Preparation And Application Of Co-based Materials For Lithium Sulfur Batteries

Lithium sulfur?Li-S?batteries hold high theoretical energy density?2600 Wh/kg?and specific capacity?1675 mAh/g?.Sulfur is rich on earth with high quality and low price.Therefore,Li-S batteries have a promising commercial prospect.However,it is still a long way to go for the commercial application of Li-S batteries.The main problems are as follows:poor electron conductivity of sulfur and lithium sulfide,high volume expansion and serious shuttle effect.Herein,such issues have been well addressed through hollow Co₉S₈ and polypyrrole-coated S and Co co-doped carbon nano-cages?PSCC?materials,which were designed and delicately prepared.The detailed contents of this thesis are as follows:1.Co₉S₈ materials were successfully prepared by using solvothermal method.SEM and TEM images showed that the prepared Co₉S₈ samples have hollow monodisperse spherical feature.In addition,the effects of different solvothermal reaction time on the morphology and electrochemical properties were systematically investigated.We concluded that the formation mechanism of hollow spherical Co₉S₈materials origins from the Ostwald ripening mechanism.Compared with carbon material?KJB?,polar Co₉S₈ material holds a stronger adsorption capability of polysulfide by using visualization test.Combined with X-ray photoelectron spectroscopy?XPS?,the mechanistic insight to this phenomenon were revealed:electron transfer between polysulfide?Li₂S₄?and Co₉S₈ materials;in addition,EIS,/SEM and charge-discharge curves showed that compared with KJB,Co₉S₈ materials were more favorable to inhibit the diffusion of polysulfide,reduce the polarization and stabilize the electrode structure.Therefore,Co₉S₈ materials can afford excellent rate and cycling performance in Li-S batteries.2.PSCC materials were successfully prepared by using multi-step reaction synthesis strategy,and the effects of solvothermal reaction time and temperature on the morphology and structure of S and Co co-doped carbon nano-cages?SCC?materials were fully studied.Thus,the optimum solvothermal reaction time and temperature were determined.Besides,the formation mechanism of SCC ion-exchange Kirkendall effect was revealed.Considering the conductivity and structural stability of the material,monodisperse dodecahedral PSCC samples were prepared by low temperature oxidation polymerization.The four-probe test showed that the conductivity of PSCC was greatly improved compared with that of the precursor?ZIF-67?.Visual adsorption tests showed that there was a stronger adsorption ability between PSCC and polysulfide,superior to that of ZIF-67.Then the deep mechanism of this phenomenon were revealed by using XPS analysis:the existence of polar functional groups in PSCC was mainly responsible for the strong chemical adsorption ability between PSCC and polysulfide.Finally,the CV curves,charge-discharge profiles,middle discharge voltage and impedance analysis showed that the PSCC/S composite had a smaller impedance and polarization phenomenon in charge-discharge process of Li-S batteries.The diffusion test of polysulfide for electrodes showed that PSCC played an important role in suppressing shuttle effect for practical cycling.Therefore,PSCC materials had excellent rate and cycling performance in Li-S batteries.