Research On The Influence Of Silicon-Carbon Composite Structure On Electrochemical Performance Of Lithium-ion Batteries

With the rapid development of economy and society,it has been difficult for traditional lithium-ion batteries to meet the growing requirement for high energy density.Silicon(Si)materials have been considered as one of the best anode candidates for next-generation lithium-ion battery with high energy density,due to their high theoretical specific capacity(3579 mAh g-1 at room temperature).However,huge volume expansion of Si materials during lithiation process will cause particle pulverization,instability of solid electrolyte interface(SEI)film,and poor electrical contact.These issues would lead to rapid decline in specific capacity of Si anodes,hindering the development and application of the Si materials.This work is devoted to effectively structural construction for silicon-carbon composites using low-dimensional carbon with high mechanical strength and conductive polymers with high resilience in a facile and efficient way.Moreover,the formation mechanism of silicon-carbon composites and the interface interaction between Si and carbon have been systematically investigated.The intrinsic correlations between specific structure and the electrochemical behaviors of silicon-carbon composite anodes have been analyzed.Ultimately,silicon-carbon composite materials with high specific capacity and long cycle life have been successfully acquired.The main research contents and results are as follows:(1)To fully explore the respective advantages of Si materials and carbon fibers(CNFs)in high specific capacity and high elasticity,UV-ozone surface modification followed by electrostatic self-assembly has been developed to prepare new Si-CNFs composites.Compared with traditional acid pickling of CNFs,the UV-ozone exposure of CNFs can maintain their internal structure without contamination.Moreover,the UV-ozone surface modification technology can expand the effective contact area between the CNFs and Si materials,achieve an interwoven structure of the silicon-carbon composites,and improve the uniformity of Si distribution on the CNFs.This strategy provides a new route to prepare uniform silicon-carbon composites.(2)In order to further improve the distribution uniformity and thus enhance the cycling performance of silicon-carbon composites,two-step electrostatic self-assembly technology has been used to adhere the Si materials to either CNFs or polymer CMC chains through strong surface interactions on the basis of the study above.Subsequently,entire Si materials can be embedded in the interconnected CNFs and amorphous carbon through the heat treatment process,forming a dual protection for the Si materials.Thus,a silicon-carbon composite(Si-CNF@C)with uniform distribution and interlaced structure has been prepared.It is found that there exist effective defects and pores in the Si-CNF@C composites,which would be conducive to lithium-ion diffusion.In addition,synergistic effect of amorphous carbon and CNFs enhances the stability of SEI membrane during the cycling of Si-CNF@C composites.At the current density of 0.5 A g-1,the discharge specific capacity of the Si-CNF@C anode can be maintained at 849 mAh g-1 after 1000 cycles,and the capacity loss rate per cycle is only 0.036%.The results indicate that such an interleaving structure with dual protections for Si materials can effectively ensure the structural integrity and electrochemical stability.It thereby provides a theoretical and experimental basis for the preparation of high-performance silicon-carbon anodes.(3)As for the enhancement of the interfacial adhesion between silicon-carbon composites,a new silicon-carbon composite(Si@C/PANI-CNT)with three-dimensionally hierarchically interconnected architecture has been synthesized via in-situ polymerization,electrostatic self-assembly followed by heat treatment technologies.In the Si@C/PANI-CNT composites,Si particles can be encapsulated in a three-dimensional conductive network composed of conducting polymer polyaniline(PANI),flexible carbon nanotubes(CNTs),and amorphous carbon.At the current density of 0.5 A g-1,the discharge specific capacity of the Si@C/PANI-CNT anode can be maintained at 879 mAh g-1 after 500 cycles,the volume expansion rate is only 37.6%at 0.5 A g-1 after 50 cycles.Even if using high current density of 1 A g-1,the Si@C/PANI-CNT anode still maintains a high specific capacity of 605 mAh g-1 after 1000 cycles.The results demonstrate that the three-dimensionally hierarchically interconnected architecture can well inhibit the volume change of Si particles during the lithiation/delithiation process,thus ensuring the structural stability and cycle durability of the silicon-carbon anodes.(4)With the purpose of the suppression of microcracks caused by volume changes of Si anodes and the ensurance of the effective electrical contact of the active materials with a collector,silicon-carbon composites(Si-PAA-PANI)in a dual-network structure with both conductive and self-healing functions have been successfully fabricated by using Si materials,polyacrylic acid(PAA),and aniline monomer though in-situ polymerization technology.PAA-PANI polymers with excellently mechanical properties(the strain can reach 690%)can effectively relieve the stress generated by the Si-PAA-PANI composites during the process of lithiation and delithiation.The interfacial interactions of hydrogen bonding and electrostatic attraction between Si and PAA-PANI enable to inhibit the microcrack propagation on the Si-PAA-PANI anodes.At the current density of 0.5 A g-1,the discharge capacity of the Si-PAA-PANI anode can be maintained at 952 mAh g-1 after 600 cycles.The discharge specific capacity of 600 mAh g-1 can thereby remain at 2 A g-1 after 800 cycles.The results confirm that the Si-PAA-PANI anodes with dual-network structure possess good cycle stability at high current density.The relationship between structural stability and electrochemical performance has been revealed through density functional theory calculation(DFT).It provides a new approach and strategy for constructing high-performance of silicon-carbon anode materials with high specific capacity and long cycle life.