| Pure silicon theory lithium storage capacity up to 4200 mAh / g,low discharge voltage,abundant reserves and excellent safety performance have became the research hotspot of anode materials for lithium ion batteries.However,Si in deep lithium alloy/dealloy cycles generate huge volume expansion,electrode material becoming easy to chapped and causing the loss of electrical contact between active materials and the electrode framework,to make the the poor cycle stability;and Si materials first coulombic efficiency is low,its poor conductivity all shortcomings limits the lithium ion battery commercial application.Improve the conductivity of silicon-based materials,buffering volume expansion in the process of lithium insertion to improve the cycle stability becoming the main direction to overcome the shortcomings of silicon-based materials.In this paper,micro SiO,NanoSi as the silicon source using different methods to prepare silicon / carbon composite materials,as the cathode material for lithium ion batteries;in addition,the different binder of silica based materials were also studied.Through the use of a thermogravimetric analyzer(TGA),X ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),scanning electron microscopy(SEM)understanding the structure and morphology of materials before and after processing.As the anode electrode materials for lithium ion batteries,the electrochemical performances of the composites were measured by the constant current charge and discharge,cyclic voltammetry(CV)and electrochemical impedance spectroscopy(EIS),and the electrochemical performances of the composites were studied.The ball milled SiO was coated with carbon source and then disproportionate,at the same time,carbon source carbonization,hydrofluoric acid etching at different time(Or first disproportionate,then etching,and then coated with carbon source,finaly carbonization).SiO composites were finally obtained,mixed with PAN then coating,electrode at different temperature heat treatment.After testing and analysis of the composite,it is concluded that the SiO disproportionate at 1100°C then carbon coating,etching 10 min,the eletrode heat treatment at 500°C to obtain the best electrochemical performance of the treatment conditions.And the capacity of sample su@SiO.10.500 after 100 discharge cycles is 771.3mAh/g,sample dis-su@SiO.10.500(sample number details see 2.5,the same below)100th discharge capacity is 763.7mAh/g;So in the best treatment of carbon source coating sequence,the electrochemical performance of the composite is not much affected.SiO disproportionate at 950°C then carbon coating,etching 0min,the eletrode heat treatment at 500°C to obtain the best electrochemical performance of the treatment conditions.The discharge capacity of sample rf@SiO.0.500 after 90 discharge cycles is 1090.4mAh/g.SiO disproportionate at 1100°C generating silicon crystal particle size in 400nm~2?m,need to be etched by 10 min to make the silicon crystal in the cycle process has sufficient buffer space,and disproportionate at 950°C of the generation of the silicon crystal particle size is only 100~200nm.The larger the Si crystal particles,the more serious the volume expansion,so it is beneficial to improve the cycle performance of the composite materials by selecting the suitable temperature to produce the silicon crystals with moderate size.The NanoSi and graphite were modified by surface modification,and the ester bond between the NanoSi and graphite was combined together,which made the composite material with stable cycle performance.The preparation of core-shell structure with NanoSi,hollow can accommodate of silicon volume expansion in cycle process.Surface modified composite electrode treatment temperature 300°C,sample m.g.300 the first time capacity is 1550.9mAh/g,the initial coulombic efficiency is only 44.6%,but the subsequent cycle performance is very stable,at 50 th cycle the discharge capacity is 634.3mAh/g.Carbon / silicon core-shell composite silicon content is 11.18%,the initial discharge capacity of sample C@Si core-shell.400 is 1280.3mAh/g,the initial coulombic efficiency is only 55.1%,but the subsequent cycle performance is stable,and the discharge capacity is 406.3 mAh/g after 100 cycles.The two kinds of composite materials have stable charging and discharging cycle,which provides a way for the practical application of NanoSilicon.However,the problem of low coulombic efficiency of silicon materials is still not solved.Adhere to the concept of greens environmental protection for silicon-based materials for the study of water-based binder.Using NanoSi/ graphite as active material,the performance of different binder being compared,and the best charge discharge cycling performance was composite binder polypropylene acid: polyvinyl alcohol =9:1.Therefore,sample PAA:PVA=9:1 this binder through further research and improvement is expected to become applicable to the silicon-based materials. |
PDF Full Text Request