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Synthesis And Electrochemical Performance Of Cathode Materials For Lithium Sulfur Batteries

Posted on:2018-10-25Degree:MasterType:Thesis
Country:ChinaCandidate:Y Q TaoFull Text:PDF
GTID:2311330515975663Subject:Material Chemical Engineering
Abstract/Summary:PDF Full Text Request
A rechargeable lithium-sulfur?Li-S?battery has been considered to be one of the most promising energy storage technologies due to its high theoretical energy density,low-cost and environmental benignancy.However,commercial applications of Li-S batteries are still greatly hindered by a series of obstacles,in particular,the insulating nature of both sulfur and its discharge products,the shuttle of intermediate polysulfides,as well as the large volume change of sulfur during cycling.To overcome these obstacles,physical confinement approaches aiming to trap sulfur within various conductive porous carbon hosts have been widely pursued,but their success in terms of capacity decay and kinetic sluggishness is still limited.Here in this work,we have updated the confinement approach tthrough integrating polar metal oxides/sulfides onto the well-developed mesoporous carbon microsphere?MCM?framework and employed the integrated structure as a highly efficient host for the sulfur cathode,which could combine the advantages of physical adsorption and chemical interaction of sulfur species and contribute to a much improved electrochemical performance.Furthermore,close collaboration between theoretical calculations and electrochemical analysis as well as characterization techniques was realized in this work.The main conclusions are summarized as follows:?1?An integrated structure of mesoporous carbon microspheres decorated with Nb2O5 nanocrystals?MCM/Nb2O5?was synthesized through a facile wet impregnation method and further employed as a new host for sulfur cathode via a classic melt-diffusion strategy.Both the adsorption experiment and theoretical calculation indicate that the MCM/Nb2O5 host has a stronger entrapment ability towards lithium polysulfides than MCM due to the synergistic contribution of physical adsorption and chemical interaction.Furthermore,an electrochemical kinetic study reveals that the Nb2O5 nanocrystals could serve as an electrocatalyst,significantly accelerating the kinetics of polysulfide redox reactions,especially for the reduction of soluble Li2S6/Li2S4 to insoluble Li2S2/Li2S.With these multiple advantages,the resulting MCM/Nb2O5/S ternary cathodes could exhibit high specific discharge capacity,excellent cycling stability and rate capabality.They could deliver a high initial discharge capacity of 1289 mAh g-1 at 0.5C with a reversible capacity of 913 mAh g-1 after 200 cycles.Moreover,the ternary cathodes demonstrate an exceptional rate capability with 887 mAh g-1 at 5C,and a high reversible capacity of 650 mAh g-1 at 2C after 500 cycles.The concept of accelerating the kinetics of polysulfide redox reactions will be significantly useful to develop Li-S batteries with high-rate and long-cycle properties.?2?Polar MoS2 and MoO2 were dispersed into the MCM framework respectively?MCM/MoS2 and MCM/MoO2 composites?via the simple hydrothermal method and the composites were further used as the hybrid hosts for sulfur cathodes.Much improved electrochemical performance could be achieved by the addition of either MoS2 or MoO2,and the better cycling stability and rate performance was realized by the MCM/MoS2/S composite cathodes.Moreover,MoS2 was proved to be electrochemically active within the working voltage range of sulfur cathode,which could contribute extra capacity for the sulfur cathode,and have a stable cycling performance without capacity degradation in DOL/DME electrolyte.The replacement of a part of electrochemically inactive carbon additives with MoS2 could thus further increase the overall energy density.The exploration of such electrochemically active host materials may provide a new possibility to improve the electrochemical performance of Li-S batteries.
Keywords/Search Tags:Lithium-sulfur battery, Sulfur cathode, Mesoporous carbon microsphere, Nb2O5, MoS2
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