Preparation And Electrochemical Performance Of Porous Silicon Anode Of Lithium Ion Battery

Silicon has always been a hot spot in the study of anode materials for lithium-ion batteries due to its high capacity,and its theoretical specific capacity is 11.3 times that of traditional graphite anodes.However,the silicon material undergoes drastic volume changes during the process of lithium insertion and removal,resulting in pulverization and deactivation of the silicon material and repeated destruction and reconstruction of the solid electrolyte interface（SEI）membrane,which greatly reduces the cycle life of silicon.Therefore,how to alleviate the volume change of silicon materials has become the focus of the development of silicon anodes.In this subject,silicon particles are the first etched into porous silicon by means of noble metal-assisted chemical etching.In order to further improve the capacity retention rate of porous silicon,this topic also carried out carbon coating on porous silicon.The specific research contents are as follows:（1）Porous silicon was prepared by precious metal assisted chemical etching method.The porous silicon was prepared by screening silicon particles（1μm,5μm and 45μm）,etching time（0.5h,1h,2h and 3h）and concentration of hydrogen peroxide（H2O2）in etching solution（1wt.%,2wt.%and 3wt.%）.To explore the best experimental conditions for the preparation of porous silicon and test its electrochemical properties.The results show that when the size of silicon particles is 45μm,the etching time is 1 h,and the concentration of H2O2 in the etching solution is 2 wt.%,the electrochemical performance of the porous silicon material is the best（Hereinafter referred to as P-SI）.Scanning electron microscopy shows that the P-Si surface of this material has evenly distributed holes.The specific surface area and pore size tests show that the specific surface area of the unetched 45μm silicon particles is only 0.9m²·g^-1,and the P-Si specific surface area is 7.3m²·g^-1,indicating that the prepared porous silicon has a rich pore structure,which is beneficial to relieving.Electrochemical charge and discharge tests show that the first reversible capacity of 45μm unetched silicon particles is 1944 m Ah·g^-1,and the first reversible capacity of P-Si reaches2525 m Ah·g^-1.After 50 cycles of a current of 0.2A·g^-1,the reversible capacity of 45μm unetched silicon particles is 63.5 m Ah·g^-1,and the capacity retention rate is only 3.3%;the reversible capacity of P-Si is 233.5 m Ah·g^-1.The capacity retention rate is increased to9.2%.Compared with unetched silicon particles,the first reversible capacity and capacity retention rate of porous silicon are significantly improved.（2）The porous silicon P-Si prepared under the above optimal experimental conditions was carbon coated to investigate the electrochemical properties of carbon coated porous silicon.The aqueous solution of sucrose（SUC）has a certain viscosity,which is coated on the surface of P-Si through physical mixing,and carbonized to obtain carbon-coated porous silicon P-Si/C-SUC.In addition,sucrose was placed in the reactor by hydrothermal reaction and coated on the surface of P-Si at 200℃.After carbonization at high temperature,carbon coated porous silicon P-Si/C-HR was obtained under the condition of hydrothermal reaction.In this paper,polyvinylidene fluoride（PVDF）is selected as the carbon source,and the hydrogen bond between the F element and the hydroxyl group on the surface of P-Si is used to prepare the porous silicon-carbon composite P-Si/C-PVDF.Finally,using the polymerization properties of dopamine（DA）,it is polymerized on the surface of P-Si to form polydopamine（PDA）,and the composite P-Si/C-PDA is obtained after carbonization.XRD results showed that the peak shapes and peak positions of the four composites were consistent with P-Si,which proved that the carbon coating process did not change the lattice structure of porous silicon.The specific surface area and pore size test found that the pore size of the composite is between 2-12 nm,which is smaller than 16.8 nm of P-Si.The charge and discharge test of P-Si/C-SUC,P-Si/C-HR,P-Si/C-PVDF and P-Si/C-PDA shows that the first reversible capacity of the composite is 1949.6,1507.7,1908.6,respectively Compared with 1883.1 m Ah·g^-1,the reversible capacities after 50 cycles are 648.8,600.0,410.4 and 833.3 m Ah·g^-1,and the capacity retention rates are 33.3%,39.8%,21.5%and44.3%,respectively.Compared with the cycle performance Porous silicon materials have been greatly improved,of which P-Si/C-PDA materials have improved the most.The carbon content of the composite was obtained by thermogravimetric analysis to be 7.7%,17.5%,7.1%and 7.9%,respectively.The enhancement effect should be related to the carbon source and the coating method of p-Si.In addition,cyclic voltammetry tests of the complexes showed that their redox peak currents were all three to four orders of magnitude lower than those of uncoated porous silicon.The results indicate that the carbon coating reduces the electrochemical reaction rate of the composite,which makes the porous silicon no longer contact directly with the electrolyte,and is conducive to the formation of stable SEI film on the surface of the composite,thus improving the cycling performance of the material.