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Preparation And Lithium-storage Properties Of Carbon/iron Sulfide Composite

Posted on:2014-02-12Degree:DoctorType:Dissertation
Country:ChinaCandidate:B WuFull Text:PDF
GTID:1221330398983418Subject:Materials Science and Engineering
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Rechargeable batteries, as an environmentally friendly energy style hasbeen generally accepted by consumers. As one kind of rechargeable batteries,lithium-ion battery (LIB) with significantly high energy density and cyclestability has become the most popular research direction in the secondarybattery. Graphite is used as the anode electrode material (with theoreticallithium storage capacity372mA h g-1) for most of the commercialized LIB,but its performance (especially the capacity) can not meet the demands fornew application fields (such as hybrid electric vehicles,pure electric vehiclesand energy-storage station). Development of new LIB system with highercapacity, good cycling stability and high-rate performance is urgent.Iron sulfides have been used as cathode materials in thermal primarybatteries owing to their facile preparation process and high lithium storagecapacity of609mA h g-1for FeS (iron monosulfide) and894mA h g-1forFeS2(ferrous disulfide). There are still many obstacles for iron sulfides when being applied as anode materials for LIBs. The advantages (structural stabilityand high conductivity) of carbon nanomaterials (such as carbon coating andcarbon nanosheets) can remedy the disadvantages (poor stability andinefficient use of active materials) of iron sulfide materials. This researchwork focus on the preparation of new carbon/iron sulfide composites withlarge lithium storage capacity and good cyclic performance (especiallyhigh-rate performance) for LIB application.First, we obtained two kinds of carbon-based iron sulfide composites“hollow-rizition” carbon encapsulated iron sulfide nanoparticles (FeS@C) andiron sulfide/carbon nanosheets aggregation (FeS/CNS) from high-pressureautoclave system by catalytic pyrolysis and solvothermal reaction system,respectively. The formation mechanism of these two structures wereinvestigated based on the measurement results of different samples from seriesreaction conditions:(1) The formation of FeS@C contains two mian steps:Carbon feedstock transformed to carbon layers via a“Adsorption-Diffusion-Precipitation” process on the surface of ironnanoparticle,then sulfur induces iron atom (in core) to transfer to the carbonshell with cavum left between core and shell.(2) The formation of FeS/CNScontains the following steps: sulfur and iron in ferrocene combined to formiron sulfide nanocrystal, followed by catalysis from carbon feedstock to CNS,quantities of CNS (with iron sulfide nanocrystals on the surface) aggregatedwith each other into micro-sized particles to reduce their high surface energy. Lithium storage capacity and cyclic stability of the above two structureswere investigated by series of electrochemical tests. We found that theutilization efficiency of iron sulfide in the two structures is very high and theyshowed high lithium storage capacity and good cyclic stability:(1) Theutilization efficiency of iron sulfide in one kind of FeS@C that used heavy oilas carbon feedstock containing15.41wt.%iron sulfide reaches91.8%, and thereversible capacity attains633.4mA h g-1under the current density of50mAg-1, with the initial coulombic efficiency68.7%and capacity fading rate percycle0.75%(in50cycles); the reversible capacity under5000mA g-1after50cycles is294.1mA h g-1, which is67.7%of the capacity got from50mA g-1after same number of cycles (434.4mA h g-1);(2) The utilization efficiency ofFeS/CNS treated by600°C annealing process (with60.80wt.%iron sulfide)is up to104.8%, and such structure shows reversible capacity977.8mA h g-1under50mA g-1with initial coulombic efficiency62.5%and capacity736.9mA h g-1after50cycles (capacity fading rate per cycle0.52%). The initialreversible capacity under500,1000,2000and5000mA g-1are782.7,734.4,555.8and541.2mA h g-1, and the capacity after50cycles are524.2,419.0,383.0and346.7mA h g-1, respectively. FeS/CNS shows good high-rateperformance.CNS in FeS/CNS structure is stacked by graphene layers in fact. Suchtremella-like graphene nanosheets (T-GNS) have potential to be a new kind ofcarbonaceous electrode materials for lithium storage. The annealing temperature could change the number of layers, sulfur contents and porestructure in T-GNS structures, which could affect the electrochemicalproperties of such material. We found that the electrochemical behavior of the1000°C annealing product (T-GNS-1000) exhibits similar lithium-storageproperties as that of monolayer graphene structure. Furthermore pore structureon the surface could also provide lithium storage capacity. The T-GNS-1000shows the reversible capacity of528.7mA h g-1under50mA g-1, initialcoulombic efficiency32.1%and capacity of413.5mA h g-1after50cycles(with capacity fading rate per cycle0.48%). The T-GNS structure possessesgood stability during high current density. The reversible capacity under5000mA g-1after50cycles is225.6mA h g-1, which is54.5%of the capacity gotfrom50mA g-1after same number of cycles (with capacity fading rate percycle0.15%).The unique structure of FeS/CNS provide good candidate for sulfur/carbonnanosheets composite (S/CNS) which could be obtained via a simplecomproportionation reaction. The structure can effectively suppress thedissolution of elemental sulfur into organic electrolyte, the intermediateproduct lithium polysulfide (Li2Sn) during the charge/discharge process couldbe adsorbed on the surface of CNS, which could reduce the loss of activematerial, so that the active material maintains good cyclic performance. Forexample, the S/CNS electrode with sulfur content of20.18wt.%exhibitsinitial capacity of2302.9mA h g-1, reversible capacity of1332.7mA h g-1at 50mA g-1, and initial coulumbic efficiency57.8%. After50cycles, thecapacity can maintain854.3mA h g-1(with capacity fading rate per cycle0.88%). The sulfur in S/CNS shows high utilization efficiency and S/CNS isexpected to be a candidate as cathode material for Li/S battery.
Keywords/Search Tags:Lithium-ion battery, anode material, iron sulfide, carbonencapsulated structure, carbon nanosheets, graphene, lithium/sulfur battery
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