| With the increasing demand for energy storage technology in the fields of portable electronic devices,hybrid and electric vehicles and energy storage power stations,the development of lithium ion batteries with high energy density,high safety and long cycle life has become the focus of research on electrochemical energy storage.Anode material is critical for lithium ion batteries,to which the energy density,safety performance and stability of the battery are closely related.Silicon-based materials have the advantages of high theoretical specific capacity,moderate reaction potential,abundant reserves and mature preparation technology,which have become the most promising anode materials for lithium ion batteries.However,the large volume effect(～400%)of silicon in the process of Li~+insertion/extraction and its own low conductivity hinder the commercial application of silicon.To solve the shortcomings of silicon-based anode materials and improve their electrochemical performance,two different silicon-based composite materials are prepared based on the surface coating modification technology and preparation method of the materials.The main research contents are as follows:Firstly,a synthesis method of Si@void@NPC composite with N/P co-doped carbon layer was developed.The silica template layer was introduced on the silicon surface by sol-gel method.The polyaniline conductive gel doped with phytic acid was used as the carbon source of nitrogen and phosphorus co-doping.After high temperature carbonization and selective ethching,the porous Si@void@NPC composite with nitrogen and phosphorus co-doping was obtained.By optimizing the experimental parameters of the concentration of polyphytic acid,the content of aniline and the carbonization temperature,the preparation conditions of Si@void@NPC composites with the best electrochemical performance were obtained:aniline content of 7 mmol,phytic acid concentration is 0.07 mol/L,carbonization temperature for 700℃.When the charge/discharge current is 0.4A/g,the initial specific discharge capacity of Si@void@NPC composite is 2411 m Ah/g,and the coulomb efficiency is 61.4%.After50 cycles,the specific discharge capacity is 924 m Ah/g,and the capacity retention rate is 38.3%.When the current density is 2 A/g,the specific discharge capacity is still 459.4m Ah/g.The improvement of electrochemical performance is mainly due to the defects formed in the carbon layer when the nitrogen and phosphorus co-doped carbon improves the electrical conductivity of the composites.These defects can provide additional lithium storage capacity and enhance the electrochemical capacity and cycling stability of the composites.Compared with nitrogen doping,the relative content of active nitrogen(pyridine nitrogen and pyrrole nitrogen)in the carbon layer was increased by N/P co-doping,and the charge transfer impedance of Si@void@NPC composite was also decreased.Secondly,using micron silicon,conductive polyaniline(PANi)and graphene oxide(GO)as raw materials,through a one-step ball milling method,silicon/polyaniline/GO composite material was efficiently prepared.The micron silicon was breaken i into nano size during ball milling and the reactivity was improved.As a buffer matrix,graphene oxide is reduced by the high temperature generated during ball milling,which improves its electrical conductivity and acts as a conductive matrix.The polyaniline acts as a"glue"that binds the silicon to GO to form a stable three-dimensional structure while inhibiting the agglomeration of the silicon.Through single factor batch experiments,it is found that the Si/PANi/GO composites have relatively high discharge specific capacity and cycle stability when the PANI content is3%and the GO content is 10～20%.In addition,electrolyte additive FEC and milling speed have great effects on the electrochemical properties of Si/PANi/GO composites.When the milling speed is 300 rpm and 10%FEC is added to the electrolyte,the composite has higher discharge specific capacity and good cycle stability.At 400 m A/g current density,the discharge specific capacity is still up to 1593 m Ah/g after 80 cycles with capacity retention rate is 53.5%. |
