Preparation,Structure And Electrochemical Performance Of Functional Interlayer And Composite Electrode Materials For Lithium-sulfur Batteries

The lithium-sulfur(Li-S)battery is considered as the next generation energy-storage battery due to its high specific energy density and low cost.This thesis focuses on the problems from polysulfides(Li₂S_x,4?x?8)formed in the electrochemical reaction process of lithium-sulfur(Li-S)batteries,which are easy to be dissolved in the organic electrolyte solution and resulted in the shuttle effects,and the poor electric conductivity of the sulfur cathode.So that,the influences of a functional interlayer of Li-S battery and the composition and structure of the cathode material on the electrochemical performance of the battery are studied.The research includes the following two parts.The first part,the electrospinning and heat treatment technology is used to prepare Ni/C and Al₂O₃/C composite nanofibers non-woven cloth as the functional interlayer between the cathode and separator of Li-S batteries,forming a novel sandwich-like structural battery,in order to inhibit the polysulfide shuttle effect and improve the electrochemical performance.The second part,the conductive polymer poly 3,4-ethylenedioxythiophene(PEDOT)/TiO₂nanocomposite material and conductive oxide Ti₄O₇/C nanocomposite material are introduced as a matrix of the active material sulfur for Li-S batteries,leading to improving the electric conductive property of the cathode material and hindering the dissolution and diffusion of polysulfides.The main research contents and results are as follows.1.Ni/C composite nanofibers non-woven functional interlayerNi/C composite nanofiber non-woven cloth has been successfully prepared by the electrospinning and thermal reduction technology and is used as the functional interlayer between the positive electrode and the separator,forming a novel type of sandwich-like Li-S battery structure,such as cathode-interlayer-separator-anode.For example,addition of 1.0 g nickel salt into the precursor for the Ni/C composite nanofibers non-woven functional interlayer,the interlayer thickness is about 230±30?m and the component is w _Ni:w _C=6.5%:93.5%approximately.The study finds that the electrochemical performance of Li-S batteries with the Ni/C composite nanofibers non-woven functional interlayer modified is mainly affected by the reduction temperature of the composite nanofiber cloth and the content of metallic nickel doped in the composite nanofibers non-woven interlayer.When the amount of nickel nitrate added to the composite nanofiber precursor is about 1.0 g and the reduction temperature is at 900°C,compared with the C nanofibers non-woven functional interlayer modified,the internal resistance of batteries with the Ni/C composite nanofibers non-woven functional interlayer modified showes a lower value.Therefore,for this battery,the initial discharge specific capacity at 1C charging/discharging rate is 1062 mAh g^-1,and the remaining specific capacity exceeds 910 mAh g^-1after 200 cycles.2.Al₂O₃/C composite nanofibers non-woven functional interlayerAl₂O₃/C composite nanofiber non-woven cloth is successfully prepared by the electrospinning--calcination method as the functional interlayer between the positive electrode and the separator.The results indicate that the addition of 1.0 g alumin?m salt into the precursor for Al₂O₃/C composite nanofibers non-woven functional interlayer,the interlayer thickness is 230±30?m and the component is w _Al2O3:w _C=4%:96%,approximately.The reserach discoveres that the electrochemical performance of Li-S batteries with modification of Al₂O₃/C composite nanofibers non-woven functional interlayer is mainly influnced by the calcination temperature of the composite nanofiber cloth and the active alumina content in the composite nanofibers non-woven interlayer.Although the internal resistance of the battery is slightly higher than that of batteries with the single-phase carbon nanofibers non-woven functional interlayer modified,when the aluminum nitrate added into the composite nanofiber precursor is about 1.0g,and the calcination temperature