| The lithium-sulfur battery is a lithium battery using sulfur and metallic lithium as positive and negative electrode active materials,respectively.The theoretical specific capacity of sulfur is as high as 1672 mAh/g,and the theoretical specific capacity is as high as 3850 mAh/g,making the theoretical energy density of lithium sulfur battery could reach up to 2600 Wh/kg,which is 6-13 times of the commercial lithium-ion battery.Besides sulfur is abundant in nature,low cost,and environment friendliness,all of which making the lithium sulfur battery is considered to be one of the most potential next-generation high-energy density secondary batteries.However,the electronic insulation of sulfur and its discharge product lithium sulfide leading to a low utilization of the active material.In addition,the shuttle effect caused by lithium polysulfide dissolved in the ether electrolyte as well as the volume change of the sulfur electrode during charge and discharge severely restrict the cycle stability of sulfur electrode,thereby reducing the electrochemical performance of the lithium-sulfur battery.After the research of references,this dissertation aim at increasing the utilization of active material and improving the cycle stability of the sulfur electrode.Using a commercial carbon black and carbon nanotube as electric matrix,different method were employed to prepare the carbon/sulfur composite with different sulfur content,as well as carbon/sulfur electrode with varied sulfur loading.By optimizing the preparation method of the composite and the preparation process of electrode,a high sulfur content composite and a high sulfur loading electrode with excellent performance were obtained.On the basis of the preliminary work,a lithium sulfur pouch cell engineering experiment line was established and in which lithium sulfur pouch cell was designed and prepared.By studying the influence of the sulfur electrode structure on the specific capacity,energy density and cycle performance of lithium sulfur battery,the preparation process of pouch cell was optimized and the technological parameter of high energy density pouch cell was obtained.All of which provide a basis understanding for the large-scale preparation of lithium pouch cells.Specific work and results are as follows:Commercial Super-P conductive carbon was used as the sulfur-bearing matrix to prepare sulfur/carbon composite(S/SP-TC)with different sulfur content by thermal compounding method.It was found that,when the sulfur content is 90%,the initial discharge capacity of the composite is 1080.2 mAh/g at 0.1C rate,the utilization of the active material is 64.6%,and the capacity retention is 56.4%after 100 cycles.An in-situ solution method was employed to prepare the sulfur/carbon composites(S/SP/CNT-in-situ)with high sulfur content and well dispersed particle size.It was found that the sulfur/carbon composite prepared by in-situ method possess a more cycle stability and with no clearly efficiency decay after cycles.The 90%sulfur content composite shows an initial discharge capacity of 1201.4 mAh/g,and a utilization rate of 71.9%at 0.1C.The 100th discharge capacity is 692.6 mAh/g with a 57.6%retention rate and a 96%coulombic efficiency.The fading mechanism of the high sulfur content composite is explored by comparing the charge-discharge profile,cycle performance,coulombic efficiency,and the change of the morphology and EIS.It was showed that the sulfur electrode surface was covered by the irreversible deposition of lithium sulfide after cycles,the polysulfide could not diffuse back to the interface of the porous electrode to participate the following reactions,which aggravates the shuttle effect of polysulfide,leading to a massive loss of the active material and capacity fading.Besides the sulfur content in the composite,the sulfur loading of electrode is also an key parameter affecting the specific energy density of the battery.Utilizing the above-mentioned S/SP/CNT-in-situ composite,electrode with different sulfur areal density loading were prepared by controlling the electrode thickness.And the electrochemical performance of the S/SP/CNT-in-situ electrode with different sulfur loading was studied.The results shows that the electrode with a 7.0 mg/cm2 sulfur loading shows a relatively inferior cycle performance,while the electrode with 6.0 mg/cm2 and 5.0 mg/cm2 sulfur loading shows relatively good and approximate cycle performance.In ordered to obtain a high energy density lithium sulfur battery,the cycle stability and sulfur loading should both take into account,so a 90%sulfur content composite,and a 6.0 mg/cm2 sulfur loading S/SP/CNT-in-situ electrode was chosen.The results shows that the initial discharge capacity is 1161.5 mAh/g,and 623.3 mAh/g after 100 cycles.To further reduce the dissolution of polysulfide,suppress the shuttle effect,improve the electrochemical performance of the lithium sulfur battery,a layer of porous carbon is coated on the surface of the S/SP/CNT-in-situ electrode which has a 90%sulfur content and 6.0 mg/cm2 sulfur loading.The initial discharge capacity of the electrode is up to 1191.6 mAh/g.After 100 cycles,the discharge capacity still remains 759.1 mAh/g,and 554.2 mAh/g at 300th cycle.Based on the above high sulfur content composite and high sulfur loading electrode,a lithium sulfur pouch cell is designed and prepared.The electrode thickness,electrolyte and active substance sulfur ratio(E/S ratio)as well as the porous carbon coating layer are practically optimized in the pouch cells.With the electrode thickness being 120 ?m and the sulfur loading being 6 mg/cm2,the highest specific energy is obtained which is consistent with the results of the previous coin cells.And when the E/S ratio is 3.3,the cycle performance of the pouch cell is optimal.The application of the coating layer in the pouch cell also effectively inhibited the polysulfide solution and migration,thus improving the cycle stability of pouch cell.The final prepared lithium sulfur pouch cell has an energy density up to 360 Wh/kg,and remains 253.7 Wh/kg after 100 cycles. |
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