Preparation As Well As Application In Silicon Anode Of Carboxymethyl Cellulose-silk Fibroin Binder

Silicon(Si)is the first choice for anode materials of next-generation high energy density lithium ion batteries,due to its high theoretical specific capacity(4200 m Ah?g-1),low reaction potential and abundant natural resources.However,there is a huge volume change(>300%)in the process of lithiation and delithiation of the Si electrode,which leads to the cracking and pulverization of particles,the destruction of the electrode structure and the unstable solid electrolyte interface film(Solid Electrolyte Interface,SEI),which limiting its further application in commerce.The electrode can be regarded as a composite material,which is mainly composed of active material,conductive agent and binder.Although the amount of the binder as a component of the electrode composite material is small,it plays an important role in the electrochemical performance of electrode materials with volume effect.The interaction between the silicon active particles,silicon and the binder,and silicon and the current collector can be enhanced,buffer the volume expansion in the process of charge and discharge,maintain the integrity of electrode structure in the process of charge and discharge,by designing the molecular structures of binder,thereby improving its cycle and rate performance.Carboxymethyl cellulose(CMC),as a common water-based binder for silicon-based materials,has the advantages of good stability,low price,safety and environmental protection,as well as good dispersion of conductive agent.However,CMC has low viscosity,high brittleness,poor flexibility,and easy to crack during charge and discharge.In order to better adapt to the volume change of silicon-based materials,it needs to be further modified.In this theis,silk fibroin(SF)with good mechanical properties and rich surface functional groups was used to modify CMC water-soluble silicon-based binder.The mechanical properties of CMC and its interaction with silicon particles were improved by molecular structure design,crosslinking method and surface and interface control of silicon particles,so as to improve the cyclic stability of silicon-based materials.The main research contents and innovations are as follows:1.The CMC-SF composite binder was prepared by blending SF with CMC.The effects of the addition amount of SF on the viscosity,mechanical properties,dissolution swelling,peel strength,and theelectrochemical performance were investigated.The research results show that the mechanical properties of CMC binder are significantly improved and the interaction between the binder and the surface of silicon particles is enhanced by the addition of SF.When the addition amount of SF is 6 wt%,the optimal electrochemical performance is obtained.The specific capacity of 1182 m Ah?g-1 can be achieved at the current density of 5 A?g-1,and the specific capacity of 1138 m Ah?g-1 after 50 cycles at a current density of1 A?g-1.However,there is only hydrogen interaction between the binder and the surface of silicon particles,which leads to weak force and limited performance improvement.It is necessary to further improve the interaction mode between the binder and the silicon surface to improve its cycle stability.2.In view of the weak interaction between the synthesized CMC-SF binder and the surface of Si nanoparticles,borax was used as a cross-linking agent to chemically crosslink SF and CMC to prepare a binder with a three-dimensional structure.The research results show that the addition of the cross-linking agent can cause the chemical cross-linking between SF and CMC,and the covalent interaction between the binder and the active material during the preparation of the slurry.The strong interaction between the binder and the active material increases the stability of the silicon electrode during the cycling and maintains the structural integrity of the electrode,which is beneficial to the formation of stable SEI film.The constructed three-dimensional structure binder has good rate performance and cycle performance when used as a binder of the silicon-based electrode materials.The specific capacity reaches1652 m Ah?g-1 at the current density of 5 A?g-1,and the specific capacity of 1272 m Ah?g-1 after 100 cycles at the current density of 1 A?g-1.3.On the basis of the above research,in order to further increase the interaction sites between the Si particles and the synthesized composite binder,polydopamine modified Si particles were prepared.Because of the catechol structure in the polydopamine,it can provide a stronger binding force with the binder and collector.At the same time,a certain thickness of polydopamine modification can also be used to buffer the volume expansion of silicon,avoiding silicon and electrolyte direct contact,thereby improving the electrochemical stability of the silicon anode.