Storage Properties

Lithium-ion batteries(LIBs)are widely applied in various applications such as electronic devices,electric vehicles(EVs),and renewable energy storage.However,graphite anode materials are currently used in commercial lithium-ion batteries,unable to match the future demand for high energy density batteries owing to their low theoretical specific capacity.Silicon(Si)anodes are of great interest for next-generation high-capacity LIBs owing to their suitable operating potential(<0.5 V versus Li+/Li),high theoretical specific capacity(3579 mA h g-1),and abundant natural reserves.While Si materials possess poor electrical conductivity and also exhibit huge volume expansion(300%)upon cycling,which promotes pulverization of particles and fractures of the electrode structure.Such issues have hindered the further commercial application of silicon-based anode materials.A lot of Efforts have been made to address these issues through the design of silicon anode material microstructures,the design of silicon composites,and the exploration of silicon oxide materials(SiOx).This dissertation aims to fabricate varieties of high-performance Si/C composites from inexpensive raw precursors with rational structure optimizations through considerations of easy and controllable synthesis strategies.Furthermore,the electrochemical properties of as-prepared Si/C composites were further investigated in detail.The main contents are as follows.(1)Graphene-like carbon nanosheets-wrapped SiOx/carbon submicrospheres were prepared by a simple sol-gel method using inexpensive silane coupling agents as raw materials,followed by adding citric acid as carbon source and further carbonized by molten salt-assisted pyrolysis.The SiOx/C sub-micron spheres are coated with amorphous and twodimensional carbon layers at the outer surface,which significantly enhance the electrical conductivity of SiOx.Meanwhile,the two-dimensional carbon layer at the outer surface of SiOx submicron spheres can effectively mitigate the pulverization and fragmentation effect due to the volume expansion of the material and maintain the integrity of the electrode structure.The as-prepared electrode material maintains a specific capacity of 548 mA g-1 and capacity retention of 88%after 1000 cycles at a high current density of 1000 mA g-1,demonstrating its excellent cycling performance.Moreover,performance of full-cell was further investigated with commercial lithium iron phosphate as the cathode(LFP),which also exhibits with good stability upon 100 cycles,further confirming the feasibility of the material in practical applications.(2)With considerations of more simple and efficient synthesis process,carbon coated SiO nanoparticles embedded in hierarchical porous N-doped carbon nanosheets were prepared with bulk SiO,g-C3N4 and coal tar pitch with simple ball milling,followed by calcination process.The ball milling process could effectively reduce the particle scale of SiO particles.Furthermore,g-C3N4 precursor was not only served as a self-sacrificing template for the 2D structure but also as a nitrogen source for doping the carbon structure.The introduction of the nitrogen-doped two-dimensional carbon nanosheet structure greatly improves the electrical conductivity of the material,enhances the diffusion rate of lithium ions,and assists in maintaining the structural integrity of the electrode during electrochemical cycling.The asprepared material exhibits a high specific capacity of 690 mAh g-1 after 400 cycles at a high current density of 1 A g-1.The potential for practical application has also been evaluated by assembling full cells with LFP as cathode.(3)Based on the previous study,the synthesis of silicon-carbon composites usually requires high temperature annealing process.With the considerations exploring more facile and energy-efficient synthesis process,a carbon nanotubes-supported porous silicon microparticles composite structure is fabricated in low-temperature molten salt process.The carbon nanotubes uniformly distributed around porous silicon and formed interconnected network,which addresses the poor conductive issue of silicon particles.Meanwhile the abundant pore distribution on the surface and inside the silicon microparticles also effectively provides sufficient buffer space upon the lithiation process of the material,further avoiding the pulverization and fracture of the composite structure.The composite exhibits a reversible specific capacity of 796.6 mAh g-1 after 100 cycles at the current density of 0.5 A g-1.After matching with cathode material of lithium cobaltate(LCO),the full cell still proves the high stability after 200 cycles.(4)Based on the above synthetic strategy and combined with the environmentally friendly concept of waste recycling,silicon nanoparticles were prepared from industrial waste fly ash by a simple pretreatment and followed by solid-phase magnesium thermal reduction reaction in a low-temperature molten salt system.Material characterizations demonstrate that the particle size of as-obtained silicon nanoparticles is small,which helps avert the pulverization of particles upon the lithiation process,as well as benefits the rapid diffusion of lithium ions.Electrochemical analysis reveals that the silicon nanoparticles have excellent electrochemical stability,and the reversible capacity maintained at 1030.4 mAh g-1 after 500 cycles at a current density of 3.6 A g-1,indicating a stable cycling performance.