Research Of Enhancing The Cycling Performance Of Sulfur-carbon Composite Materials

Higher discharge capacity and easier access to cheaper raw materials are the advantages of lithium sulfur batteries over lithium ion batteries, but lithium sulfur batteries have the defect of relatively poor cyclye performance, hence systematic experiments should be arranged to improve the cycling performance of lithium sulfur batteries. Due to the very poor electrical conductivity of sulfur(S), to achieve good electrochemical performances of sulpur cathode, carbon materials with good electrical conductivity are commonly utilized as electron conducting framework additives to the cathode of lithium sulfur battery. In this dissertation, activated carbon(AC) with very high pore volume are firstly synthesized as carrier of sulfur, and then the electrochemical stable activated carbon/sulfur composites(AC/S) with high discharge capacity were prepared. Secondly, the as-synthesized AC/S were further wrapped with graphene. By altering the synthesis method of graphene wrapping layer and adjusting the morphology and amount of graphene wrapping layer, the influences of graphene wrapping layer on the cathode material performances are discussed so as to obtain cathode materials with excellent electrochemical performances for lithium sulfur batteries.A high-pore-volume activated carbon material AC800 synthesized by high temperature activation method is impreganated with sulfur to make a composite material with 60wt% S loading(AC800/60S). The initial discharge capacity of AC800/60 S at 0.1C is as high as 1165 m Ah/g. After 300 cycles at 1C, the discharge capacity of AC800/60 S still remains 375 m Ah/g. AC900 prepared at higher activated temperature has a high pore volume of 2.076 cm3/g. The highest discharge capacity of AC900/70S(70wt% sulfur loading) reaches 1100 m Ah/g, and the discharge capacity of AC900/70 S still remains 328 m Ah/g after 600 cycles at 1C. AC900/70 S is a superior and stable material which lays a solid foundation for further improvements.As for carbon/sulfur materials with single graphene wrapping layer, the initial dischage capacity of AC800/60S@GNS(X) at 0.1C reaches a very high value of 1596 m Ah/g, and the average dischage capacity of AC800/60S@GNS(X) at 3C high discharge rate still remains a high value of 699 m Ah/g. After 500 cycles at 1C, the discharge capacity of AC800/60S@GNS(X) still remains 640 m Ah/g. AC900/70S@GNS(Y) with higher sulfur content and lower graphene amount also displays a very high discharge capacity of 1395 m Ah/g for the first cycle at 0.1C, and after 600 cycles at 1C the discharge capacity still remains 458 m Ah/g. The frozen drying method can retain the highly porous structure of graphene wrapping layer, which can adsorb more electrolyte to alleviate diffusion polarization, and thus greatly enhance the high rate capability. Compared with AC900/70 S, AC900/70S@GNS(X)#C shows a high average discharge of 454 m Ah/g at 3C. AC900/70S@r GO#HI with a graphene wrapping layer obtained by HI reduction method mainly improves the high rate performance over 1C rates.As for carbon/sulfur materials with dual graphene wrapping layers, the initial discharge capacity of all these materials are all above 1217 m Ah/g at 0.1C. At 3C high discharge rate, P(CNT)-C#GNS(D) displays a high average discharge capacity of 510 m Ah/g, and after 560 cycles at 1C the discharge capacity still remains a high value of 482 m Ah/g, and the subsequent capacity decay rate is very small. With the same graphene amount in the wrapping layers, the P-C materials with a porous layer inside and compact layer outside have better electrochemical performaces than the C-P materials with a compact layer inside and porous layer outside. The addition of CNT makes the porous graphene layer even more porous, which can improve the diffusion process and thus the high rate capability. ’When the amount of graphene wrapping layer is doubled, the improvement is more marked.