Design And Application Of Functional Materials For Separator Modification Of Lithium-Sulfur Batteries

With the rapid development of electric vehicles,drones,smart grids,and other emerging industries,lithium-ion batteries have been unable to satisfy the ever-increasing demands due to their limited capacity and high cost.Among the candidates for nextgeneration secondary batteries,lithium-sulfur(Li-S)batteries are very competitive due to their high theoretical energy density(2600 Wh kg-1)and low price.However,the low electrical conductivity of sulfur and Li2S,the shuttle effect induced by the high solubility of lithium polysulfides(LiPSs)in electrolyte,and the large volume change of the sulfur cathode during charging and discharging process result in low utilization of active substances and rapid capacity decay,which hinder the commercialization of Li-S batteries.As an indispensable component of Li-S batteries,constructing a modified coating on separators’ surface is an effective strategy to inhibit the shuttle of LiPSs and improve the secondary utilization of sulfur,thus improving the performance of the batteries.In this paper,based on transition metals and their compounds,separator modification materials with physical/chemical adsorption and catalytic capacity towards LiPSs were designed and prepared,while the adsorption/catalytic mechanism and electrochemical performance were investigated by combining characterization techniques such as synchrotron radiation and electrochemical tests.The specific research contents are listed as follows:(1)Co3Fe7 alloy embedded into nitrogen-doped hollow carbon sphere(CoFe/NHCS)was synthesized by hard template and impregnation method,and applied as functional materials for separator modification.The Co3Fe7 alloys exhibit strong chemical adsorption and excellent electrocatalytic conversion ability for LiPSs,while nitrogendoped carbon hollow spheres provide ample pathways for Li+/electron transport and physically restrict LiPSs.Due to their synergistic effect,Li-S batteries assembled by CoFe/NHCS modified separators exhibit excellent cycling stability and rate performance.Even increasing the sulfur loading to 6.7 mg cm-2,a high areal capacity of 4.45 mAh cm2 is maintained after 100 cycles at 0.1 C.In addition,the catalytic mechanism of CoFe/NHCS toward LiPSs during cycling was characterized by X-ray absorption spectroscopy(XAS),and its crystal structure and microstructure stability were confirmed by X-ray diffraction(XRD)and scanning electron microscopy(SEM).(2)MnV2O6 with surface defects(D-MVO)was successfully prepared by sol-gel and solution etching method,and used as a pre-catalyst for separator modification.The existence of V vacancies on D-MVO surface is elucidated by X-ray photoelectron spectroscopy(XPS)and XAS,which is used as an inducement to increase the sulfurization rate during the cycling process.Then the electrochemical analysis demonstrates that the sulfided D-MVO as a real reaction catalytic center has a much higher catalytic activity than before sulfidation,and it is mainly manifested in a lower Li2S decomposition barrier.Therefore,Li-S batteries assembled with D-MVO modified separators show excellent cycling stability and rate performance.When increasing the sulfur loading to 5.5 mg cm-2,a high areal capacity of 4.21 mAh cm-2 can be maintained after 50 cycles at 0.1 C.(3)Super P-supported amorphous Cr2O3(aCr2O3/SP)composites were successfully prepared by a simple impregnation-pyrolysis method,which were employed as functional materials for separator modification.Amorphous Cr2O3 provides abundant chemisorption sites and catalyzes the conversion of LiPSs,while excellently conductive Super P provides pathways to Li+/electron transport.Therefore,aCr2O3/SP modified separator can effectively inhibit the shuttle of LiPSs,and the assembled battery shows a high areal capacity of 4.96 mAh cm-2 after 100 cycles at 0.1 C with a high sulfur loading of 8 mg cm-2,indicating its potential application in high-energy density Li-S batteries.