Performance Study Of Lithium Sulfur Battery Cathode Materials Constructed Based On Oxidation Sulfur-loading Method

Lithium sulfur(Li-S)battery is regarded as one of the most promising high energy rechargable batteries,mostly owing to its high theoretical gravimetric energy(2600 Wh Kg-1)that is five times higher than that of the commercial lithium ion batteries(500 Wh Kg-1).However,the inherent insulation of sulfur,shuttle effect of dissovled intermediates,and the volumetric variation during cycling,lead to low utilization of cathode active material,inferior rate performance and shortened cycle life,which seriously hinder the practical application of Li-S batteries.Herein,based on an innovative inside-out sulfur loading method,a series of microporous carbon coated sulfur composites with hollow structure are designed and synthesized.Additionally,the host material with dual sulfur confinement effects is introduced for its strong chemical adsorption functions,to improve the cycling performance of Li-S batteries.A facile in-situ oxidation method is proposed.As the structure template and sulfur source,the zinc sulfide(Zn S)nano sphere is firstly prepared,fol lowed by microporous carbon coating.Then,the Zn S is in-situ oxidized into element sulfur inside the carbon shell,employing the iodine as the oxidation agent.The generated sulfur is tightly anchored on the inwall of the hollow carbon sphere,forming a carbon coated sulfur composite with double-hollow-sphere structure.This unique structure provides enough internal space for the volumetric expansion during charging process,meanwhile,enables superior physical blockage to the dissovled intermediates of lithium polysulfide(LPS)by the microporous carbon shell.Therefore,the composite exhibits interesting cylcing performance with the dacay rate of 0.05 % per cycle during 1000 cycles at 1 C-rate.Even without the protection of the additive Li NO3,it also displays the coulombic efficiency of about 93 %,which indicates excellent immobilizing effect of microporous carbon shell to the LPS.Based on the in-situ approach,the controlable structures and sulfur confinement functional designs of the hollow sulfur/carbon composites are achieved through adjusting the micromorphology of Zn S precursor.The surfactants are adopted to synthesize a clustered-Zn S(C-Zn S)precusor by changing the surface energy state.After the microporous carbon coating and in-situ oxidation,a novel microporous carbon coated sulfur composite with clustered microcapsule structure is obtained.The high specific surface area and the in-situ preparation guarantee the homogeneous distribution of sulfur inside the carbon frame,even under the sul fur loading of as high as 86 wt%.The particular clustered structure enables the layer by layer spatial blockage to the LPS,which leads to improved cycling performance.In another part of work,the Zn S nano-rods are vertically planted on the surface of Zn S sphere to prepare a rambutan-like Zn S precursor through the solvothermal reaction and single source decomposition.The hollow rambutan shaped hollow sulfur/carbon composite is synthesized after the microporous carbon coating and in-situ sulfur preparation.The composite contains three kinds of pores that are central hollow macropore,marginal straight-through meosopores connected with the central macropore,and the micropores distributed on the outer carbon shell,respectively.The three kinds of pores are seperated spatially,perform their respective single function:(i)Macropores for accommodating the volumetric expansion,(ii)mesopores for sulfur storage,(iii)micropores for LPS blockage.Meanwhile,they also synergistically collaborate to suppress the sulfur loss.Therefore,it delivers outstanding electrochemical performances.A kind of organic metal sulfides,containing the elements of metal,sulfur,carbon and nitrogen at the same time,is employed as the the precursor to prepare the nitroge-doped carbon encapsulating sulfur composites,via single source precursor decomposition and in-situ oxidation process.During the calcination,the metal and sulfur are expected to form metal sulfide nano particles.And the carbon and nitrogen are transformed into nitrogen-doped carbon frame,coating outside of the metal sulfide particles.The nitrogen-doped carbon encapsulating sulfur composites with two dimensional and three-dimensional unit structures are respectively prepared from the organic ferro-compound and organic zinc-compound.The porous carbon frames of the composites provide physical sulfur confinement effect,and the rich well-distributed polar functional groups enable excellent chemical adsorption capability to LPS.Hence,they demonstrate excellent cell properties.A polar urchin-shaped tungsten oxide(W18O49)is prepared and employed as the sulfur host material,owing to its strong chemical bonding capability to LPS.Sulfur is chemically deposited on W18O49,followd by PPy coating,forming a sandwich structured W18O49@S@PPy composite.This unique composite possesses both chemical anchoring function of W18O49 and spatial confinement effect of PPy.The dual sulfur confinement effects enable W18O49@S@PPy with perfect cyclic stability.Decay rate of 0.006 % and 0.005 % per cycle are obtained during 1000 cycles,at 1 C and 1.5 C respectively.Outstandingly,the dacay rate of 0.005 % per cycle is also achieved at 2 C during 2000 cycles.