Study On Structural Design And Electrochemical Performance Of High-Performance Silicon-Carbon Composite Anode Materials

Lithium-ion batteries（LIBs）are widely used in many applications such as portable devices,photovoltaic energy storage and new energy vehicles due to their advantages of high output voltage,long cycle life and no memory effect.However,with the increasing requirement for long-range applications,the task of improving the energy density of battery systems is urgent.Increasing the specific capacity of materials is one of the most important ways to improve their energy density among many measures.For anode materials,silicon（Si）anodes are considered as ideals for high energy density battery systems due to their abundant reserves,environmental friendliness and high theoretical specific capacity(4200 m Ah g^-1).However,the huge volume change of the Si-based anode during cycling leads to structural pulverisation of the electrode material,which can destroy the cycling stability of the battery.This problem has become a"bottleneck"to limit the scale up of the Si-based anode.Therefore,the design of Si-based composites with high capacity and long cycle stability has become focus of current research.In this thesis,four different silicon/carbon（Si@C）composites with excellent performance were prepared firstly,and the key mechanisms for the comprehensive performance enhancement of the composites were investigated systematically.Then,the nano silicon/graphene/carbon（SiNPs@graphene@C）with a"popcorn"structure has the best overall performance.The initial coulomb efficiency（ICE）and cycling performance were further improved by pre-lithiation with a home-made Li-aromatic complex.Finally,the compatibility of the newly developed silicon-carbon composites with high nickel cathode materials was verified by assembling full cells.Details of the study are as follows:（1）To suppress the volume change of the silicon anode electrode during cycling and improve the electrochemical performance of the electrode material,the effect of the porous structure introduced in situ in mitigating the volume expansion of the silicon electrode was investigated.Specifically,porous silicon/carbon（porous Si@C）composites were prepared in situ by a high energy ball milling process using silicon nanoparticles as the active material,recyclable Li Cl as the template agent and high softening point bitumen as the carbon precursor.The electrochemical performance test results show that the prepared porous Si@C has the best overall electrochemical performance when the template dosage ratio is 10%.The material still has a high reversible specific capacity(642 m Ah g^-1)after 50 cycles at a current density of 0.2 C.By characterising the thickness change of the electrode before and after cycling,it was found that the electrode expansion of the porous Si@C composite（15.38%）was much lower than that of both pure silicon（162.46%）and conventional Si@C negative electrodes（40.24%）.This indicates that the porous structure provides an effective buffer for the volume expansion of silicon and reduces the volume expansion of the electrode during charging and discharging,thus inhibiting the destruction of the solid electrolyte interface（SEI）film at the porous Si@C electrode interface and maintaining the stability of the electrode structure and interface.（2）To eliminate the influence of the high specific surface area of porous materials on the formation of SEI film,a composite silicon/graphite/carbon anode material（Si@Graphite@C）with a"sandwich"structure was prepared by a combination of mechanical moulding and wet process using graphite with good electrical conductivity and low specific surface area instead of porous carbon.The in situ electrochemical impedance spectroscopy（EIS）and other characterisation means were used to investigate the mechanism of the electrolyte/electrode interfacial reaction on the electrochemical performance at different ambient temperatures,and the results showed that the ambient temperature is critical to the formation and evolution of the SEI film.At room temperature（25°C）,the formation of SEI film on the electrode surface was relatively stable and favourable to charge transfer.At a high temperature（55°C）,a large amount of organolithium salts dissolved,causing a continuous reorganisation of the SEI structure and leading to a more prominent inorganic salt component.At a low temperature（0°C）,the lower solubility of the lithium salts leads to poorer kinetic properties and consumes a larg e amount of Li⁺in the first cycle,forming a relatively dense SEI and leading to a reduction in the coulomb efficiency of the material.（3）In view of the above work,it is found that although the"sandwich"structure has a good buffering effect,the specific capacity of the material is low due to the small specific surface area of the graphite particles,resulting in a limited loading of silicon particles on the surface.Increasing the loading amount will affect the stability of the electrode structure.Based on this,SiO_xparticles with a larger particle size were chosen to replace the nano-Si,and the SiO_x particles were anchored to the surface of the graphite particles by a combination of chemical vapour deposition and mechanical moulding to form a composite particle with a"mosaic"structure.The composite particles combine the high electrical conductivity of graphite with the low expansion of SiO_x,successfully increasing the bonding strength between the electrode particles and improving the overall electrochemical performance of the material.The material exhibits a high reversible capacity（2698.8 m Ah）and excellent cycling stability（76.9%capacity retention after 500 cycles）in full battery applications.（4）These results demonstrate that the presence of a porous structure can effectively improve the structural stability of the material,the introduction of graphite can improve the overall electrical conductivity of the electrode material,and SiO_x can reduce the volume change of the material itself.Based on this,in this section,silicon nano particles（SiNPs）/graphene/carbon composites（SiNPs@graphene@C）with a"popcorn"structure were prepared by a combination of spray drying and wet process using highly conductive graphene as the substrate.The structure combines the properties of porous structure（popcorn shape）,high electrical conductivity（graphene substrate）and low expansion（SiNPs and SiO_x）,which effectively improves the overall performance of the electrode material.The results show that the SiNPs@graphene@C electrodes exhibit excellent multiplicative properties and good electrochemical stability.The charge specific capacity reaches 777.2 m Ah g^-1 at a current density of 500 m A g^-1.After 200 cycles,the discharge specific capacity reaches 692.8 m Ah g^-1 with a capacity retention rate of 83.6%.（5）Based on the above four parts,it is found that the electrochemical performance of Si-based anode materials has a common shortcoming of the low ICE.In order to improve the ICE of Si-based anode,a pre-lithium reagent suitable for Si-based composite was prepared to improve its ICE.The lithium aromatic complex was prepared by reacting ortho-terphenyl with excess lithium metal in a 2-methyl tetrahydrofuran solvent.The application of the reagent to SiNPs@graphene@C composites with a"popcorn"structure shows that the pre-lithium reagent has a high reducing ability and can increase the ICE of SiNPs@graphene@C composites from 65.7%to 86.5%.Finally,the compatibility of the newly developed SiNPs@graphene@C anode material with high nickel cathode material was verified by assembling the full battery.The results show that the SiNPs@graphene@C||NCM 811 batterie has a higher energy density(340 Wh kg^-1)and good cycle stability.