| The specific capacity of commercial graphite/carbon anodes severely limits the further application of lithium-ion batteries,while silicon has the highest theoretical specific capacity of 4200 m Ah g-1,and it has a wide range of sources which can be regarded as the most potential alternative material.However,in the alloying process,the electrode exhibits the huge volume variation and internal stress,which causes the unstable material structure,the fracture of silicon material and the ever-increasing SEI.The semiconductor property of silicon also leads to poor electron and ion transport efficiency,which restricts its industrial application.In order to keep high capacity as well as ensure the integrity and the high rate performance of the electrode,we provides the internal voids to accommodate the volume change in response to the inherent defects of the silicon electrode,and the micro characteristics of the external carbon component can be easily tuned by the pitch types and heat treatment temperature.The carbon component with different microstructure was applied to the composite materials and the correlation between carbon microstructure and electrode performance was explored to acquire the best cycle stability and electron and ion transport properties.The specific research contents and results are as follows:(1)The industrial silicon and pitch were applied as the silicon and carbon source,the yolk-shell silicon-carbon composite material was prepared through a simple and controllable process;The different void/silicon core volume ratios were obtained by adjusting the oxidation parameters reasonably,and the cycle performance was explored.The research shows that:through the characterization means,the yolk-shell composite is successfully prepared,and the reserved voids can accommodate the volume expansion of silicon core,which improves the cycle performance.The electrochemical tests show that the material retains high capacity and good cycle characteristics when the oxidation degree is 70%,which can be used as the preparation process for subsequent research.(2)Taking three types of pitch with large differences in softening points as carbon sources,yolk-shell composites were prepared.The high structural adjustability of soft carbon structure made the composite electrode show different electrochemical behaviors.The research shows that:the structures of the three types of pitch pyrolytic soft carbon are different,and the carbon layer structure has an important influence on the cycle performance and rate performance of the electrodes when applied to the composites.Among them,P2-1050-Si@V@C shows the best electrochemical performance,with an initial discharge specific capacity of 1362 m Ah g-1,and it still retains a reversible capacity of 755 m Ah g-1 after 200 cycles.The rate performance and EIS result also showed the highest electron and ion transmission efficiency.(3)The microstructure of carbon layer was further fine-tuned,and the yolk-shell composites were prepared with P2 pitch precursor at a series of heat treatment temperatures.The optimal carbon structure was obtained,there is the correlation between the carbon component and the performance of the electrode.The research displays that:the graphitization degree of the carbon component increases with the increase of pyrolysis temperature gradually,and the pore channels decrease with the polymerization process,while Si C inert interlayer begins to form at 1150?,which seriously restricts the electrochemical performance of the electrode.As a result,P2-1050-Si@V@C shows the best cycle performance,rate performance and electrochemical reversibility,which can be attributed to the high crystallinity and suitable pore characteristics of the carbon shell which ensure the fast transmission of electron ion and the good storage characteristics of lithium ions.The material characterization results show that the void structure is still maintained after 70 cycles. |
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