The Study Of High Stability And Capacity Lithium Sulfur Battery

Lithium sulfur batteries (Li-S) is regarded as next generation batteries due to its high theoretical capacity of 1675 mAh g-1 and high energy density of 2500Wh/kg, and the sulfur is abundant, low cost, environment friendly. However, several problems are hindering the practical realization of Li-S batteries. The insulating nature of sulfur causes low active material utilization, "shuttle effect" resulted from the dissolution of polysulfides in the electrolyte which leads to low coulombic effiency, the volume would expand during the charge-discharge process, and the low electrode loading and low sulfur loading decrease the energy density and capacity by area.Firstly, a multi-functional separator (NP/Celgard) was obtained which was consist of Li+-Nafion and PEO coated on the Celgard according to dissolved intermediate and "shuttle effect". Li+-Nafion was cation exchange membrane could reject the shuttle of polysulfides and PEO could adsorb electrolyte to decrease the charge transfer resistance and in favour of high rate cycle, while Celgard could adsorb little electrolyte and have high ion transfer resistance. SNP/Celgard was get after adding Super P in NP/Celgard which could reused the dissolved polysulfides additional. Therefore, SNP/Celgard could improve lithium sulfur batteries cycling performance for the synergistic effect. Moreover, the lithium sulfur batteries delivers high capacity when the percentage of sulfur was improved from 49% to 70% or enhanced the loading to 3.6 mg cm-2.The study of membrane demonstated that it could effectively improve the capacity of the battery, but the rate of cycle declining didn’t change obviously. Therefore, commercial LiCoO2 which was added as additive in the sulfur cathode was studied, it could react with polysulfide to form thiosulfate (Li2S2O3) and cobaltpentlandite (Co9S8) by preliminary verification. Both Li2S2O3 and Co9S8 could enhance polysulfide absorption and suppress polysulfide shuttle effect. The LiCoO2 addition decreased sulfur lithiation impendence in order to promote lithiation kinetics. The cell maintained a reversible capacity of 830mAh/g which was about 70% of initial capacity after 100 cycles at 0.2C. Moreover, reversible capacity of 630mAh/g was still obtained at 5C. Decreasing the ratio of LiCoO2 after ball mixing can improve the capacity by the electrode is also explored.Last, in order to further improve the cycling performance, LiCoO2 additive and SNP/Celgard were combined together based on the above. The composite lithium sulfur battery delivered a specific capacity of 681 mAh/g and high capacity by area of 3.5 mAh cm-2 fter 50 cycles, with a cathode containing 68% sulfur and approximately 8 mg cm-2 sulfur loading on electrode, which was 79.4% of initial capacity and higher than commercial lithium ion battery. In addition, soft package lithium sulfur batteries were explored. Different shape and loading single lithium sulfur battery were prepared and the feasibility of parallel battery pack was verified.