Research On Sulfur-based Cathode Materials For Rechargeable Lithium Batteries

Rechargeable lithium batteries with high energy density are the keyresearch and development directions in the chemical power sources field.Elemental Sulfur (S) is a promising cathode material with high theoreticalspecific capacity and specific energy density for rechargeable lithium batteries.Other advantages of employing sulfur as the cathode material for lithiumsecondary batteries are its natural abundance, low cost, and environmentalfriendliness. However, low electrical conductivity of sulfur, dissolution ofpolysulfides in electrolyte and volume expansion of sulfur during cyclinggreatly limit the cycle life of the sulfur cathode and impede the widespreadpractical application of rechargeable Li-S batteries. In order to overcome theabove-mentioned problems, sulfur was combined with kinds of electricallyconductive species with sulfur-locked function to form various sulfur-basedcomposites. However, the cycle stability and rate properties of the compositeswere not satisfactory.Aiming at the above-mentioned problems of sulfur-based cathode materials, we undertook the research work from optimizing the nanostructureof the composites. We prepared three kinds of composites by introducingexcellent electrically conducting materials into sulfur-based composites via insitu polymerization or spray drying methods. In addition, we designed andprepared a dual-mode sulfur-based composite. The effects of differentadditives on the composite structure, electrochemical performance, and therelationship between them were studied. The main results were summarizedas follows:A novel pPAN-S@MWCNT composite material was prepared by usingPAN@MWCNT tube-shell composite as precursor. And the tube-shellstructure could be maintained in the sulfur-based composite. The cyclestability of the cathode was observably improved. And relatively stablecapacity450mAh g^-1compositewas obtained at current rate of4C. Thehomogeneous dispersion and integration of MWCNT in the composite createdan electronically conductive network and reinforced the structural stability. Asa result, the cycle and rate performances were significantly improved ascathode material for rechargeable Li-S batteries.Polyacrylonitrile/Graphene (PAN/GNS) composites have beensynthesized via in situ polymerization method for the first time, which serveas precursors to prepare cathode materials for high-performance rechargeableLi-S batteries. The PAN nanoparticles, less than100nm in size, were anchored on the surface of GNS, and this unique structure was maintained inthe sulfur-based cathode material. The electrochemical test results showedthat it exhibited a relatively stable cycle performance at0.1C. Even up to6C,competitive capacities370mAh g^-1composite(ca.800mAh g^-1sulfur) wereobtained. The superior performance of pPAN-S/GNS could be attributed tointroduction of GNS and the evenly composite structure. The GNS in thecomposite materials worked as three-dimensional (3-D) nano current collector,which could act not only as electronically conductive matrix, but also as aframework to improve the electrochemical performance.For the aspect of granulation, a novel porous nanostructuredpPAN-S@GNS composite material was prepared by using PAN@GNS porouscomposite as precursor, which was available via spray drying method from amixed aqueous suspension of PAN nanoparticles and graphene nanosheets.The pPAN-S primary nanoparticles embedded in micro-sized sphericalsecondary particles were wrapped homogeneously and loosely with a GNS3-D network. The electrochemical test results showed that the cyclingstability and rate performance of the cathodes were observably improved.After100cycles, about90%of the initial reversible capacity could beretained at0.1C. The capacities of300mAh g^-1compositecould be obtainedwhen the rate was enhanced to10C. The smaller grain sizes of the pPAN-Sparticles shortened the path of the ion diffusion in the particles. And the unique structure of pPAN-S@GNS composite created an electronicallyconductive network and reinforced the structural stability, leading to theoutstanding electrochemical performances as cathode materials forrechargeable Li-S batteries.A novel dual-mode sulfur-based material pPAN-S/mGO-S compositewas prepared via method of fractional steps. The composite demonstratedunique dual-mode electrochemical performances as a cathode material for therechargeable Li-S battery. The sulfur content of dual-mode composite couldbe obviously increased to65wt.%compared with pPAN-S compositecontaining less than50wt.%. Sulfur utilization and cycle stability wereevidently improved compared with mGO-S composite.The above-mentioned results indicated that the introduction of excellentelectrically conducting materials into sulfur-based composites couldobservably enhance the electrochemical performance of them, and theoptimization of the preparation technologic could make rechargeable Li-Sbatteries more close to practical realizationï¼›In addition, the concept ofdual-mode sulfur-based composite may promote the development of newsulfur-based composites with high performance.