The Study On Structure Regulation And Inhibition Of Polysulfide Shuttle Effect In Aramid Nanofiber Based Lithium-Sulfur Battery Separator

As one of the key components of a battery,the separator has the function of separating the positive and negative electrodes to prevent short circuits in the battery and ensure ion transmission within the battery.It determines the interface structure and internal resistance of the battery,and directly affects the safety performance,capacity,and service life of the battery.However,currently widely used polyolefin separators(PP,PE)have a smooth surface without active groups,low surface energy,poor wettability between electrolytes,and large pore size of the separators.The "shuttle effect" generated by poly sulfides during the charging and discharging process of lithium sulfur batteries leads to the loss of active substances,resulting in problems such as rapid battery discharge capacity degradation,poor cycling,and stability.At the same time,the growth of lithium dendrites can easily penetrate the polyolefin separator,seriously threatening the safety characteristics and service life of batteries.Therefore,the application of traditional polyolefin separators in the field of high energy density lithium sulfur batteries is limited.It is worth noting that the aramid nanofibers(ANF)and aramid nanopapers developed by our team in recent years have unique three-dimensional nanonetwork structures,excellent mechanical properties,and thermal stability,providing new ideas and strategies for the development of new lithium sulfur battery separators.Based on the above analysis,this thesis uses aramid nanofibers as the structural construction unit of a new type of lithium-sulfur battery separator,and conducts systematic research from four aspects:nanofiber coating modification,pore structure regulation of ANF separator,functional design of ANF separator,and construction of ANF-based composite separator.It clarifies key scientific issues such as pore structure regulation mechanism of ANF separator,internal relationship between structural design and ion transport,and inhibition mechanism of polysulfide shuttle effect,provide a new idea and theoretical basis for the preparation of high-performance ANF based lithium sulfur battery separators.The specific research content is as follows:Aiming at the problems of poor temperature resistance and large pore size of polyolefin separators,ANF and Al2O3 composite coatings were prepared,and Al2O3/ANFs@PP composite separator with sandwich structure was constructed on the surface of PP separator by coating method.The effects of nano Al2O3 dispersion,addition amount,and coating process parameters on the microstructure of composite separator and battery charge/discharge capacity were studied,revealing the effective regulatory mechanism of nano coating on the structure construction of PP separator.The research shows that due to the excellent mechanical properties and thermal stability of ANF,the rich three-dimensional nano network structure,and the introduction of nano Al2O3 with excellent hydrophilicity,the electrolyte wettability,temperature resistance,and battery cycle performance of the PP separator have been significantly improved.The initial discharge capacity of the battery during charging and discharging cycles at a current density of 0.5 C is 1233 mAh·g-1,and after 200 cycles the discharge capacity is 832 mAh·g-1,the average capacity attenuation per turn is 0.16%.Due to the fact that the ANF coating substrate is a PP separator,temperature resistance is still limited,while aramid nanofibers have advantages such as high temperature resistance and good film forming ability.Therefore,this chapter constructs ANF based separators with micro/nano porous structures and reveals the mechanism of pore structure regulation,aiming at the barrier of lithium ion transport caused by strong intermolecular hydrogen bonding forces and dense pore structure of ANF separator fibers.The effects of various strategies such as porogen,solvation,and drying methods on the pore structure of ANF separators were studied.The study shows that the pore diameter of the separator obtained by supercritical drying(SCD-ANF)method is 44 nm,and the ionic conductivity is 1.71×10-4 S/cm,which is 5 times and 1.9 times that of pure ANF separator,respectively.The regulation of pore structure can appropriately hinder the hydrogen bonding of ANF molecules.During the forming process of ANF separators,fibers undergo a certain degree of orientation and alignment,forming multilayer micro nano porous ANF separators with uniform pore size and orderly structure.The initial discharge capacity of the separator at a current density of 0.5 C is 1108 mAh·g-1,and after 200 cycles the discharge capacity is 566 mAh·g-1.The average capacity attenuation rate per cycle is 0.24%,and the coulomb efficiency is 99.25%,which can reach the current commercial PP level.Through structural control,the polarization of the redox reaction in the battery is reduced,the chemical reaction and the migration rate of Li+are accelerated,and polysulfides are difficult to penetrate the ANF separator with a small pore structure.The utilization rate of active substances is improved,and the battery has a higher discharge capacity.In response to the problem of fast capacity degradation and poor stability of batteries due to the shuttle effect of polysulfides,functionalization of the ANF separator was achieved through surface modification with perfluorosulfonic acid and the structural design of sodium polyphenylene sulfonate(PSS)with an additional self-assembled layer.Carboxyl and amino functional groups as well as electrostatic attraction layers were successfully introduced into the ANF separator,achieving chemical adsorption of poly sulfides.The research shows that the initial discharge capacity of the ANFs/PSS separator is 1064 mAh·g-1 at a current density of 0.5 C.After 200 cycles of charging and discharging,the discharge capacity is 860 mAh·g-1,the capacity retention rate is 80.83%,and the average capacity attenuation rate per cycle is 0.1%.Through the functional modification of the surface of the ANF separator and the structural design of an additional selfassembled layer,the adsorption and binding of poly sulfide ions are achieved.The functional separator has the dual effect of selectively penetrating lithium ions and blocking polysulfide ions at the positive electrode side,achieving efficient recycling of active sulfur,significantly improving battery capacity.Further,metal skeleton compounds(MOFs)with high specific surface area and high reactivity were successfully grown into ANF nanonetwork structures using in-situ growth methods to construct ANFs@MOFs composite separator with high ionic conductivity(1.23×10-3 S/m)while containing a large number of metal active sites on the surface,which can hinder the shuttle of polysulfides through physical adsorption and coordination with polysulfide ions.The analysis of battery charging and discharging shows that the initial discharge capacity of the battery is 1082 mAh·g-1,after cycling 200 cycles at a constant current density of 0.5 C,the discharge capacity is 876 mAh·g-1.The capacity retention rate is 81%,the average capacity attenuation rate per cycle is 0.1%,and the coulomb efficiency is 97.72%.In addition,the mechanism of inhibiting polysulfide shuttling by functional composite separators has been revealed:during the charging and discharging cycle,MOFs with high specific surface area and a large number of metal active sites can hinder the free movement of polysulfide between positive and negative electrodes through physical adsorption and coordination between polysulfide ions,impeding polysulfide shuttling,and a new type of high discharge capacitance ANF based composite separator has been developed,the process of transforming the ANF separator from simplification to diversification and functionalization has been realized.This thesis breaks through the controllable preparation technology of aramid nanofiber battery separators with micro/nano porous structures,reveals the pore structure regulation mechanism of ANF separators and the inhibition mechanism of their functionalization on the polysulfide shuttle effect,solves the bottleneck problem of commercial polyolefin separators limiting the development of lithium sulfur batteries to higher energy density and discharge capacity,and promotes the application of aramid nanofibers in the field of battery separators.It provides a new idea and method for the preparation of high-performance lithium-sulfur battery separators,and has broad application prospects.