is at 900°C,the cycle performance and charge/discharge performance show the better results(the cathode with S load of 1.85 mg cm^-2),majorly due to the formation of chemical bonds between the active alumina and polysulfides.So that with this battery,the initial discharge specific capacity at 1C charging/discharging rate is 1360 mAh g^-1,and the remaining specific capacity exceeds993 mAh g^-1after 200 cycles.3.PEDOT/TiO₂nanocomposite modified cathodeA simple soft chemical method combined with in-situ oxidation technology is used to produce the PEDOT/TiO₂nanocomposite positive electrode matrix material(w _PEDOT:w _TiO2=85%:15%),which is then mixed with the sublimed sulfur to form the cathode material of PEDOT/TiO₂/S.The results show that the electrochemical cycling stability and rate performance of batteries with PEDOT/TiO₂nanocomposite modified cathode of PEDOT/TiO₂/S are related to the size of nano-TiO₂particles in the composite matrix.When the size of nano-TiO₂particles in the composite matrix is about5 nm,the battery shows a better cycling and rating performances(The load of S in cathode is 1.75 mg cm^-2.),which is mainly due to a high specific surface area of nano-TiO₂and the conductive polymer PEDOT uniformly coated on nano-TiO₂particles with a core-shell-like structure.With this battery,at 1C charging/discharging rate,the initial discharge specific capacity is 859.7 mAh g^-1,after 300 cycles the remaining specific capacity reaches 495.9 mAh g^-1.4.Ti₄O₇/C nanocomposite modified cathodeBy use of tetrabutyl titanate and polyvinyl alcohol(PVA)as the starting materials,Ti₄O₇/C nanocomposite cathode matrix material(w _Ti4O7:w _C=94%:6%)has been successfully prepared after reduction at 1000°C for 4 hours by the in-situ hydrolysis and thermal reduction method.This matrix material is then mixed with the sublimed sulfur to form the Ti₄O₇/C/S composite cathode.The research results show that the carbon content in Ti₄O₇/C nanocomposite matrix material obtained by the in-situ coating and thermal reduction method is about 6%,and the Ti₄O₇nanoparticles formed during this process are uniformly dispersed in the amorphous carbon,with a carbon film-coated structure.Because of the effects of space confinement and separation from the coated carbon on Ti₄O₇nanoparticle formation process,the Ti₄O₇nanoparticle size is effectively controlled,with the particle diameter between 200-400 nm,and consequently there is no obvious Ti₄O₇nanoparticle agglomeration phenomenon occurred.Based on analyses of CV,EIS,and cycling performances,it is found that the size and distribution uniformity of Ti₄O₇particles in the composite matrix material have a significant effect on the electrochemical performance of the battery.At 1C charging/discharging rate,the initial discharge specific capacity of the battery with the Ti₄O₇/C nanocomposite matrix modified cathode is 781.5 mAh g^-1(The load of S in cathode is 1.80mg cm^-2.),the remaining specific capacity of the battery exceeds 465 mAh g^-1after 500 cycles,and the single-cycle specific capacity decay is less than 0.081%.5.Electrochemical performance of batteries with synergistical modification of Al₂O₃/C composite nanofibers non-woven functional interlayer and PEDOT/TiO₂nanocomposite matrix cathodeBased on the above research results,by use of PEDOT/TiO₂/S cathode material and Al₂O₃/C composite nanofibers non-woven functional interlayer,a novel structure of lithium-sulfur battery is fabricated.The results show that the synergistical modification of Al₂O₃/C composite nanofibers non-woven functional interlayer and PEDOT/TiO₂nanocomposite matrix cathode confirms the positive improvement of the electrochemical performance of lithium-sulfur batteries,which is proved to be an effective route for the electrochemical performance improvement of Li-S batteries.Finally with this comprehensive modification battery,at 1C charging/discharging rate,the initial discharge specific capacity reaches1430.2 mAh g^-1(The load of S in cathode is1.75 mg cm^-2),after 200 cycles the remaining specific capacity can still reach 1082.3 mAh g^-1,and the single-cycle specific capacity decay is less than 0.12%.