Research On Interface Replacement And Performance Optimization Of Silicon-based Anode Materials For Lithium Ion Batteries

The development of portable electronic equipment to electric vehicle technology has created a huge market demand for lithium ion batteries?LIB?.Compared with other battery systems,lithium-ion batteries have higher energy,power density and better long-term cycle performance,so it has been continuously concerned by researchers.The structure of a lithium-ion battery is very simple.It is composed of a positive electrode,an electrolyte,a separator,and a negative electrode.At present,the negative electrode material mainly based on graphite has been difficult to meet people's storage requirements due to its maximum theoretical specific capacity upper limit(372 mAh g^-1).So the development of higher specific capacity anode materials has become a key factor in breaking the bottleneck of lithium-ion battery development.Silicon,which has abundant reserves in the earth's crust and has a very high theoretical specific capacity(4200mAh g^-1),is considered to be the most likely to replace graphite as the anode material of the next-generation lithium-ion battery.However,the silicon anode material will undergo a great volume change?up to 300%?during charging and discharging,which will cause the silicon material on the electrode surface to pulverize and fall off from the current collector,so that the active materials will loss of electrical contact with electrode and continue to form a new solid electrolyte membrane?SEI?,which will eventually result in reduced coulomb efficiency,rapid capacity decay,and reduced electrochemical performance.In order to overcome these problems,this article has conducted a series of explorations and researches on the SEI film of the silicon anode material.The thickness,composition and performance of the SEI film are improved by means of electrolyte additives and surface coating,and a relatively stable SEI film and better electrochemical performance are obtained.The main research contents are as follows:?1?First,we used triallyl phosphate?TAP?as a lithium-ion battery electrolyte additive in silicon/graphite composite electrodes.The Coulomb efficiency,long-term cycle and rate test shows that the 3 wt%TAP has the best electrical properties.Cyclic voltammetry tests show that TAP has a significant improvement on the polarization of silicon/graphite electrodes.SEM observed that the SEI film formed by TAP is more uniform.XPS test shows that the composition of SEI has changed after the formation cycle,which is believed to be caused by the electrochemical polymerization reaction of TAP on the surface of the pole piece.?2?The second work investigated the vinyl sulfate?DTD?as an electrolyte additive to improve the electrochemical performance of silicon/graphite composite anodes.After adding different amounts of DTD to the electrolyte solvent with EC/EMC/DMC,it can be observed that the first Coulomb efficiency,long-term cycle and rate performance have been significantly improved.Through the cyclic voltammetry test,it can be found that the DTD is successfully involved in the formation of the SEI film.The electrochemical impedance test shows that the electrolyte containing the DTD has a lower film formation resistance after high-rate cycling,indicating that the resulting SEI film is more stable and thinner.Scanning electron microscopy observed that in the electrolyte with 3wt%DTD added,the SEI film of the silicon/graphite electrode is more uniform and the cracks are smaller.X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyzed the change of DTD on the composition of the SEI film,and confirm from the side that the SEI film grown by the electrolyte containing DTD additive is thinner.?3?The third work applies layer-by-layer self-assembly technology?LBL?to the surface coating of silicon-based negative electrode materials for the first time.The structure and composition of SEI is controlled by controlling the number of different coating layers and selecting compounds with different functions.Through high-resolution TEM,EDX,infrared spectroscopy and other characterization methods,it can be clearly observed that the multilayer self-assembled film is successfully assembled on the surface of the nano-silicon particles.From the long-term cycle,rate and first Coulomb efficiency electrical performance tests,it can be seen that the self-assembled membraneasd with four-layer structure has the best electrochemical performance.Through the analysis of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy,the layer-by-layer self-assembly technology can adjust the structure and composition of the SEI film to obtain more excellent electrochemical performance.This paper proposes some new electrolytic additives and a new coating strategy?LBL technology?to overcome the problems of low coulombic efficiency and poor cycling performance of commercial Si nanoparticles.Through these methods,the thickness,properties,composition and structure of the SEI film can be effectively adjusted,so that the rate performance and cycle performance of the SI-based negative electrode material can be improved,and a new vision is provided for the commercialization of the silicon-based negative electrode material.