Preparation And Electrochemical Properties Of Nanocomposite Si/C Electrode Materials

Recently,the rapid development of electric vehicles and mobile electronic devices needs higher-energy-density commercial lithium-ion batteries?LIBs?.It is necessary for us to develop next-generation high-performance LIBs.The electrode material is a significant factor to improve energy densities.Owing to the low theoretical specific capacity(372 mA h g^–1),the commercial graphite-based anode is limited to accommodate the rapid development of LIBs.Silicon with a high theoretical specific capacity(⁴200 mA h g^–1)is a promising candidate as anode materials for next-generation electrochemical energy-storage devices.The advantages such as high specific capacity,low discharge potential and environmental friendness have attracted enormous scientific and industrial attention.However,poor cycle life derived from huge volume change?>300%?during charge and discharge processes has greatly hindered the utilization of silicon.In this thesis,we mainly focus on the electrochemical application of Si/C nanocomposite materials.We have successfully developed several effective methods to improve the cyclability of micro-/nano-Si particles.In addition,we synthesized amorphous Si/C nanocomposites by a ball-milling route.The as-prepared Si/C nanocomposite electrode material shows outstanding electrochemicalperformances in sodium-ion batteries.The main contents and results of this thesis are listed as follows:?1?The Si/GO films were fabricated,taking advantages of the flexibility and self-supporting features of the graphene oxide.We have developed a low-cost method for the preparation of Si/rGO electrode materials using directly the commercialized micrometer Si power as the Si source.Meanwhile,PMMA was used as a pore-forming agent to alleviate the volume changes of micron-scale Si in the charge and discharge process.The decomposition of PMMA and the reduction of GO were carried out through the thermal treatment in the Ar/H₂atmosphere.When used as the electrode for LIBs,the electrodes made of the resuling m-Si/rGO porous films exhibit improved cyclability.Typical,the reversible specific capacity could be maintained at 1000 mA h g^–1 after 200 cycles.In this work,the structural design of micron-sized Si powders has been investigated,and the electrochemical properties of the Si/C anode materials have been improved by a low-cost and high-practicality method.?2?The Si/C anode material with a core-shell structure was prepared by using in situ generated Fe₃O₄ nanoparticles as the template.Fe₃O₄ nanoparticles could catalyze the decomposition of the low-concentration acetylene?C₂H₂?gas.The problem that the low-concentration C₂H₂ gas is difficult to deposit directly on the Si surface.Fe₃O₄ nanoparticles could be easily removed.The Si/C anode materials with a core-shell structure were successfully prepared by optimizing the reaction conditions such as Fe/Si ratio,C₂H₂ reaction time and C₂H₂ decomposition temperature.As the negative electrode of LIBs,the obtained Si/C electrode material shows excellent electrochemical performances.The specific capacity is achieved at 1000 mA h g^–1 after 100 cycles at 200 mA g^–1,while the pure Si electrode rapidly decays below 700 mA h g^–1.In this study,a new type of template with a catalytic activity is proposed,which provides a new idea for the design of Si/C nanostructures.?3?Amorphous Si/C nanocomposites were easily prepared from commercial silicon and graphite powders by a simple mechanochemical approach,serving as a low-cost anode material for room-temperature sodium-ion batteries.By adjusting the milling time to control the crystallization and size of Si/C nanocomposites,the appropriate milling time afforded the original Si/C material,without sodium ion battery activity,a good sodium ion battery electrochemical performance.The Si/C nanocomposites showed a specific capacity of 280 mA h g^–1 after 100 cycles at 100 mA g^–1.At a higher current density of 1000 mA g^–1,the specific capacity was 170 mA h g^–1 after 500 cycles.For the first time,this paper proposes the preparation of Si/C material by ball milling as the anode material of sodium ion battery,which provides a new idea for the application of silicon-based anode materials in the direction of electrochemical energy storage.