Study On Fabrication And Properties Of Porous Si@C Composite In Lithium Ion Anode Materials

Lithium-ion batteries?LIB?have excellent performances,such as large capacity,high conversion efficiency,environmental friendliness and good cycle life,which have been attracted attention by many researchers.Based on the common optimization of the various components inside the battery,the performance of lithium-ion batteries can be improved.The negative electrode material,as one of the important components of the lithium-ion battery system,is significant for the improvement of the performance of lithium batteries.Silicon material has the characteristics of high specific capacity and low lithium insertion potential,which can be one of the most promising materials for anode materials.However,the alloying/de-alloying during charging and discharging is limited for the application and development due to its serious volume effect.In this work,we fabricated a lithium-ion battery composite material aiming at solving the defects of silicon anode materials.Based on the spray drying process,a core-shell structured silicon-carbon composite anode material was obtained after the metal silver was induced corrosion by low-cost micro-silicon and porous silicon particles was coated by a layer of pyrolytic carbon shell,which can further improve the performance of the composite material by changing the type of the binder.The microstructure and phase composition of the composites can be characterized by BET,XRD and SEM.The electrochemical properties,including the constant current charge and discharge,impedance analysis and cyclic voltammetry,are listed as follows:?1?Using micro-silicon etching by metal-induced chemical etching,nano-mesopores can be successfully formed on the surface of silicon to obtain a three-dimensional sponge-like porous silicon anode material.The pore structure can alleviate the volume effect of the silicon anode material to some extent,and improve the conductivity of the material and the diffusion rate of lithium ions between the positive and negative electrodes.Compared with the unetched micron,the first capacity and the cycle life of the porous silicon electrode,with a good corrosion degree of 2mol AgNO₃+3mol HF,is greatly improved.However,the battery capacity decays rapidly as the number of charge and discharge increases.?2?Using a water-soluble sulfonated pitch as a carbon source,the three-dimensional porous silicon is coated to form a spherical core-shell structured silicon-carbon composite anode material.The porous silicon was coated by the sintered pyrolytic carbon layer under the high temperature,enhancing the diffusion performance of lithium ions and the ability to accommodate volume expansion voids.The Si@C composite electrode has a specific capacity of 3810mAh/g for the first time.Meanwhile,the specific capacity of the discharge at 100C for 1C current density can still be 824mAh/g.In addition,the coulombic efficiency is maintained at99%.Moreover,the specific capacity of the discharge to 0.1C was maintained at1210mAh/g under the rate test.The results of SEM indicates the morphology of the electrode was favourable and there is no separation of the active material from the current collector and cracking of the pole piece,meanwhile,the capacity is maintained at 1210 mAh/g under the discharge ratio restored to 0.1C.?3?The effects of SBR+CMC,PVDF and PVP adhesives on the electrochemical properties of Si@C pole pieces were investigated based on the bonding system changed by active materials with Si@C composites.The observation of SEM showed as follows:The surface of the PVP adhesive group was very rough and many cracks exist in the surface;The PVDF adhesive group can obviously observe the conductive network formed by the spherical Si@C composite and carbon black.Furthermore,the performance of film formation in the SBR+CMC adhesive system is best due to the hydrogen bonding force in spite of the active material is covered by a film.The electrochemical and rate properties and impedance test indicate that the PVDF adhesive group has the highest specific capacity for the first time,and the SBR+CMC bonding system group is far superior to the other two groups in the cycle performance.The specific discharge capacity after 30 cycle test was increased by 29%compared with the PVDF adhesive group under the 30 mA/g current density;At the 2C rate,the first specific capacities of the three sets of half-cells?SBR+CMC,PVDF,PVP?were668.6mAh/g,401.3mAh/g and 257.2mAh/g,respectively,and charge transfer impedance(R_ct)is 28?,37?and 68?,respectively.