Study On Cyclodextrin Modified Acetate Fiber Membrane And The Inhibition On Shuttle Effect Of Li-S Battery

With the rapid advancement of electronic equipment and new energy vehicles,higher demand has been put forward for power storage devices.Lithium sulfur(Li-S)batteries are the candidates for the next generation of high energy density secondary batteries due to the advantages of their high theoretical specific capacity equivalent which is about 5 times of lithium ion batteries,abundant natural resources of positive active substance sulfur,low toxicity and low cost.However,the internal reaction characteristics of lithium sulfur batteries also make the battery system face many problems,the shuttle effect of polysulfides,lithium dendrite,etc.limit its commercialization.As an important component of the battery,the separator has an important influence on the performance of the battery.In this work,cyclodextrin(CD)and graphitic carbon(C)double-sided modified electrostatic spinning acetate fiber membranes were prepared as high-performance separators for lithium-sulfur batteries.Firstly,acetate fiber membrane(CA),cyclodextrin-benzoyl chloride independent film(CD-TMC)and cyclodextrin-modified acetate fiber membrane(CD/CA)were prepared.In order to improve the porosity and hydrophilicity of separator,the acetate fiber membrane was prepared by electrostatic spinning technology.The influence of spinning parameters on the appearance and diameter of fibers was investigated and the process parameters were optimized by orthogonal experiment.The optimized parameters were used to obtain the nanofiber membranes with an average fiber diameter of 180 nm,and the membranes were partially deacetylated.With CD aqueous solution as the aqueous phase and TMC solution as the organic phase,the independent films of CD-TMC were constructed by the interfacial polymerization method.And analyzed the morphology structure and component structure of the film.The results showed that the cross-linking between CD and TMC was successful,the film had selfsupporting structure and certain strength,and the pores were <1.0 nm.CD-TMC film was constructed in situ on the surface of acetate fiber membrane as the substrate,and the effects of different reaction times on the morphology of the composite film were investigated.SEM observation results showed that the composite film surface was smooth and dense when the reaction time was 5 min.Secondly,the electrochemical performance of CD modified acetate fiber membrane applied to lithium sulfur battery was studied.The lithium sulfur battery was assembled with CD/CA membrane as separator.The electrolyte affinity,interface impedance,ionic conductivity,cyclic voltammetry,charge-discharge performance,cyclic performance and rate performance were tested and with PP separator as the control.The results showed that: The porosity and absorbency of CD/CA membrane were 2.20 times and 2.70 times that of PP membrane respectively;Interface impedance is 67.2 ?,only 55.35% of the PP separator,ionic conductivity is 4.89 times of the PP separator;The initial discharge capacity of CD/CA composite separator is 1320.40 mAh/g higher than that of PP separator 471.39 mAh/g.The inhibition effect of separators on polysulfides was observed by using self-made diffusion device.The results of uv-vis and XPS showed that the composite separator could effectively inhibit the diffusion of polysulfides.The DC impedance was used to study the effect of CD's structure on ion transmission.The results show that the DC impedance of the battery when lithium ions are transferred from the large port to the small port is greater than that of the battery when the lithium ions are transferred from the small port It is about 14% smaller,indicating that the conical structure of CD has a certain funnel effect,which can accelerate the transmission of lithium ions and provide a unusual idea for the rapid charging or discharging of the battery.Finally,in order to improve the dendrite growth of the lithium negative electrode of lithium-sulfur batteries,the CD/CA separators were modified to face the negative electrode side,and graphite carbon nanoparticles were deposited on the surface of the separator by magnetron sputtering technology to prepare CD/CA/C high performance composite separator.SEM and EDS showed that graphite carbon was uniformly deposited on the fibers surface.The electrochemical performance of CD/CA/C composite separators in lithium sulfur batteries were tested.The results showed that: After 500 cycles,the capacity of CD/CA/C composite separator remains 863.78 mAh/g,the capacity attenuation rate of each cycle is only 0.075%,and the capacity retention rate is 3.89% higher than that of CD/CA separator;The rate performance test of the batteries showed that the specific capacity retention rate of CD/CA/C composite separator battery reached 42.69% at the current density of 5 C,that is 32.79% higher than that of PP separator.The observation results of the lithium anode and separators on the negative side after the cycle show that: The surface of lithium which CD/CA/C membrane as separator is the most smooth,and the separator surface is the cleanest,there is basically no dead sulfur deposition,indicating that the excellent conductivity of graphite carbon can evenly deposit lithium and reduce the generation of lithium dendrites.EDS-Mapping showed that the signal of element S on the surface facing the negative side of the lithium sheet and the separator was the weakest,indicating that the composite separator effectively blocked the polysulfide in the cathode area,which was conducive to maintaining the healthy state of the lithium cathode and improving the electrochemical performance of the batteries.