| Compared to intermittent solar and wind energy,renewable rechargeable battery system has been serving people for hundreds of years.With the extensive application of portable electronic devices and electric vehicles,lithium-ion batteries become the current popular energy storage devices.As a branch of lithium-ion batteries,since the 1990s,lithium-sulfur batteries have aroused the concentrated interest from scientists.Due to the high theoretical specific capacity of 1675 mAh g-1 and the theoretical energy density of 2600 Wh Kg-1,which is almost 6 times of the current commercial LiCoO2 battery(387 Wh Kg-1),lithium-sulfur battery is considered to be the next generation of energy storage system.However,the commercialization of lithium-sulfur batteries still faces with two major technical challenges,low sulfur utilization and inferior cycle life.The insulated property of sulfur leads to the capacity loss of inner sulfur,resulting in low utilization.Discharge intermediates,lithium polysulfides,easily dissolve into ether-based electrolyte and undergo the subsequent migration,causing short cycle life and serious sulfur loss.A variety of porous carbon material as matrices for sulfur,not only improve the electron conductivity of sulfur electrode but also offer strong capillary adsorption force to ease the diffusion of lithium polysulfides into electrolyte.In spite the application of carbon material can largely improve the electrical conductivity of sulfur electrode,but the capillary adsorption of porous carbon matrices can not absolutely eliminate the escape of lithium polysulfides.Based on capillary physical adsorption,chemical binding is introduced for sulfur/carbon composites to improve the sulfur utilization and cycle life,combining physical adsorption and chemical coupling to inhibit the migration and diffusion of lithium polysulfides,In our reasearch work,on the basis of capillary adsorption from porous graphene in sulfur/graphene composite,we adopt a series of effective strategies to modify,functionalize and optimize sulfur electrode including functionliazing graphene,adding functional additives,modifying sulfur/graphene composite with polymer and inserting the functional interlayer.Finally,graphene were recycled from the cycled sulfur/graphene composite and applicated in oxygen reduction reaction,which effectively exploits the new potential application for the recycled graphene.The detailed works are as follows:(1)Graphite and sulfur were encountered high energy ball milling assisted with dielectric barrier discharge plasma and prepared into ultrafine nano sulfur anchored on oxygen doped graphene in one step.Compared with bulk graphite,synthesized porous graphene can provide large specific surface area for uniformly dispersing ultrafine nano sulfur and increasing the electron conductivity of sulfur,and offer aboundant pores to accommodate lithium polysulfide by chemical adsorption.Nano sulfur is benefical to shorten the transport pathway of ion and electron arriving to the internal sulfur and improve the sulfur utilization.The produced oxygen-contained groups on graphene further provide more chemical binding sites for capturing and confining the escape of lithium polysulfide from graphene matrix.(2)Commercial molybdenum powder was used as the additive to modify sulfur/graphene composite,introducing a functional additives towards lithium polysulfide.On the one hand,commercial molybdenum powder can enhance the electrical conductivity of the sulfur electrode.On the other hand,oxides on the outer layer of molybdenum particles can effectively alleviate the diffusion of lithium polysulfide and enlarge the cycle life of lithium-sulfur batteries through the chemical binding with lithium polysulfides.The analysis results show that the valence state of the outer oxides on molybdenum particles has been changed during charge and discharge processes.Although molybdenum trioxide on molybdenum particels was reduced into molybdenum dioxide after discharge,both molybdenum oxides are beneficial for the continuous chemical bonding towards lithium polysulfides.(3)Quaternary ammonium-containing polyd(imethyldiallylammonium)chloride(PDDA)polymer was ultilized to functionalize sulfur/graphene composite with the aim of limiting the diffusion of lithium polysulfides via electrostatic interaction between ammonium cations and polysulfides anions.Chain-like polymer coating formed a fishnet-like protective screen,which can alleviate the diffusion rate of lithium polysulfides migrating to lithium anode.Although PDDA polymer is a nonconductive polymer,only small amount of that,in fact,were modified on sulfur/graphene,which hardly affect the electron transport.All analysis results show that sulfur/graphene composite functionalized with 10 times the mass of PDDA polymer have the optimal performance.The corresponding capacity retention is almost two times that of sulfur/graphene.In addition,we adjusted the functionalization sequence of PDDA polymer,which modify graphene first and then load sulfur.It was found that PDDA polymer directly modifying sulfur/graphene show better electrochemical properties.(4)Hierarchically porous cobalt disulfide(CoS2)was grown on carbon fiber paper and used as interlayer inserted between sulfur/graphene electrode and separator.CoS2/CP interlayer was inserted in the traditional battery configuration,which can introduce chemical binding and physical barrier towards lithium polysulfides on the basis of the capillary adsorption derived from graphene in sulfur/graphene.Hierarchically porous CoS2 layer with metal-like electron conductivity acts as upper current collector to increase the utilization and physically blocks the diffusion of the lithium polysulfides.Most important,Co4+ and S2-of CoS2 can respectively couple with polysulfides anions and lithium cations of lithium polysulfides,providing robust chemical binding.Electrodeposition precursor,cobalt hydroxide,was undergone subsequent calcination and leaf-like and single layer of CoS2 nanoparticels were garnered,where CoS2 nanoparticels furnish enough active sites for capturing lithium polysulfieds.Finally,the visual adsorption experiment intuitively shows that CoS2 has the advantage of the rapid adsorption and strong chemical bonding towards lithium polysulfides.(5)Sulfur/graphene composite is served as the cathod material in lithium-sulfur battery and repeately cycled for 100 cycles.The cycled graphene were rinsed and recycled,and used to work as the electrochemical catalyst for oxygen reduction reaction(ORR).Electrochemical charge and discharge process realized the sulfur doping on graphene due to the sulfur species environment.Sulfur-doped graphene has a higher oxygen reduction potential and a reduction current in contrast to undoped graphene,and the catalytic reaction pathway is closer to efficiently one-step reaction.Compared with the commercial 20 wt%Pt/C electrochemical catalyst,the obtained sulfur-doped graphene has more stable catalytical ability and methanol durability.All the results show that the sulfur-doped graphene achieved from electrochemical cycling has superior catalytical performance for ORR,which is mainly attributed to the charge transference of carbon atoms on sulfur-doped graphene,and the enhancement of electron transfer rate and adsorption capacity oxygen on graphene. |
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