| In recent years,with the development of new energy vehicles and more electronic devices,even if the lithium-ion battery capacity reaches the theoretical value,it can not meet the urgent needs of these electronic devices for energy.Therefore,scientific research workers are striving to find and develop more.High energy density and longer cycle life battery system.The lithium-sulfur battery is a secondary battery system in which sulfur is uesd to the positive electrode and lithium is used to the negative electrode.The lithium-sulfur battery not only has a high theoretical specific capacity(1675 mAh g-1)but also has a considerable energy density(2600 Wh kg-1),so it has received more extensive attention.However,the widespread practical applications of Li-S batteries are still largely plagued by its inherent issues.For instance,the natural insulation of sulfur and lithium sulfide limit the electron transfer during the electrochemical reactions,leading to poor utilization of active materials,and the dissolution of lithium polysulfide?LiPS?intermediates in the liquid electrolytes cause the well-known“shuttling effect”,and the significant volume expansion during charge/discharge process may also lead to shed of active material and cause the poor cycle stability.Biochar is generally a carbon-based material that has evolved from biological materials.It has attracted more attention due to its wide variety of sources,low cost and excellent electrical conductivity.To solve the current problems of the lithium-sulfur battery described above,this paper adopts different biomass carbon materials to modify the commercial separator.The main research contents of this paper are as follows:?1?The research on lithium-sulfur battery separator modified by sweet potato-based biomass carbon.In this experiment,sweet potato biochar material?SP-CA?is prepared through various steps of hydrothermal,freeze drying and high temperature carbonization.The battery performance with the SP-CA modified separator and the commercial PP separator are compared.It is confirmed that SP-CA is a porous carbon material containing mesopores by SEM,TEM,BET,etc.,so the modified separator coating layer is also a loose porous carbon layer.The coating layer not only facilitates the contact between the positive electrode and the separator,reduces the interface resistance,but also can effectively form a certain blocking effect on the lithium polysulfide and reduce the internal“shuttle effect”of the battery.The experimental results show that the initial discharge capacity is 1216mAh g-11 at 0.1C,the reversible discharge capacity is maintained at 431mAh g-11 after 1000 cycles at 1C with the coulombic efficiency over 95.3%.?2?The research on Lithium-sulfur battery separator modified by pomelo-based biomass carbon material.In this experiment,pomelo peel is used as raw material to prepare pomelo-based biomass carbon material?PPCA?.The PPCA is applied to the modification of the lithium-sulfur battery separator,and the improvement of the performance of the lithium-sulfur battery with modified separator is investigated.The modified separator coating layer is observed as a loose porous carbon layer by SEM and TEM.The coating layer not only inhibits the"shuttle effect"of polysulfide intermediates during cycling,reduces battery resistance,but also acts as an upper current collector to increase sulfur utilization.The discharge specific capacity of the battery after 500 cycles at 1C still reached 457.9 mAh g-1.When the magnification is increased to 2C,the battery discharge capacity is still maintained at 644.1mAh g-1.?3?The research on lithium-sulfur battery separator modified by elemental doping pomelo-based biomass carbon material.In this experiment,impregnation method and high-temperature carbonization method are used to dope nitrogen and boron into the skeleton of pomelo-based biomass carbon material.The BET test results show that the specific surface area of the material have been greatly improved after doping,which has played a certain role in promoting the adsorption of polysulfide.XPS test shows that the nitrogen and boron elements form a good combination with the skeleton of the carbon material,indicating that elemental doping has achieved the desired effect.The doped carbon material is used as a coating layer for modifying commercial PP separator.On the one hand,the coating layer can adsorb polysulfide through physical action,and can also form a certain chemical adsorption between the coating layer and polysulfide through chemical action,thereby it can fix the polysulfide more effectively and inhibit the occurrence of the“shuttle effect”.The battery discharge capacity increased from 457.9mAh g-11 to 581.1mAh g-11 compared to undoped after 500 cycles at 1C,and the discharge capacity at 2C also increased from644.1mAh g-11 to 748.7mAh g-1. |
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