Two-Dimensional Nanomaterials Modified Separators For Boosting Performance Of Lithium-Sulfur Batteries

Rechargeable lithium-ion batteries（LIBs）are now pushing forward a revolution in the field of energy storage technology.The era of“burning oil for driving,burning coal for generating station”will come to an end in the next few decades.LIBs have become the“energy source”of our daily life.However,the theoretical specific capacity and energy density of traditional LIBs are relatively low,which are still insufficient to meet the growing energy demand of various devices.With the development of smart portable devices,zero-emission electric vehicles,and efficient energy storage systems,the demand for batteries with high energy density,long cycling life,and high security performance is continuously increasing.Therefore,it is urgent to develop high-energy-density battery systems（Li-S battery,Li-O₂ battery,etc.）as candidates for the next-generation energy storage devices.Researchers have demonstrated that the laboratory-level lithium-sulfur batteries（LSBs）can reach an energy density of 400-600 Wh kg^-1,which is higher than those present commercial LIBs can do.Although LSBs have many advantages,their commercial development still faces many challenges.The main problems are as follows:（1）the low conductivity of sulfur/discharge products（Li₂S）leads to poor electrochemical performance;（2）the lithium polysulfides can easily be dissolved in the electrolyte,resulting in shuttle effects and various side reactions;（3）the large volume change of sulfur usually results in irreversible structural destruction of electrodes.The electrochemical performance of LSBs can be improved by properly des igning the host material of sulfur cathode,electrolyte composition,and the modifier layer of separator.In this dissertation,we developed two-dimentional nanomaterials as separator modifiers for high-performance LSBs.Then,we explored the effects of different modifiers on the performance of batteries;the main contents are shown as following:1.Few-layered MoS₂ embedded in N-doped carbon sphere as separator modifier for LSBs.We designed few-layed MoS₂ embedded in N-doped carbon sphere（MoS₂@NC）as separator modifier for LSBs and investigated the electrochemical performance.The MoS₂@NC modifier is composed of hierarchical spheres.Few-layed MoS₂ nanosheets have strong chemical interactions with polysulfides and the N-doped carbon matrix can efficiently improve the conductivity of modifier layer.At the same time,MoS₂@NC modifier has abundant heteroatom and interfacial defects,which can be used as captures for polysulfides.Electrochemical tests demonstrate that the MoS₂@NC modifier can suppress the shuttle effect and accelerate the redox reaction kinetics of electrode.As a result,the LSBs with MoS₂@NC-modified separators exhibit improved cycle performance and capacity.LSB reaches a reversible capacity of 874 mAh g^-1 after 80 cycles at 0.2A g^-1,and a capacity of 685 mAh g^-1 after 160 cycles at 0.5 A g^-1.2.A dual-functional MoS₂/graphene modifier as polysulfide barrier for high-performance LSBs.We have developed a dual functional MoS₂/graphene composite as separator modifier for LSBs.MoS₂/graphene modifier consists of MoS₂ nanoflowers wrapped with graphene nanosheets.The MoS₂/graphene modified separators are prepared using a vacuum-filtration method.The unique structural and chemical properties of the MoS₂/graphene modifier are beneficial to improve the electrochemical performance of LSBs.Functional MoS₂/graphene modifier can physically block LiPSs with graphene nanosheets and chemically suppress the dissolution of LiPSs with MoS₂ nanoflowers.Furthermore,the MoS₂/graphene modifier can form a 3D network,acting as an upper current collector which facilitates electron and ion transfer during cycles.XPS analysis suggests that MoS₂/graphene modifier has strong chemical interactions to polysulfides.The surface morphologies of the metallic Li anodes in the sycled LSBs further demonstrate the suppressed shuttle effect by MoS₂/graphene modifier.Therefore,the obtained LSB with the MoS₂/graphene modifier manifests both a superior rate capacity and an improved cycling capacity.LSB reaches a reversible capacity of 689 mAh g^-1 after 200 cycles at 0.5 A g^-1,and a capacity of620 mAh g^-1 after 200 cycles at 1.0 A g^-1.3.Few-layered MoS₂ nanowall/carbon cloth nanocomposite as multifunctional intercalation for high-performance LSBs.We prepared few-layered MoS₂ nanowall/carbon cloth（MoS₂/CC）nanocomposite as intercalation for LSBs.The vertical MoS₂ nanoflakes were directly grown on the surface of commercial CC by a simple hydrothermal growth method.MoS₂ nanoflakes interlace with each other to form a three-dimentional interconnection network structure.A single MoS₂ nanoflake is composed of 4-10 layers.The results indicate that the MoS₂/CC intercalation can inhibit the diffusion of polysulfides and reduce the shuttle effect through chemical adsorption and physical barrier.Moreover,the conductive intercalation provides a good contact with the surface of the sulfur cathode,acts as an upper current collector and greatly improves the transportation of elctrons/ions.MoS₂/CC can greatly improve the electrochemical performance of LSBs.LSB with the MoS₂/CC intercalation exhibits excellent capacity of 490 mAh g^-1 after 500 cycles at 1.0 A g^-1..LSB with the areal sulfur loading of 1.54 mg cm^-2 achieves a reversible capacity 810 mAh g^-1 after200 cycles at 0.5 A g^-1.4.Nb₂O₅/RGO nanocomposite modified separators with robust polysulfide traps and catalytic centers for boosting performance of LSBs.Heterostructual Nb₂O₅/RGO nanocomposite modified separator has been synthesized via a simple method and developed as an efficient LiPSs trapper and redox accelerator for LSBs.With the strong chemical interactions between Nb₂O₅ and LiPSs as well as superior catalytic nature of the Nb₂O₅ nanocrystals,the Nb₂O₅/RGO modifier shows high trapping efficiency and efficient electrocatalytic effects to the long-chain LiPSs,especially for the conversion of long-chain LiPSs to short-chain Li₂S₂/Li₂S.Moreover,the Nb₂O₅/RGO nanocomposite has abundant sulfophilic sites and defective interfaces,which are beneficial for the nucleation and growth of Li₂S as evidenced by in-situ XRD and analys is of the cycled separators.The effective regulation of LiPSs conversion endows the LSBs with excellent rate capability(816 mAh g^-1 at 3 A g^-1)and cyclic performance(628 mAh g^-1 after 500 cycles).More importantly,even at an elevated temperature of50°C or with higher sulfur loadings,the LSBs with the Nb₂O₅/RGO modified separator still exhibit high specific capacities and stable cycling performance.