Research On Preparation And Stability Of Carbon/Silicon Anode Materials

Lithium-ion batteries are widely used in portable electronic devices due to their long cycle life,high discharge voltage,low cost,and low pollution.The rise of electric vehicles has driven the development of high energy density lithium-ion batteries.Graphite is commercialized due to its good cycle stability and high safety performance,but its theoretical specific capacity is low(372 m Ah g^-1),which cannot meet the demand of high energy density lithium ion batteries.Silicon is the most attractive lithium-ion battery anode material for the next generation of graphite replacement because of its abundant resources,low cost,low lithiation/delithiation voltage（<0.5 V）and high theoretical capacity(4200 m Ah g^-1).However,silicon has the following two problems:First,silicon undergoes huge volume changes during multiple charge and discharge processes,and loss of electrical contact with the current collector,formation of an unstable SEI film,resulting in poor cycle stability of silicon.Second,the electronic conductivity of silicon is poor,and these two disadvantages severely limit the application of silicon.Based on the above problems,in this paper,Si/C composites containing silicon-manganese alloy phase are prepared by thermal reduction method.The presence of silicon-manganese alloy and soft carbon material can not only buffer the volume change of silicon during charging and discharging,but also improve the electrical conductivity of the composite.This paper mainly explores the effects of different factors（carbonization temperature,carbon content,manganese addition amount,KOH activation treatment,carbon source type,etc.）on the stability properties of composites.The conclusions reached are as follows:（1）The Si/C composite containing silicon-manganese alloy prepared by carbonization at 850°C at a ratio of silicon to manganese of Si:Mn=9:1,a carbon content of 10 wt%is a typical core-shell structure.The silicon and silicon-manganese alloys are used as the core,and the carbon coating on the surface is the outer shell.The combination of silicon-manganese alloy and core-shell structure can alleviate the volume change of silicon during cyclic charging and discharging,at the same time,and the presence of carbon material improves the conductivity of the composite.The Si/C composite containing silicon-manganese alloy phase has a specific capacity of up to 960 m Ah g^-1after 100 cycles at a small current density（100 m A/g）,and still has a reversible specific capacity of nearly 460 m Ah g^-1at a high current density of 1 A/g.The composite has good cycle stability and rate performance.（2）The core-shell structure of the Si/C composite material containing silicon-manganese alloy remains unchanged after KOH treatment,but the carbon coating on the outer surface is changed from the original dense non-porous to the loose porous,and the structure is more advantageous for the migration of lithium ions during the lithiation/delithiation of the electrode material.The Si/C composite with PVP added and the KOH activation treatment has excellent cycle stability,rate performance and electrical conductivity.It has a stable specific capacity of more than1300 m Ah g^-1after 100 cycles at 100 m A/g,even with a high current density of 1 A/g,it still has good lithiation/delithiation capacity with a reversible specific capacity of approximately 730 m Ah g^-1.（3）The introduction of graphene makes the Si/C composite material more uniform and improves the electrical conductivity of the composite material.The Si/C composite materials with phenolic resin,polyacrylonitrile and polyaniline as carbon sources are non-porous core-shell structure,porous structure with small pore diameter,and porous structure with large pore diameter,respecctively.The Si/C composite prepared with polyaniline as carbon source has higher cycle stability,better rate performance and lower impedance.The cycle specific capacity was maintained above1350 m Ah g^-1at a current density of 100 m A/g,and the reversible specific capacity at a high current density of 1 A/g was about 800 m Ah g^-1.