Synthesis And Electrochemical Performance Of Polymer Composites For Lithium-sulfur Batteries

The rapid development of portable electronic devices,electric vehicles as well as large-scale grid energy storage stimulates the demands for higher energy storage capability.However,the existing lithium-ion(Li-ion)batteries exhibit limited energy density and cycle life,and thus is insufficient to fulfill the rising demands.Lithium-sulfur(Li-S)batteries,which exploit reversible conversion reaction of sulfur with lithium ions,could theoretically achieve the energy density several times higher than that of the lithium-ion batteries.Moreover,sulfur is cheap and readily abundant in the Earth's crust.Thus,the combination of lithium and sulfur has shown great promise as an alternative energy storage system.Despite the enticing characteristics of lithium-sulfur batteries,several challenges associated with sulfur cathodes still need to be addressed.Besides the low electronic and ionic conductivities of sulfur and Li2S,lithium-sulfur batteries also suffer from signifcant volumetric change,?80%during charging and discharging cycles.Besides,the shuttling effect arising from dissolved polysulfde intermediates causes an inevitable loss of sulfur,self-discharging and low Coulombic efficiency during cycling.All of these problems result in limited use of active materials,reduced effciency as well as poor cycle performance.Although encapsulating sulfur within porous carbon hosts has been developed to solve the above issues,the majority of the current works are targeted at a low sulfur loading of less than 2 mg cm-2.Increasing sulfur loading results in the crack and delamination of cathode materials from current collector.In addition,due to the large void space inside and/or large fraction of nonactive carbons,the high sulfur loadings are realized at the cost of high electrolyte/sulfur ratios which are used to fully wet the electrodes,reducing the overall energy density.So it is still critical to construct feasible Li-S batteries with high sulfur loading and low electrolyte/sulfur ratio.Based on the above problems,a variety of sulfur based cathodes are prepared to investigate the cycling performance.We also discuss the positive effect of conductive polymer binder on improving the high-loading performance of sulfur cathode.In addition,the protetive effect of SEI film that is in situ formed on the sulfur particles is studied.The detailed research contents are listed as follows:We present a new strategy to improve the Coulombic efficiency by using nitryl grafted sulfur cathode(Nitryl-S),which realizes the in-situ formation of SEI layer during the intial charging-discharing process.Significantly,in-situ X-ray diffraction measurements allow us to clearly observe the formation process of SEI layer on sulfur cathode,which serves as polysulfide's barrier to retain sulfur active material.As a result,the Nitryl-S cathode enables a high Coulombic efficiency of?99.5%at various current rates from 0.1 to 2 C.In addition,the Li-ion conductive SEI layer could only allow the effective transport of Li ions,while the penetration of electrolyte through the SEI layer is prevented.Thus the soluble polysulfide is unable to diffuse out of the compact SEI layer and trapped in the cathode,which is favorable for the cycling stability.These results were further proved by in-situ UV/Vis spectra,showing the significantly reduced concentration of soluble polysulfide.We develop a new strategy to covalently stabilize the discharge products of lithium-sulfur batteries by covalently binding the sulfur onto the thiol-terminated polymeric(AFG/S).Signifcantly,DFT calculations demonstrate that the fracture of crosslinked sulfur chains in the present copolymer is prone to happen in the middle position(S4-S5 bond),with the lowest dissociation energy of-16.14 kcal mol-1,forming polysufide that is firmly immobilized on the graphene.Further cleavage produces short-chain polysulfdes as the only product.Similarly,in situ UV/Vis spectroscopy,coupled with the derivative curves,has also illustrated that only short-chain polysulfides are produced throughout the charging and discharging process.We report a new strategy to improve the Coulumbic efficiency and rate capability through the synthesis of new sulfur cathodes by a radical reaction between the alkylene radicals(allyl-grafted graphene,allyl-G)and sulfur,finally enabling the covalent attachment of sulfur onto graphene nanosheets.Allylamine molecule is chosen as the medium to react with graphene oxide and simultaneously introduce alkylene radicals.Meanwhile,graphene nanosheets are used as the host to immobilize the sulfur species,benefting to the electrical conductivity as well as specific capacity.As a result,the integrated molecular structure makes it an ideal cathode material for Li-S battery,exhibiting - 100%Coulumbic efficiency and excellent capacity retention of 92%over 200 cycles.In addition,the allyl-G/S cathode presents high discharge capacities of 1300,1091,963,834,757 and 702 mAh g-1 as well as a high Coulombic efficiency of?100%when the current density increases from 0.2 to 6 C.In situ UV/Vis spectroscopy and DFT calculations demonstrate that only short-chain polysulfide formsA novel double-chain polymer network binder was synthesized by polymerizing 4,4'-biphenyldisulfonic acid connected pyrrole monomer onto viscous sodium carboxyl methyl cellulose matrix,yielding a primary crystal structure.The DCP binder is shown to assist the interparticle physical connection as well as improved Li+/e-transportation,leading to high sulfur loading,superior rate performance and excellent cycling stability.Moreover,crack-free electrodes with high sulfur loading(9.8 mg cm-2)are obtained,which allows a high area capacity of 9.2 mAh cm-2.In addition,in-situ UV/vis measurements demonstrate that DCP binder impedes the polysulfide dissolution,thereby giving rise to improved cycling stability.