| Along with the continuous development of social economy,the energy crisis and environmental problem have pushed the traditional fossil fuels to the forefront of public.At the same time,in the facing of intermittent and uncertainty of the clean renewable energy,the exploitation of efficient energy storage and conversion devices is becoming the key method to achieve sustainable development.At present,among many advanced energy storage systems,the lithium/sodium sulfur(Li/Na-S)batteries have attracted much attention due to their abundant resources(sulfur and lithium/sodium),non-toxicity,low cost,high energy density,and so on.However,even after decades of research,there are still many inherent difficulties and challenges on the road of the practical application of Li/Na-S batteries,such as insulation of sulfur and its discharge products(Li2S/Na2S),volume expansion of active material,dissolution and shuttle of intermediate polysulfide(Li2Sn/Na2 Sn,4 ? n? 8),as well as the dendrite problem of Li/Na anode,etc.Therefore,improving the conductivity of cathode materials,realizing the constraint and rapid transformation of Li2Sn/Na2 Sn are the preferred strategies to improve the electrochemical performance of Li/Na-S batteries.In recent years,the Fe,Co,Ni chalcogenides have been considered as the most promising "catalytic" materials for Li/Na-S batteries due to their strong affinity with Li2Sn/Na2 Sn,which as hosts of S can assist the active component to receive ions or electrons,further reducing the electrode overpotential and speeding up the conversion reaction kinetics of Li2Sn/Na2 Sn.Meanwhile,those polar hosts can also be compounded with the traditional carbon-based material to improve the overall conductivity of the cathode.As a result,combining the physical limitation and chemical adsorption together can inhibit the shuttle phenomenon and lay the foundation for the long cycle life for batteries.In addition,on the basis of the above analysis,designing and synthesizing of the hollow structure not only can increase the sulfur loading of electrode,but also provide adequate space to accommodate the volume change of sulfur during the charging and discharging process.In view of the above analysis,this thesis aims to limit the dissolution and diffusion of polysulfide and further enhance the electrochemical reaction kinetics of Li/Na-S batteries.Based on this,a series of novel Fe,Co,Ni chalcogenides are prepared and the S is encapsulated into them through melt diffusion method.Then,the batteries are assembled and their electrochemical properties are accordingly studied.The main contents are summarized as follows:(1)In order to improve the conductivity of sulfur composite cathode,Fe Co2S4 nanotube arrays are uniformly grown on the surface of the carbon cloth(CC)through two-step hydrothermal methods.Then,after the molten diffusion of sulfur(S),a flexible and self-supporting positive electrode(Fe Co2S4/CC@S)is obtained for the assembly of Li-S battery.Among the rest,the interconnected carbon fiber skeleton of the composite electrode ensures the basic electrical conductivity,whereas the polar Fe Co2S4 nanotube arrays not only boost the electron and electrolyte ion transfer but also inhibit the dissolution of polysulfide due to their strong chemical adsorption.Meanwhile,the hollow structures of those arrays can provide a large inner space to accommodate volume expansion of sulfur.More significantly,the developed composite electrode also reveals a catalytic action for accelerating the reaction kinetic of the Li-S battery.As a result,the Fe Co2S4/CC@S electrode delivers a high discharge capacity of 1384 m A h g-1 at the current density of 0.1 C and simultaneously exhibits a stable cycling ability at1.0 C(300 cycles)along with a high Coulomb efficiency(? 98%).