| With the rapid development of renewable energy and electric vehicles,large-scale development of advanced energy storage equipment has become a top priority.Lithium-ion batteries have become the focus in the research of new energy storage devices due to their high energy densities,excellent long-cycle stability,and outstanding rate capacity performance.However,because of low theoretical specific capacity and poor rate cycling performance,traditional graphite anode materials have severely limited the performance improvement of lithium-ion battery equipment.Carbon nanofibers have become one of the new electrode materials that have attracted much attention due to their ultra-high surface area,outstanding electrical conductivity and excellent electrochemical stability.At the same time,as the second most abundant and cheap natural biopolymer on the earth,lignin is one of the most attractive raw materials for the synthesis of carbon nanofibers.Therefore,with sodium lignosulfonate as carbon precursor,PVP as spinning aid,lignin-based carbon nanofibers were prepared by pre-oxidation and carbonization.In addition,lignin-based carbon nanofibers were used as anode materials for lithium ion batteries and their electrochemical properties were studied.First,by adjusting the mass ratios of lignin/PVP and the carbonization temperature,carbon nanofiber membranes were successfully prepared and then used as lithium ion anode materials.Then,in order to further improve its electrochemical performance as the negative electrode of a lithium ion battery,ammonia gas was introduced during the pyrolysis process to increase the nitrogen content in the material,and the effect of nitrogen doping time on the performance of the carbon nanofiber membrane was explored.The main results are as follows:(1)By adjusting the mass ratio of lignin/PVP and carbonization temperature,lignin-based carbon nanofibers were prepared and their electrochemical properties as anode materials for lithium-ion batteries were tested.The results shows that the average diameter of the obtained carbon nanofibers is distributed between 200 and 300 nm.The best ratio is when the mass ratio to lignin is 2:1,and the best carbonization temperature is 900°C.At this time,the specific surface area of carbon nanofibers is as high as 614m~2/g,showing a rich microporous structure.When used as the negative electrode of a lithium-ion battery,the reversible capacity is as high as 659 mA h/g at a current density of 100 mA/g.When the current density is restored from 5000 mA/g to 100 mA/g,a reversible capacity of 658 mA h/g can still be obtained.Reflects excellent rate performance and reversibility of charge and discharge.At the same time,in the long cycle test,even if it is cycled for 100 cycles at a high current density of 200 mA/g,it still maintains a reversible capacity of 440 mA h/g,showing good cycle stability.(2)Nitrogen-doped lignin nanofibers were prepared by using ammonia gas in the process of carbonization and heat preservation,and their electrochemical properties as anode materials for lithium-ion batteries were tested.The results show that the diameter of carbon nanofibers doped with nitrogen is reduced compared with carbon nanofibers without nitrogen doping.The best nitrogen doping time is 2 hours.At this time,the LCF-3-900-N2 produced has the largest specific surface area and pore volume(1270m~2/g and 0.7131 cm~3/g).Its electrochemical performance is the best,the reversible specific capacity is stable at 640 mA h/g after 100 cycles at a current density of 200mA/g,and the coulombic efficiency at this time is 99%,which has good cycle stability.When testing the rate performance,it still has a high reversible specific capacity under a high current density of 5000 mA/g,and the capacity recovery rate is nearly 100%when the current density is restored to 100 mA/g,and it has a small internal resistance,showing Excellent electrochemical performance. |
PDF Full Text Request