The Application Of Metal Sulfides For Cathode Materials In Lithium Sulfur Batteries

Based on the drawbacks of current cathode materials in lithium sulfur batteries,this thesis developed several methods which combines with metal sulfide.On the one hand,simple material synthesis methods were applied to construct cathode materials for lithium sulfur batteries with metal sulfide and sulfur.On the other hand,we have studied their electrochemical performance by studying their CV,EIS,GITT and charge/discharge properties.Meanwhile,the methods in this thesis are easy to scale up,and the original materials are normal and cheap,which is significantly important for their further application from the perspective of the industry.Specifically,the main research content of this thesis includes the following aspects:1.By applying a simple method called "melting-diffusing-reacting" method,we have prepared S/NiS2-C composites successfully with nanostructure.In this composite,NiS2 particles are uniformly distributed across the matrix of carbon,while the elemental sulfur is anchored in the structure with NiS2 and C.Owing to the preparation of composite and the formation of NiS2 are at the same time,each component can be distributed uniformly in this composite.In this cathode material,NiS2 has different roles to play.On the one hand,NiS2 can provide an extremely strong interaction towards polysulfides during the cycling of lithium sulfur batteries,so polysulfide can be anchored at the cathode side as well as improved electrochemical cycling performance can be achieved.On the other hand,because of the superior catalytic properties towards pyrite-type?FeS2?compounds and polysulfides,NiS2 in this composite can accelerate the transformation between sulfides,which significantly improves the rate performance of lithium sulfur batteries.Besides,we studied the interaction between NiS2 and polysulfide?Li2S4?by the computational method.Finally,the as-prepared S/NiS2-C electrodes showed a good performance,maintaining 730 mAh g^-1 after 200 cycles at a current density of 0.5 C,and 544 mAh g^-1 after 500 cycles at a current density of 2 C,respectively.2.From tin powder,porous carbon and elemental sulfur,this thesis prepared SnO2/S/C composite and SnS2/S/C composite with the help of a baked-in-salt or sealed-in-vessel approach,respectively.In each composite,both SnO2 and SnS2 can provide an anchor interaction towards polysulfides through their strong chemical polarity,while porous carbon can absorb polysulfides physically and improve the conductivity of composites directly.Meanwhile,with the help of the computational methods,we have compared the interaction between polysulfides?Li2S4?and two metal compounds?SnO2 or SnS2?and guess the better material for good electrochemical performance.Besides,from the perspective of the experiment,this thesis compared the cycling performance of SnO2/S/C composite and SnS2/S/C composite in lithium sulfur batteries.SnS2/S/C electrodes showed a better performance,which can maintain 780 mAh g^-1 after 300 cycles at a current density of 0.5 C with 78%sulfur loading in the composite.Then,SnS2/S/C electrodes were charged and discharged at different current densities,which indicates their superior rate performance.Finally,for the further development of this composite,full batteries were assembled with SnS2/S/C cathode and Ge-C anode after pre-lithiation process,maintaining 613 mAh g^-1 after 50 cycles at a current density of 0.5 C.