Research On The Key Technology And Basic Application Of Lithium Sulfur Battery

The sustainable development of economy and society is be in great demand for high energy density electrochemical energy storage system.The theoretical energy density of 2600Wh kg^-1 for lithium sulfur battery is 6-8 times that of lithium ion battery.In addtion the sulfur is abundant in nature,cheap and non-toxic,low cost.Therefore,in recent years,lithium sulfur batteries become research hotspot at home and abroad.However,the poor stability of the lithium-sulfur batteries has become a bottleneck restricting its commercial application.The fundamental reason is:?1?the conductivity of elemental sulfur is very poor(25?,conductivity of 5×10^-30s m^-1),the utilization of active sulfur at electrochemical reaction is not high;?2?The intermediate products(Li₂S₈,Li₂S_6,Li₂S₄)of elemental sulfur electrochemical reaction are easily dissolved in the electrolyte leading to the irreversible capacity loss of active sulfur in the electrode and destroy the consistency and stability of the sulfur composite electrode structure.On the other hand,with the role of concentration gradient and electric field force,the polysulfides dissolved in the electrolyte reciproclly migrate between the cathode and anode,thus result in"shuttle effect",which directly give rise to low charging efficiency,or even incomplete charging process;?3?The reduced product?nonconductive solid Li₂S₂,Li₂S?is deposited on the surface of the porous sulfur composite cathode leading to the electrode passivation and the serious polarization during electrochemical reaction;Furthermore,this resultes in the deterioration of the charge and discharge and cycling performance of lithium-sulfur batteries.In order to solve the problem of the commercial application bottleneck and speed up the application process of lithium-sulfur batteries.In this paper,we systematically researched the following aspects:the preparation for carbon-sulfur composite material;modification of carbon-sulfur composite materials;structure-activity relationship;the key technologies and process parameters for soft-package assembly;optimization of the electrolyte components and electrolyte content;application and adhesion mechanism of a new type of binder.Specific research contents and conclusions are as followings:?1?Carbon nanotubes-sulfur composites were respectively prepared by innovative vacuum solution impregnation method or sulfur vapor infiltration approach.The vacuum solution impregnation method can fully realize the nanocrystallization of sulfur and uniformity of active sulfur into the whole carbon matrix material,compared with the traditional thermal melt method.The nanoscale"cable"structure of the composites was constructed by the mechanism of nucleation and growth of the active sulfur in the solution at the surface active sites of carbon nanotubes.Thus,the as-prepared carbon nanotube-sulfur composites prepared by the vacuum solution impregnation method have a smaller impedance,higher electrochemical activity and reversibility,in contrast to the carbon sulfur composite prepared by the thermal melt method.The specific discharge capacity reached 1334 mAh g^-1 with an increase of 23.2%on average,and a stable cycle performance and high coulomb efficiency were obtained.Also,sulfur vapor infiltration approach can not only fill small sulfur molecules into carbon nanotubes,but also lead to strong interaction between carbon nanotubes and sulfur.The synergistic effect of these two aspects can restrain the"shuttle effect",thus significantly improves the cyclic stability of the carbon nanotube-sulfur composite.The capacity retention rate was 75%at 0.2 C after 50 cycles,which was about 21%higher than that of the vacuum solution impregnation method.In addition,the effects of the microstructure of different carbon substrates?activated carbon,superconducting graphite,carbon nanotubes and carbon nanofibers?on the morphology and electrochemical properties of carbon-sulfur composites were investigated.The results showed that structure and morphology of carbon-sulfur composites and the uniform dispersion of sulfur are closely related to the microstructure of carbon matrix materials.Carbon nanotubes with moderate specific surface area,large pore volume and 3D conductive network structure are the most suitable matrix for sulfur loading among these four kinds of carbon materials.?2?The effects of high temperature KOH etching,polypyrrole coating and carbon coating on the properties of carbon nanotube-sulfur composites were studied.The carbon nanotubes can form a rich mesoporous structure after etching by KOH at high temperature,and the pore volume and specific surface area increase by 156%and 53%.The capacity retention rate of the raw carbon nanotube based composites is about 56%at 0.2 C after 50 cycles,meanwhile,the capacity retention rate of the etched carbon nanotube based composites is 75%,and the cycle stability is significantly improved.For in situ polypyrrole coated carbon nanotube-sulfur composites,the dissolution of the polysulfide from the sulfur electrode and"shuttle effect"are suppressed,as a result of the physical adsorption barrier effect of the polypyrrole coating effectively improving the cycling stability of the lithium-sulfur battery.The capacity retention rate of carbon nanotube-sulfur composites with the polypyrrole coating increased by 13.2%at 0.2 C after 50cycles.The carbon-sulfur composite can be prepared by one-step method,at the same time,the carbon coating and nitrogen doping can be successfully realized.Carbon coating increases the micro pores of the carbon matrix material and enhances the electrical conductivity of the carbon-sulfur composites.In addition,the synergistic effect of carbon coating and nitrogen doping better inhibits the dissolution of polysulfide from the sulfur electrode and maintains the electrode stable structure.Thus,a significantly improved cycle stability and rate performance of lithium sulfur battery were obtained.?3?The effects of the electrolyte concentration and the volume ratio of the mixed solvent on the ionic conductivity were studied,in addition,the influence of the amount of electrolyte on the discharge performance of the lithium-sulfur battery was also investigated.The results showed that when the volume ratio of ethylene glycol dimethyl ether?DME?to 1,3-dioxolane?DOL?in the mixed solvent was 2:1,and the concentration of the electrolyte was 1M,the comprehensive performance of the electrolyte was the best.Moreover,when the electrolyte is insufficient,it will lead to the"depression"at the conversion of the high and low voltage platform of the lithium-sulfur battery discharge curve.The overall electrochemical performance of the lithium-sulfur battery is best when the ratio of electrolyte volume to active sulfur mass is about 21?L mg^-1.?4?The application of a new binder of sodium alginate in lithium-sulfur battery was studied.The results showed compared with polyvinylidene fluoride?PVDF?binder,discharge specific capacity of the lithium sulfur battery with sodium alginate as the binder increased by 15.6%at 0.1 C for the initial cycle,the capacity retention rate increased about 16%after 50 cycles,which may be related to the flexible molecular chain structure,larger Young's modulus and flexibility.Sodium alginate can better adhere to the various ingredients together in sulfur composite cathode to avoid the electrical contact"failure"during repeated expansion and contraction of the sulfur electrode;In addition,the better flexible structure can buffer the accumulation of stress and maintain the consistency of electrode structure?5?For assembly of soft-package lithium sulfur battery,the key technologies and process have been studied.Large-scale preparation of carbon-sulfur composites with uniform distribution of sulfur throughout the carbon matrix could be obtained by the vacuum solution impregnation method;the larger sulfur content in slurry and sulfur surface area density of the sulfur composite cathode films,the worser electrochemical charge-discharge and the cycling performance for lithium-sulfur batteries.Also,the process parameters of sulfur composite slurry preparation and coating are optimized,and the as-prepared sulfur composite electrode with high adhesion,the uniformity of coating and the excellent quality can meet the demand of assembly of soft-package cell.By optimal structure design,the successful introduction of conductive interlayer in the soft-package lithium-sulfur battery was realized;the optimum range of the ratio of electrolyte volume to active sulfur mass is 18-20?L mg^-1 for soft-package lithium-sulfur battery.The assembled soft-package lithium-sulfur battery shows better discharge performance and cyclic performance,the capacity retention rate is about 70%after 100 cycles.