Preparation Of Micro-Nano Porous Silicon/Carbon And Performance As Anode For Lithium Ion Batteries

The rapid development of electric vehicles,mobile electronics has required higher demands for lithium-ion batteries.It is a top priority to develop a high energy density,high safety,low-cost Li-ion battery anode material in industry.Silicon has a high theoretical specific capacity(3580mAh g^-1)and a low lithium insertion potential?0.4V vs Li/Li⁺?,which has attracted a lot of researches'attention.However,the silicon material occurs a volume change of more than 300%during the process of lithium extraction,resulting in silicon particles fracture during the charging and discharging process.Further,Si particles lose contact with the electrode,becoming an inactive substance.What's worse,volume expansion also causes the break and re-formation of SEI layer,leading to irreversible capacity loss.Those limit the commercial application of silicon materials as anode materials for Li-ion batteries.The porous structure inside the porous silicon can relieve the volume expansion effect and provide a transmission channel for lithium ions,which has high practical value.In this paper,based on micro-nano porous silicon,we design and prepare high-performance silicon-based anode materials through a simple and low-cost synthesis process,and study their structures and electrochemical performance.The results are as follows.We successfully synthesized three Si/C composites by combining micro-nano porous Si with three commercial carbon materials,including artificial graphite,flake graphite and soft carbon.The synthetic route is simple,low-cost and suitable for industrial production.All of the Si/C composites show a porous structure,which can not only release the suppression of volume expansion related to silicon but also provide transport routes for electrolyte and facilitate the diffusion of Li⁺.Compared to flake graphite and soft carbon,artificial graphite is the optimal matrix for porous Si,it can form a well-distributed mixture during the ball milling.The amorphous carbon coating on the surface of the composites can remarkably improve the cycling stability of Si/AG composite.The well coating amorphous carbon layer of Si/C-AG can promote the formation of stable SEI layer on the composites surface,which is advantageous for the long cycle life and better rate performance.The content of Si in Si/C-AG is about 21.5%.The Si/C-AG composite exhibits a reversible specific capacity of 473 mAh g^-1 and maintain 445 mAh g^-1 after 200 cycles at 0.5 A g^-1.It also shows excellent rate capability with 350 mAh g^-1 retention at a high current density of2 A g^-1.The morphology of the electrode sheet remained basically unchanged after 100cycles,showing excellent structural stability.This work will provide the reference for the selection of carbonaceous materials on facile and large-scale synthesis of Si/C composites.Using ball-milled micro-nano porous silicon as the silicon source,N-doped carbon cage-coated porous silicon?Si-NCC?materials were prepared by spray drying and pyrolysis carbonization.The Si-NCC material is based on a spherical carbon cage.Silicon is coated on a carbon matrix.NaCl is a sacrificial agent to create a large number of pores in the carbon cage.It can accommodate the volume expansion of silicon,reduce the pressure on the carbon cage,and maintain the carbon cage and SEI layer on the surface,reducing the irreversible capacity loss.The carbon cages are in close contact to form a three-dimensional conductive network.The internal pores are interconnected to form a network of holes,which provides a channel for the transmission of lithium ions and electrons,increasing the diffusion rate and enhances the conductivity of the material.N-doping makes more defects in the carbon structure,raise the reactive site,and improve the dynamic properties of the material,and ensure the excellent cycle performance and rate performance of the material.The content of Si in Si-NCC is about 48.5%.The initial capacity of Si-NCC material is 947 mAh g^-1,and the capacity is still 867 mAh g^-1 after 100 cycles.It still has 560 mAh g^-1 under the high current density of 5A g^-1,showing the prospect of commercial application.