Preparation And Electrochemical Performance Of Silicon/Carbon Composite Materials For Lithium Ion Bstteries

Silicon has been regarded as one of the most promising anode materials for next generation lithium ion batteries,due to its high theoretical specific capacity(4200 mAh g-1),low lithiation/delithiation potential and natural abundance.Howener,because of its large volume expansion(>300%)during lithiation-delithiation,silicon anode may potentially suffer from mechanical cracking,pulverization and instability of the solid electrolyte interphase,which significantly limits its cycle performance.And the poor conductivity of silicon will affect its rate perpformance.Gernrally,preparing nanosized Si and combing Si with second phase to form composite anode are demonstrated as efficient approach to improve the electrochemical performance.In this paper,we fabricate Si@C composite materials based on the industrial silicon powder(600 mesh)via a high energy ball milling combing with in-situ carbon coating(carbonization)methord.Then Si@C/graphite(Si/C)composite anode materials are prepared by a simple mechanical ball-milling approach.As a control,nano-Si/C composites(nm-Si/C)are prepared using the same method.At the same time,the effects of carbon coating content and proportions of Si/graphite on eletrochemical performance are investigated systematically.Compared with nano-Si/graphite composites,the cycle performance and rate performance of Si@C/graphite composites can be significantly improved.Si@C materials is the composite of amorphous C and crystal silicon,the primary particles size is about 100-200 nm.Compared with the original industrial micro-size silicon material,the cycle performance is improved.Carbon coating has singnificant effects on the morphology and electrochemical performance of the composite materials.Carbon coating is able to effectively suppress the aggregation of Si nano-particles formed during high energy balling,and improve the electronic conductivity of Si,thus improve the electrochemical performance of Si@C compostie materials.Si/C-2-1 composite shows high reversible specific capacity,good rate performance and cycling stability.When cyclyed at 0.1C,Si/C-2-1 shows a reversible capacity of 492 mAh g-1,and a capacity retention of 85.8%after 100 cycles.When cyclyed at 1C,the reversible discharge capacity reachs 369.7 mAh g-1,corresponding to 73.9%retention of that at 0.1 C.Si/C-2-3 which contains 20%silicon shows a higher reversible capacity of 600.4 mAh g-1 whencyclyed at 0.1 C.However,the cycling stability of these composites decreases with increasing Si content,indicating that the graphite content plays an important role to improve the cycle performance of the composite.Si/CNT/graphene(Si/CNT/G)ternary composites are synthesized by a simple spray drying technique,the effects of nano carbon tubes(CNTs)and graphene sheets on electrochemical performance of the composites are investigated.The as-prepared Si/CNT/G composite shows a three-dimensional structure,and silicon nano-particle are uniformly distributed in this three-dimensional structure formed by graphene sheets and CNTs.After compositing with graphene sheets and CNTs,the electrochemical performance of nano-Si is obviously improved.Si/CNT/G shows improved cycling stability and rate capability.The dQ/dV and EIS results show that Si/CNT/G composites can reduce the impedance of battery and improve structure stability of the electrode during charge and discharge process,thus improve the electrochemical performance,especially the rate performance and the cycling stability.