| Due to their high specific capacity(1675 m Ah g-1)and energy density(2600 Wh kg-1),lithium-sulfur batteries have become one of the hotspots in current research.However,the"shuttle"of the polysulfide between the positive and negative electrodes makes the system poor in cycle stability,high self-discharge rate,and rapid capacity decay.The"shuttle"process is the diffusion process of polysulfides,and the diffusion and mass transfer of polysulfides is an indispensable link between the optimization of the design of cathode materials and the improvement of the electrochemical performance of lithium-sulfur batteries.In this regard,by studying the diffusion and mass transfer process of polysulfides in different carbon materials,this article further understands the mechanism of the"shuttle effect"of lithium-sulfur batteries,The following are the main research contents of this thesis:（1）We applied commercial activated carbon with rich and varied pore structures to lithium-sulfur battery systems.By studying the process of diffusion and mass transfer of polysulfides and their effects on the performance of lithium-sulfur batteries,we obtained the relationship between the structure of carbon material,polysulfide adsorption/diffusion and impacts on the lithium-sulfur batteries.Studies have shown that the diffusion coefficients(Dps)of polysulfides in GYAC,XFAC,and KLLAC are1.64*10-5,2.17*10-6,and 1.83*10-6 cm2 s-1,respectively.The specific discharge capacities of GYAC/S,XFAC/S,and KLLAC/S are 876.6,1295.4,and 1102.4 m Ah g-1,respectively.After 150 cycles at 0.2 C,the capacities remain at 433.2,511.5,and540.1 m Ah g-1.The trends of diffusion coefficients of polysulfides in various carbon materials are consistent with the electrochemical performances of the corresponded lithium-sulfur batteries and the size of pore is closely related.（2）Through exploration,we know that the diffusion and mass transfer process of polysulfides is not only related to the specific surface area and pore volume of the carbon material,but also has a close relationship with the pore size.Here,we synthesized several carbon materials with unique pore size from micro（ZTC）,meso（OMC）to macro（OMAC）.Studies have shown that the diffusion coefficients(Dps)of polysulfides in ZTC,OMC,and OMAC are 6.3*10-6,2.97*10-6,and 1.83*10-5 cm2s-1,respectively.ZTC/S,OMC/S,and OMAC/S have specific discharge capacities of1327.3,1332.4,and 1260.3 m Ah g-1 in the first cycle,respectively.After 150 cycles at0.2 C,the capacities remain at 619.3,659.1,and 513.6 m Ah g-1.The trends of diffusion mass transfer coefficients of polysulfides in various carbon materials are consistent with the electrochemical performances of their corresponded lithium-sulfur batteries.Among them,ordered mesoporous carbon materials exhibit good electrochemical performance due to their unique pore channels.（3）Experimental results show that mesoporous carbon can effectively inhibit the"shuttle effect"of lithium-sulfur batteries,but the cycle performance of the batteries still needs to be improved.To this end,we loaded Pt and Au nanoparticles on the surface of ordered mesoporous carbon（RF-2-A）,and used them as electrocatalystes to improve the cycle performance of lithium-sulfur batteries.Experiments show that the diffusion coefficients(Dps)of polysulfides in RF-2-A,RF-2-A-Pt,and RF-2-A-Au are2.31*10-5,8.60*10-6 and 1.03*10-5cm2 s-1,respectively.The specific discharge capacities of RF-2-A/S,RF-2-A-Pt/S,and RF-2-A-Au/S are 1371.5,1446.2,1409.5m Ah g-1,and 150 cycles at 0.2 C,the capacity remains at 572.4,670.4,621.8 m Ah g-1.We can see that the cycling performance of the materials with Pt and Au nanoparticles is better than that of the simple RF-2-A/S.Pt and Au nanoparticles have an electrocatalytic effect,which can accelerate the transformation of polysulfide in the electrolyte during the charging and discharging process and inhibit the"shuttle effect"at the source. |
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