Study On The Performance Improvement Of Aprotic And All-solid-state Lithium-sulfur Batteries Through Material Design And Structural Optimization Of Sulfur Electrode

Developing energy storage devices with high energy densities and high safety can promote the development of many industries,including transportation,renewable energy and distributed energy storage.Among next-generation energy storage systems,lithium-sulfur battery has attracted much attention due to its high theoretical energy density of up to 2600 Wh kg-1,sulfur's low cost and tremendous reserves.However,the practical application of lithium-sulfur battery still suffers from huge challenges,e.g.the insulation problems of sulfur,large volume changes of active material during redox reaction,shuttle effect of polysulfides and safety problems caused by the growth of lithium dendrites,leading to the poor utilization and low loading of active sulfur,as well as the decreased actual energy density,the rapid capacity decay and the shortened cycling life of the whole lithium-sulfur battery.Structural design of electrode has a significant impact on its performance.Improving the interfacial conditions between the active material,the conductive additive,the binder and the electrolyte in the composite sulfur electrode by material design and structural optimization of electrode plays a key role to enhance the performance of lithium-sulfur battery.Aiming at the problems existing in sulfur electrode with traditional aprotic electrolytes,a triblock copolymer named as polystyrene-poly(n-butyl acrylate)-polystyrene(SnBAS)is firstly designed and synthesized by employing macro-RAFT emulsion polymerization.This copolymer presents a better absorbency property to aprotic electrolyte,strong binding energies with polysulfides and good mechanical properties.Secondly,the reduced graphene oxide(rGO)foam is prepared via employing freeze-drying method as a three-dimensional conductive matrix for sulfur loading.The rGO foam is one kind of ultra-light material with various micron-sized pores,which can substantially increase the sulfur loading.While providing fast electron-transmission pathways for composite electrode,the rGO foam also offers active sites for electrochemical reactions.Finally,the w/o Pickering emulsion is designed and prepared for fabricating a 3D rGO foam-sulfur(3D rGO-S)composite electrode,which can introduce three phases including the active material sulfur,the triblock copolymer binder SnBAS and the conductive polymer PEDOT:PSS together into the rGO foam in one step.Compared with the 2D electrode prepared by coating slurry on aluminum foil,this 3D rGO-S composite electrode exhibits higher specific capacity,better cycling performance and coulombic efficiency even though its sulfur loading is twice that of the 2D coated electrode.In lithium-sulfur battery with aprotic electrolyte,the 3D rGO-S composite electrode with sulfur loading of 3.25 mg cm-2 delivers a discharge specific capacity of 758 mAh g-1 at 1 C and a discharge specific capacity of 623 mAh g-1 at 2 C.The 3D rGO-S composite electrode with sulfur loading of 4.83 mg cm-2 demonstrates a reversible specific capacity of 590 mAh g-1 at 0.2 C after 150 cycles,with coulombic efficiency remaining above 97%.Aiming at solving the problem of poor electrode kinetics in all-solid-state lithium-sulfur batteries,the chain structure of active material sulfur is designed.Poly(ethylene glycol)methyl ether methacrylate(PMEMA)is chosen as a functional monomer for transferring lithium ions,and 1,3-diisobutenylbenzene(DIB)is used as a crosslinker,both of which are copolymerized with ring-opening sulfur diradicals,thus the polymeric sulfur material poly(S-PMEMA-DIB)is synthesized with a sulfur contenet of 75 wt%.Polymeric sulfur material poly(S-PMEMA-DIB)is further empolyed in all-solid-state lithium-sulfur battery as positive material.In the meantime,NASICON-type inorganic ceramic Li1.5Al0.5Ge1.5(PO4)3(LAGP)particles are selected to be dispersed in a polyethylene oxide(PEO)matrix for the preparation of inorganic/organic composite all-solid-state electrolyte.Because poly(S-PMEMA-DIB)can disperse well in acetonitrile and shows good compatibility with LAGP and PEO,the electrode can be prepared easily with one uniform structure.Plus the intrinsically ionic conductivity of this polymeric sulfur,the interfacial resistance of the prepared electrode is decreased,leading to the better battery performance than that of all-solid-state lithium-sulfur battery prepared by elemental sulfur with the same composition.All-solid-state lithium-sulfur battery made from poly(S-PMEMA-DIB)delivers a reversible specific capacity of 1140 mAh g-1 at 0.1 C after 100 cycles with coulombic efficiency remaining above 98%,and a reversible specific capacity of 608 mAh g-1 at 0.5 C after 150 cycles,with an increased ultimate charge/discharge rate of up to 1 C.