(2)Through the research in chapter one,it is found that the adsorption capacity of the host to Li2 Sn is the critical factor to the cycling stability of Li-S battery.Therefore,in this chapter,the yolk shell hollow carbon spheres(Si O2-HC)are first prepared through sol-gel method and high-temperature pyrolysis process.Then,a layer of polar Co Se2 nanosheets are vertically grown on the surface of Si O2-HC and meanwhile a large amount of Si O2 is removed to obtain THC-Co Se2 hosts.After loading S,the S@THC-Co Se2 electrode not only possesses a large enough cavity with closed feature to relieve the volume expansion of active substance,but also has firm “three lines of defense” to prevent the outward diffusion of Li2 Sn through physical and chemical interactions.In addition,the outermost Co Se2 with good conductivity is also able to catalyze the transformation of Li2 Sn anchored on its surface,which further effectively accelerates the elimination of soluble intermediates and weakens the shuttle phenomenon.Based on above advantages,the initial discharge capacity of the Li-S battery assembled by S@THC-Co Se2 cathode is as high as 1230.0 m Ah g-1 at the current density of 0.1 C,while after 1000 cycles at 1.0 C,the capacity attenuation rate of each cycle is as low as 0.034%.(3)In the above two works,the S with poor conductivity is used as cathode to assemble the Li-S batteries,and the utilization rate of the active substance needs to be improved.Hence,selenium disulfide(Se S2)with better conductivity than S is selected as the active material and encapsulated it into a double-walled N-doped carbon(NC)@Ni Co2S4 hollow capsules(Ni Co2S4@NC-Se S2)at 180 °C to obtain high-performance Li-Se S2 batteries.On the basis of ensuring excellent electrical conductivity,the hollow Ni Co2S4 lining and its NC coating can not only suppress the shuttle effect of polyselenium/polysulfide by combining the chemical adsorption and physical constrain merits,but also are able to buffer the volume expansion of Se S2.Moreover,the polar Ni Co2S4 is beneficial to enhance the redox kinetic of lithium polyselenium/polysulfide.As expected,the Ni Co2S4@NC-Se S2 cathode delivers high reversible capacities of 1205.1 m Ah g-1 at 0.1 C and 673.5 m Ah g-1 at 2.0 C.More importantly,the hybrid cell shows an ultralong cycling life up to 800 cycles at 1.0 C with the attenuation rate as low as 0.038 % per cycle,which is more superior to that of Ni Co2S4-Se S2 and NC-Se S2 contrast eletrodes.(4)The storage mechanism of room-temperature(RT)sodium-sulfur batteries(Na-S)is similar to that of Li-S batteries,and the content of Na element in the earth's crust is richer than that of Li element and the price is cheaper.Therefore,in this chapter,we apply the research theories of Li-S batteries to RT Na-S batteries and study its electrochemical performance.However,the energy density of room-temperature RT Na-S usually rely on the efficient conversion of Na2 Sn during discharging and sulfur recovery during charging,which also is the rate-determining step in the electrochemical reaction process of Na-S batteries.In this work,a three-dimensional network(Ni-NCFs)host composed by nitrogen-doped carbon fibers(NCFs)and Ni hollow spheres is synthesized through electrospinning method.In this novel design,each Ni hollow unit not only can buffer the volume fluctuations of S during cycling,but also can improve the conductivity of the cathode along the carbon fibers.Meanwhile,the result reveal that a small amount of Ni is polarized during the sulfur loading process forming polar Ni-S bond.Furthermore,combining with the nitrogen-doped carbon fibers,the Ni-NCFs composite can effectively adsorb soluble polysulfide intermediate,which further facilitates the catalysis of Ni unit for the redox of sodium polysulfide.In addition,the in-situ Raman is employed to supervise the variation of Na2 Sn during charging and discharging process.As expected,the freestanding S-Ni S2@Ni/NCFs cathode exhibits outstanding rate capability and excellent cycle performance.In summary,this thesis designed and prepared four kinds of iron,cobalt,nickel chalcogenides(Fe Co2S4,Co Se2,Ni Co2S4 and Ni S2)and carbon composite materials by using different methods.After that,those composites are regarded as hosts to encapsulate elemental S or Se S2 and further assembled the Li-S,Li-Se S2 and RT Na-S batteries in turns.Meanwhile,the adsorption and catalysis during the charging and discharging process of the polar substrates to Li2 Sn,Li2Sen and Na2 Sn are fully investigated to improve the cycle stability,rate capability and electrochemical reaction kinetics of batteries.Furthermore,this thesis provides some useful research strategies to solve the challenges of Li-S and RT Na-S batteries. |
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