Study On Functional Separator For Lithium/Sulfur Batteries

Lithium-sulfur battery is attracting widespread concerns,due to its high specific energy density,natural abundance and environmental friendliness.The theoretical energy density of the lithium-sulfur batteries could reach 2600 Wh/Kg.However,the current practical energy density of the lithium-sulfur batteries is far from the theoretical level,and lithium sulfur batteries have short cycle life,which is difficult to meet the requirements of pratical application.The development of lithium-sulfur batteries is facing big challenges.The intermediate products lithium polysulfides generated during discharge process could dissovle in the electrolyte,which causes the shuttle effect.Moreover,the final products Li2 S and Li2S2 are insoluble in the electrolyte,which leads to the inactivation of sulfur-carbon skeleton.The two problems above have important impacts on its battery cycling performance.Aiming at alleviating these problems,the carbon nanotubes paper and the polymer electrolyte membrane were introduced into lithium-sulfur batteries to inhibit the shuttle phenomenon and improve the battery performance,including cycle life and coulombic efficiency.Compared to those of the lithium-sulfur batteries using Celgard separator,the cycle life and coulombic efficiency of lithium sulfur batteries using carbon nanotubes paper were significantly improved.The discharge capacity is 1537.6m Ah/g in the first cycle and maintains 1189 mAh/g after 80 cycles for the lithium sulfur batteries using carbon nanotubes paper,while the discharge capacity is only 1180 m Ah/g in the initial cycle and fades to 950mAh/g after 80 cycles for the lithium sulfur batteries using Celgard separator.The carbon nanotubes paper which was introduced between the sulfur cathode and Celgard separator not only can improve the specific capacity of lithium-sulfur batteries but also improve the cycle performance of the battery.The impacts of the four types of carbon nanotubes paper on the electrochemical performance of the lithium sulfur batteries were studied.It was found that using the M3 type carbon nanotubes paper could bring the best battery performance.The discharge capacity reaches 1523.2 mAh/g in the first cycle and maintains 1231.1 mAh/g after 45 cycles.The four type carbon nanotube paper after 20 cycles were investigated with elemental mapping and XPS measurements,the elemental mapping results indicate the M3 type carbon nanotube paper shows the most homogeneous distribution of sulfur;the XPS results suggest the oxidized sulfur content is more than 50% in the cathode,indicating the irreversible oxidation of sulfur may be one of the reasons for capacity loss during cycling for lithium sulfur batteries,the existence of sulfur on the four type carbon nanotube papers,further indicating the carbon nanotube paper could adsorb active sulfur.The M3 type carbon nanotube with one capped end has a tube diameter of 20 nm and a tube length of 10~30um.Because of the optimum of the tube diameter and length,the M3 type carbon nanotube paper has the best effect of adsorbing active material and confinement of active material,the elemental distribution after cycling is the most homogeneous,the electrochemical reversibility and stability is the best,and the absorbed material is the most,so using M3 type carbon nanotube paper could lead to higher discharge capacity and more stable cycling performance.Li-PFSD ionomer was casted at 120?,140?,160?,180?,the result showed that when the temperature is 180?,the structure of Li-PFSD membrane is dense and smooth.With the increasing temperature,porosity and aspiration rate fell down.After 6h,the membrane casted at 180? was swelled completely.The aspiration rate reached to 56.25%.The ionic conductivity of the Li-PFSD memebranes under different membrane forming temperature were investigated.It found that the membrane made in 180 ? has the best ionic conductivity,reaching to 10-4S/cm magnitude,which meets the ionic conductivity repuriment of usage of lithium sulfur battery.The activation of organic solvent PC for Li-PFSD membrane is also studied.When the content of PC is 2% of electrolyte,it has the best effect of activation.The ionic conductivity reaches to 2.665×10-4S/cm.