| High-performance rechargeable batteries with long driving range and short charging time are of great significance for the rapid development of the new energy vehicle industry.Silicon-based materials have a very high theoretical capacity,which can greatly meet the development needs of the new energy automotive industry.In this paper,two F-doped silicon-based composites precursors,AHFS-SiO2/GO and AF-SiO2/GO,were successfully prepared by direct addition and dipping combined with the method of supercritical drying,Using AHFS and AF as fluorine sources respectively.Then,the precursor and magnesium powder were mixed to reduce SiO2 in the material to prepare AHFS-Si/SiO2/rGO and AF-Si/SiO2/rGO composite materials.EDS analysis showed that the elements in the AHFS-Si/SiO2/rGO and AF-Si/SiO2/rGO composites were evenly distributed.The fluorine source was successfully introduced into the composite and distributed evenly in it.Scanning electron microscope(SEM)and X-ray diffractometer(XRD)analysis of the material shows that the AHFS-Si/SiO2/rGO and AF-Si/SiO2/rGO materials are stacked in the form of particles and flakes,which retains The porous network structure of the gel precursor has an average particle size of about 30 nm.The addition of F element has no obvious effect on this porous network structure.Both composite materials exhibit the typical crystalline characteristics of silicon and silicon dioxide materials.The introduction of F element does not change the crystalline form,but the Si characteristic peak of the AHFS-Si/SiO2/rGO composite has a certain small angle deviation Shift,the reduction degree of SiO2 in the material increases with the increase of the magnesiothermic reduction temperature,and there is still incompletely reacted SiO2 in the material.The precursor prepared when ammonium fluoride is used as the fluorine source will not be reduced by magnesiothermic reduction treatment.At this time,the reaction of ammonium fluoride and magnesium powder occurs to produce magnesium fluoride as a by-product.The specific surface area and pore size of the AHFS-Si/SiO2/rGO and AF-Si/SiO2/rGO composites were analyzed by nitrogen adsorption and desorption test.The analysis results showed that the specific surface area of AHFS-Si/SiO2/rGO composite decreased with the increase of AHFS content.The specific surface area of AHFS-Si/SiO2/rGO and AF-Si/SiO2/rGO composites reduced at different magnesium heating temperatures decreased with increasing temperature.Both composites retain the porous network structure of the precursor,mainly mesoporous,and the pore size is mainly distributed between 2-50nm.The electrochemical performance of AHFS-Si/SiO2/rGO and AF-Si/SiO2/rGO composites as anode materials for lithium ion batteries was tested.The results show that suitable F doping can enhance the transport of lithium ions and improve the electrical conductivity of the composite.The AHFS-Si/SiO2/rGO composite exhibited good electrochemical performance.The samples at 800? had the highest first-cycle charge-discharge capacity and first-cycle Coulomb efficiency,which were 1226 mAh·g-1,2209 mAh·g-1 and 55%,respectively.The 720? sample has better cycle stability.After 100 cycles,the reversible capacity is 778 mAh·g-1,which is 95%of the initial capacity.In addition,for the 800? sample,at a current density of 2 A·g-1,the specific capacity is still 374 mAh·g-1.After a large current impact,the capacity can still be restored to more than 85%of the original,AF-Si/SiO2/rGO The composite material also exhibits the same excellent electrochemical performance.At a current density of 0.2 A·g-1,the specific charge capacity of the first lap is as high as 1296 mAh·g-1,the Coulomb efficiency is 62%,and the reversible capacity is maintained after the 50th cycle At 606 mAh·g-1.The specific capacity at 2 A·g-1 is 374 mAh·g-1.The good electrochemical performance is attributed to the synergy between the appropriate amount of fluorine doping,the appropriate ratio of Si to SiO2 and the stable porous network structure. |
PDF Full Text Request