Structure Design And Properties Of Silicon/carbon Anode Materials For Lithium Ion Batteries

Lithium-ion batteries have dominated the market for portable electronics such as mobile phones,laptops and digital cameras,and are considered the power source of choice for electric vehicles and fixed energy storage.Anode materials play an important role in lithium-ion batteries.Silicon has an extremely high theoretical specific capacity(4200m A h g^-1),an appropriate working potential（～0.4 V vs.Li⁺/Li）,and superior safety and energy density to commercial graphite.These advantages,combining with its mature processing process,make it superior to most other anode materials for cost-effective and high-energy lithium-ion batteries.However,commercial applications of silicon have been hampered by poor electrical conductivity and large volume change during the（de）lithiation processes.The large change in volume causes high internal stress,electrode pulverization,and the formation of unstable solid electrolyte film（SEI film）.Therefore,effective enhancement of electrical conductivity and maintenance of electrode structure stability are the key to improve the electrochemical performance of Si anode materials.Various silicon/carbon composites were prepared to enhance their electrical conductivity and to alleviate volume expansion.This paper mainly carried out the following experiments:（1）To improve the carbon coating method and design a certain structure to enhance the electrochemical performance of the whole material,S/N co-doped graphene nanoribbons（GNRs）wrapped Si nanoparticles composite（SN-GNRs/Si）was prepared by hydrothermal method.S/N doped graphene nanoribbons were prepared using thiourea as reducing agent and dopant,and Si nanoparticles were tightly encapsulated in SN-GNRs by electrostatic adsorption.the unique structure of the GNRs wrapping Si NPs and the synergistic effect aroused by S/N co-doped GNR3D network can alleviate the volume expansion effect of Si,increase the active site of Li⁺,and promote the transport of Li⁺and electrons.The experimental results show that SN-GNRs/Si has excellent cycling and rate performance and can deliver a high reversible specific capacity of 1137.8m A h g^-1 at 0.5 A g^-1 over 100 cycles.（2）In order to further improve the cyclic stability of the material and improve the material modification methods,composite of MOF-derived carbon coating silicon nanoparticles confined in graphene composite（Si/C@G）was designed and synthesized by in-situ growth method.In this structure,MOF-derived carbon is distributed on the surface of r GO,and the derived porous carbon can absorb electrolyte,which can provide a good diffusion channel for lithium ions and alleviate volume expansion.Graphene coating of Si/C further alleviates the volume expansion and poor conductivity of electrode materials.The experimental results show that Si/C@G can show excellent cycling and rate performance,and can reach a specific capacity of～677.2 m A h g^-1 after 500 cycles at 1.0 A g^-1 with the coulomb efficiency of 99.84%.（3）To reduce the production cost and simplify the preparation method,porous carbon coated Si nanoparticle/graphite composite（Si/C/Gr）was prepared by carbonization and mechanical mixing method.The carbon layer obtained by carbonization was uniformly coated on the surface of Si particles,which can effectively prevent electrolyte penetration,play the role of mechanical framework,limit the change of silicon volume,and then composite graphite,so that Si/C particles are evenly dispersed in the graphite matrix,and enhance the structural stability of the material,which can improve the electrochemical performance of Si-based materials.The results show that Si/C/Gr delivers excellent cycling performance,and the capacity of Si/C/Gr remains at 415.2 m A h g^-1 after 1000 cycles,which is higher than the theoretical specific capacity of graphite(372 m A h g^-1).