Investigations On The Nano-Sulfur Cathode And Gel Polymer Electrolyte For Lithium Ion Battery Use

Elemental sulfur is a promising cathode material for the next generation ofhigh-specific-energy rechargeable lithium batteries due to its high theoretical specific capacity.In addition, sulfur is also inexpensive, nontoxic and environmentally benign. However, thelithium sulfur cell still faces several serious challenges. On the one hand, the low utilizationof active material sulfur due to the high electrical resistivity of elemental sulfur. On the otherhand, the rapid capacity loss due to the high solubility (in organic solvent electrolytes) of thepolysulfide ions that are formed during the discharge/charge processes. In order to addressthese challenges, various kinds of carbon materials have been used to accommodate sulfurand to prepare carbon-sulfur composites by simple chemical deposition. These carbon-sulfurcomposites here can overcome the challenges of sulfur as a cathode for lithium battery.Moreover, novel gel polymer electrolytes based on an electrospun polymer membraneincorporating with room-temperature ionic liquid were prepared here to suppress thedissolution of the polysufides generated during the discharge process and to improve the cyclestability of lithium sulfur cell.Carbon particle and carbon nanofiber supported carbon-sulfur composites were preparedby a chemical deposition method, respectively. The carbon particle-sulfur cathode withcarbon nanofiber as conductor exhibits an initial discharge capacity of1200mAh/g, andretains668mAh/g after50cycles. The carbon nanofiber-sulfur electrode prepared withcarboxy methyl cellulose (CMC) and styrene butadiene rubber (SBR) binder exhibits aspecific capacity of up to1313mAh/g at the initial discharge and a specific capacity of586mAh/g after60cycles. The carbon particle and carbon nanofiber can improve theconductivity of sulfur, carbon nanofiber can provide a more effective electronicallyconductive network for sulfur electrode.Porous carbon and porous carbon nanofiber were prepared by the carbonization of thepolymer mixture. Sulfur is uniformly incorporated into the porous carbon and porous carbonnanofiber via a new simple chemical deposition strategy. The novel porous carbon-sulfurcomposite maintains a stable discharge capacity of more than740mAh/g after100cycles at0.05C(1C=1672mA/g). The porous carbon nanofiber-sulfur nanocomposite with42wt%sulfur maintains85%of its initial capacity after30cycles at0.05C(1C=1672mA/g). Theporous carbon and carbon nanofiber provide an extremely high surface area to adsorb anddisperse sulfur and ameliorate its disadvantages, such as the insulating nature of sulfur and thesolubility of polysulfide intermediates in organic solvent based electrolytes. Sulfur was immobilized to graphene oxide to prepare graphene oxide-sulfur compositeby simple chemical deposition. The graphene oxides in the heat-treated composites have goodconductivity, extremely high surface-area, and provide a robust electron transport network.The graphene oxide network also accommodates the volume change of the electrode duringthe lithium-sulfur electrochemical reaction and effectively confines any polysulfides fromdissolving. The lithium sulfur cell exhibits a specific capacity of up to1000mAh/g at theinitial discharge and a specific capacity of954mAh/g after50cycles at0.1C(1C=1672mA/g).Poly(acrylonitrile)/poly(methyl methacrylate)(PAN/PMMA) membrane with highporosity was prepared by electrospinning technique, and novel gel polymer electrolytes basedon this fibrous PAN/PMMA activated with room temperature ionic liquid were prepared. Agel polymer electrolyte based on electrospun PAN/PMMA and ionic liquidN-methy-N-butylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR14TFSI) wasprepared and used for lithium/lithium iron phosphate(Li/LiFePO4) and lithium/sulfur(Li/S)cells. In Li/LiFePO4cell, it exhibits an initial discharge capacity of134mAh/g, and retainsretains92%of its initial discharge capacity after50cycles. In Li/S cell, it has higherdischarge capacity than that uses the traditional liquid electrolyte. A new lithium-sulfur cellsystem exhibits excellent performance with a unique combination of carbon nanofiber-sulfurelectrode and gel polymer electrolyte based on electrospun PAN/PMMA membrane andlithium bis (trifluoromethylsulfonyl) imide in N-methyl-N-butylpiperidiniumbis(trifluoromethanesulfonyl)imide (PPR14TFSI)-poly (ethylene glycol) dimethy ether(PPR₁₄TFSI-PEGDME,1:1, by weight) electrolyte. The discharge capacity of this cellremains at760mAh/g after50cycles.