Preparation And Electrochemical Properties Of Cathode Materials Of Lithium/Sulfur Battery

Composite powder of carbon nanofibers （CNFs） and iron was synthesizedby hot filament chemical vapor deposition with iron powder as catalyst. Thereacting gas was the mixture of H₂and C₃H₆O. Then the composite powder ofCNFs/Fe was used as electrically conductive substance and adsorbing agentin the electrode containing sulfur to improve its electrochemical properties.Surface morphology and crystal structure of CNFs/Fe composite powder werecharacterized by scanning electron microscopy and X-ray diffractionseparately. Besides, elemental mass ratio of the composite powder wascharacterized by energy dispersive spectra. Electrochemical properties oftested by methods of constant current charge-discharge and measurement ofimpedance. Results indicate that the ratio of CNFs increases as the volumeflow rate between H₂and （H₂+C₃H₆O） decreases. When the rate between H₂and （H₂+C₃H₆O） is80/80(mL·min^-1/mL·min^-1), lots of CNFs grow withrelatively high degree of crystallizing and diameter of200nm. Cyclic tests ofcharge and discharge show that discharge specific capacity and cyclicdischarge stability of electrodes containing sulfur rise as content of CNFs/Fecomposite powder increases in electrodes containing sulfur. When masscontent of CNFs/Fe composite powder is35%and current density is200mA/g, initial discharge specific capacity of the electrode containing sulfur is821mAh/g and the discharge specific capacity is545mAh/g after40cycles.In the atmosphere of H₂and C₃H₆O, carbon fiber film deposited onto ironfoil by hot filament chemical vapor deposition. Then the film was sulfurizedby heating to form composite film of sulfur and carbon fibers, which was usedas cathode materials of lithium/sulfur battery. Morphology and crystalstructure of carbon fiber films were characterized by scanning electronmicroscopy and X-ray diffraction separately. Electrochemical performance ofthe composite film was tested by methods of constant current charge-dischargeand measurement of impedance. Results indicate that as distance betweenfilament and substrate of iron foil decreases, the density of carbon fibers increases firstly and decreases afterward. And the vertical orientation ofcarbon fibers turns good firstly and bad afterward. With the diminishingvolume flow ratio between H₂and （Ar+C₃H₆O）, the density of carbon fibersincreases and the vertical orientation of carbon fibers turns good. When thedistance between filament and substrate is6mm and the volume flow ratio is30/40(mL·min^-1/mL·min^-1), dense carbon fiber film with thickness of50μmdeposits onto iron foil. Those carbon fibers with diameter of3μm are mainlyvertically oriented and have relative high degree of crystallizing. After theprocess of sulfurizing, the composite film of sulfur and carbon fibers whichdid not contain binder was synthesized. When current density is200mA/g,initial discharge specific capacity of the composite film is1050mAh/g and thedischarge specific capacity is759mAh/g after50cycles.In addition, sulfur sol was fabricated through the reaction of Na₂S₂O₃andHCl at room temperature. Porous activated carbon was used to adsorbnano-sized sulfur particles in sulfur sol to form composite materials of sulfurand carbon. The composite materials were then used as cathode materials oflithium/sulfur battery. Morphology and crystal structure of the compositematerials were characterized by scanning electron microscopy and X-raydiffraction separately. Electrochemical performance of the composite film wastested by methods of constant current charge-discharge and measurement ofimpedance. Results indicate that diameter of the nano-sized sulfur particles isabout50nm and they are dispersed in the matrix of activated carbon. XRDresult shows that the composite materials of sulfur and activated carbon areamorphous. Cyclic tests of charge and discharge indicate that when currentdensity is200mA/g, initial discharge specific capacity of the compositematerials of sulfur and carbon is784mAh/g and the discharge specificcapacity is510mAh/g after50cycles.