Study On New Cathode Material And Electrolyte Additive For Lithium-Sulfur Battery

With the rapid development of society and economy,the demand for batteries in the fields of consumer electronics,electric vehicle energy storage power systems,and aerospace technology continues to increase,which not only requires batteries to have high specific energy but also meet the requirements of safety,cheapness and environmental protection.However,limited by the energy density of traditional transition metal oxide cathode materials,the Lithium-ion battery can no longer meet people’s needs.Lithium-sulfur battery are considered the most promising nextgeneration energy storage systems due to their high theoretical specific energy(2500 Wh kg-1).However,due to the insulating properties of sulfur cathodes and the shuttle of polysulfides,the low utilization rate of active materials,low Coulombic efficiency,and short cycle life of the Lithium-sulfur battery seriously hinder their commercialization.Based on this problem,various strategies have been developed to improve Lithium-sulfur battery.Among them,organosulfides with a high specific capacity,low cost,environmental protection,and functional group tunability have attracted much attention.Organosulfur compounds are not only promising cathode materials but also serve as useful additives in electrolytes to improve the performance of sulfur cathodes and lithium metal anodes.In this work,fluorinated macrocyclic organic disulfides(F-MCDS)were designed and synthesized in dimethyl sulfoxide using 2,5-difluorobenzene-1,4-dithiol via a simple one-step oxidation method.Among them,the synthesized product is composed of dimer,trimer,and tetramer of 2,5-difluorobenzene-1,4-disulfide insoluble in ether electrolyte.Tests in Li half-cells found that due to the electron-withdrawing effect of fluorine atoms,F-MCDS showed high discharge voltage plateaus at 2.45 and 2.2 V and could provide a discharge of 268.6 mAh g-1 at 0.1 C rate,the capacity retention rate after 200 cycles is 78.1%,and the Coulombic efficiency is close to 100%.In addition,it can be cycled 1000 times at a rate of 0.5 C,proving its excellent cycling stability.This work adds a new member to the family of organic electrodes and will stimulate more interest in the rational design of organosulfides with rechargeable battery performance.Second,to control the dissolution behavior of polysulfides,thiol-based electrolyte additives such as 2,3,5,6-tetrafluorobenzene-1,4-dithiol(4F-BDT)were used in Lithium-sulfur battery to improve the capacity retention of Lithium-sulfur battery.During battery cycling,the 4F-BDT additive can not only make Lithium-sulfur battery have a higher Lithium-ion diffusion coefficient but also interact with polysulfides to form fluorine-containing cyclic phenyl polysulfides that are insoluble in electrolytes,effectively suppressing the shuttle of polysulfides and improving the kinetics of the dissolution process,enabling Lithium-sulfur battery to exhibit better cycling stability during cycling.The Lithium-sulfur battery with 4F-BDT added has a specific discharge capacity of 1232.6 mAh g-1 in the first cycle at a cycle rate of 0.5 C and still has a specific discharge capacity of 839.8 mAh g-1 after 250 cycles,with a capacity retention rate of 66.8%.