Modified Preparation And Electrochemical Performance Of Lithium-sulfur Battery Cathode Materials

Lithium-sulfur battery has been extensively considered as a new type of energy storage devices due to its ultra-high theoretical energy density(2600 Wh kg-1),low cost and non-toxicity of electrode materials,and it has become one of the most promising candidates for next generation rechargeable batteries.However,there are still several critical challenges existing in the sulfur cathode presently.Firstly,sulfur and its discharge products have low electronic conductivity.Secondly,rapid dissolution and diffusion of polysulfides will cause serious shuttle effect.And thirdly,a large volume expansion of sulfur can cause collapse of the electrode structure during charging-discharging process.All of these problems severely restrict the practical applications of lithium-sulfur batteries.In order to solve the above problems,this thesis intends to improve electrochemical performance of the lithium-sulfur battery by changing the electronic conductivity of sulfur cathode by addition of conductive carbon materials and enhancing the adsorption of polysulfides to reduce shuttle effect via nitrogen doped carbon material and metal sulfides.Firstly,the conductive carbon black Vulcan XC-72(XC-72).which pore size mainly distributed in the mesoporous range,is used as the support of sulfur to improve the conductivity and utilization of the sulfur electrode;then the NiCo2S4/C composite is prepared as a host.The reaction of sulfur ions in polysulfide with Ni and Co ions in NiCoS4 can relieve the shuttle effect of polysulfides.Finally.the urchin-like NiCo2S4 is synthesized by two-step hydrothermal method,and it is solely used for the host of sulfur to improve the conductivity and inhibit shuttle effect.The obtained NiCo2S4/S composite delivers excellent electrochemical performance with high initial capacity.good rate performance and cycle stability.The detailed contents and results are as follows:1?Carbon black XC-72 and nitrogen-doped carbon material(N-C)were selected as the hosts of sulfur.Firstly.XC-72 with high mesoporous porosity and good conductivity was adoptd as the host of sulfur.The effect of mass content of sulfur(35%,50%.65%,70%)on electrochemical proformence was investigated.Due to the addition of carbon material,the electronic conductivity and the utilization of sulfur in the electrode material increase.However,as the sulfur content increases,more sulfur can not be distributed in the mesopores of XC-72,and thus it results in lower utilization.When sulfur content increases up to 50%,the electrode material shows initial capacity of 1061 mAh g-1 and has a capacity of 405.6 mAh g-1 after 100 cycles at current density of 0.1 C.Considering nitrogen-doped carbon material not only has good electronic conductivity but also has stronger chemic-al adsorption with intermediates of polysulfides,thus it can effectively alleviate the shuttle effect and improve the utilization ratio of active material,and then lead to improved cycle stability and rate performance.For the first time,riboflavin with advantages of cheap and nontoxic carbon/nitrogen source was used to synthesize nitrogen doped carbon material(N-C),and further applied as the sulfur host.The N-C/S composite electrode shows initial capacity of 773 mAh g-1 at 0.5 C.After 100 cycles,the capacity retention rate is 76.6%.Compared to the XC-72/S electrode with same sulfur content,the capacity retention rate is improved by 30%.2?NiCo2S4/XC-72 composite acts as host for sulfur.Due to the stronger bonding force between metal ions and sulfur ions,metal sulfides can inhibit the polysulfides shuttle effect better than nitrogen-doped carbon materials.Using XC-72 as carbon source,NiCo2S4/XC-72 was obtained by uniformly depositing NiCo2S4 nanoparticles at the surface of carbon material through microwave-assisted heating method.Then the NiCo2S4/XC-72/S composite was prepared by melt infusion.As a result,NiCo2S4/XC-72/S shows high initial capacity of 1103 mAh g-1 and capacity retention of 48%after 300 cycles.Even at high current density of 1 C,the capacity retention rate is 70.5%after 500 cycles.At a larger high current density of 2 C,the specific capacity of the battery is still up to 475 mAh g-1.The cycle stability and rate performance of NiCo2S4/XC-72/S in the lithium sulfur battery are better than Co3S4/XC-72/S,Ni3S4/XC-72/S and NiCo2S4/S.The main reasons are as follows:one is carbon material increases the electronic conductivity of the electrode materials and the other is the binary metal sulfide has stronger chemical adsorption with lithium polysulfides and also can catalyze the reduction of sulfur.Due to the synergistic effects,the electrochemical performance can be enhanced.3?Urchin-like NiCo2S4 is solely used as the host of sulfur.Urchin-like NiCo2S4 was synthesized via two-step hydrothermal process,and then sulfur was melted and infused to achieve NiCo2S4/S composite.This composite delivers a capacity of 1028 mAh g"1 at 0.1C.Even at the high rate of 2 C,the specific capacity is 482 mAh g-1.It also demonstrates a high stable cycle performance with a decay rate of 0.18%per cycle at 1 C for 300 cycles.The NiCo2S4/S composite has perferable cycle stability and rate performance,the reasons are as follows:on the one hand,the unique urchin-like structure not only facilitates the storage of sulfur but also facilitates the penetration of electrolyte and the diffusion of lithium ions;on the other hand,NiCo2S4 has strong chemisorption with polysulfides and can catalyze the reaction of polysulfide to lithium sulfide,thereby alleviating the shuttle effect during charge and discharge processes.In summary,this thesis has adopted various supports as the hosts of sulfur,the hosts not only improve the electronic conductivity but also boost the adsorption of polysulfides,thus it can improve the electrochemical performance of lithium sulfur battery.It will open up a new direction for preparation of better cathodes for the lithium-sulfur battery.