Studies On High Performance Lithium-sulfur Batteries Based On Modified Separators

Owing to the rapid development of portable electronic devices and electric vehicles,finding advanced energy-storage systems going beyond lithium-ion batteries that are limited by their low energy density is an immediate necessity.Lithium-sulfur(Li-S)battery is regarded as one of the most promising candidates for next-generation energy-storage systems because of its high theoretical gravimetric capacity(1675 mAh g-1)and energy density(2567 W h kg-1).However,the "shuttle effect" that stems from the dissolution of polysulfides is the most fatal issue affecting the cycle life of lithium sulfur batteries.In order to suppress the "shuttle effect",one primary focus is to block the diffusion of polysulfides into electrolyte during discharge process.But this approach can't be fully implemented.Therefore,many studies further inhibit the "shuttle effect" by modifying the separator to hinder the transport of polysulfide to the lithium anode.In this paper,we have developed two modified separators that can effectively block the diffusion of polysulfides:1.LiF/GO solid electrolyte interface modified separator.Lithium fluoride/graphene oxide(LiF/GO)solid electrolyte interphase(SEI)is a dense layer with high lithium ion conductivity which can effectively block the diffusion of polysulfides to lithium anodes.In-situ UV/vis spectroscopy confirmed the significant effect of LiF/GO SEI in mitigating the polysulfide shuttling.Moreover,the restrained "shuttle effect" can also be confirmed by analysis of electrochemical impedance spectroscopy and characterization of lithium dendrite,which indicates that no insulating layer of solid Li2S2/Li2S was found on lithium anode surface.These features enable a remarkable cyclic property of 0.043%of capacity decay per cycle during 400 cycles.2.A covalent organic frameworks(COFs)material coated separator.Here,we propose a modified separator based on a COFs material to mitigate the effects of the "shuttle effect".The modified separator based on COFs can act as an ion sieve that can selectively sieving lithium ions while effectively inhibiting the migration of polysulfide to the lithium anode in a Li-S battery.As expected,the analysis of the electrochemical impedance spectroscopy and the characterization of the surface of the lithium anode proved that the "shuttle effect"was effectively suppressed.A battery assembled with a COFs modified separator exhibited excellent cycle performance.