The Preparation And Modification Of Li₂S-Based Lithium-Sulfur Battery Cathode Materials

Lithium-sulfur（Li-S）batteries stand out from many next-generation battery systems due to their high theoretical specific capacity and high energy density.Compared with the traditional sulfur anode,the lithium sulfide（Li₂S）cathode material has the advantages of no volume expansion effect and can match other negative electrodes other than lithium metal.However,there are still some problems such as poor conductivity of Li₂S cathode materials and shuttle effect of lithium polysulfide（Li PSs）,which lead to high activation energy barrier in Li₂S oxidation process and poor cycle life of Li-S battery.Conductive carbon materials are widely used as host materials for Li-S batteries due to their high electrical conductivity,but reasonable modification is needed to improve their adsorption and catalytic conversion of polysulfide.This paper is based on the Li₂S-based Li-S battery cathode material research,through the introduction of heteroatom,highly dispersed metal atoms,and metal oxide catalysts on conductive carbon materials,to explore the mechanism of lower activation energy barrier of Li₂S,catalytic conversion mechanism,aimed at easing shuttle effect,speed up the reaction kinetics,effectively improve the performance of Li₂S-based Li-S battery,the main research work is as follows:（1）To control the particle size and morphology of Li₂S,and reduce the manufacturing cost,N/S co-doped porous carbon（NSPC）was synthesized by the hard template method,and Li₂S/NSPC composite cathode material was obtained by reducing cheap lithium sulfate（Li₂SO₄）to Li₂S by in-situ carbothermic reduction reaction.The particle size of Li₂S was reduced effectively,the catalytic conversion capacity of non-polar carbon was improved by heteroatomic doping.The Li₂S/NSPC cathode exhibits a low activation energy barrier（2.39 V）,high initial discharge specific capacity(1036.61 m Ah g^-1 under 0.025 C),and good cycle performance(690m Ah g^-1 at 1 C and maintains 587 m Ah g^-1 after 200 cycles with the attenuation rate of 0.075%per cycle).（2）To enhance the polarity of the carbon skeleton,the porous carbon skeleton was synthesized by the molten salt method and doped with highly dispersed Mn atoms with O/N coordination.Porous carbon framework provides abundant conductive channels for ion/electron transport.O/N coordination highly dispersed Mn atom is connected with Li PSs by polar bond and Lewis acid-base interaction to inhibit the shuttle effect.Li₂S particles were sealed into the carbon framework by the solvent method so that the active site could get better contact with the active substance so that a lower activation energy barrier and better cycling performance could be obtained.The initial capacity of the Li₂S/ON-MNPC cathode at 1 C is 534 mAh g^-1,and the reversible capacity of 514.18 m Ah g^-1 can be maintained after 100 cycles,with the retention rate of 96.23%,and the decay rate of 0.04%per cycle.（3）In order to improve the reaction kinetics of lithium polysulfide conversion in Li-S batteries,cellulose graphene aerogel（CGA）modified by Ce O₂ was introduced and used as the host material of Li₂S.The rich oxygen groups and large specific surface of CGA are conducive to the anchoring of Ce O₂,while the introduction of Ce O₂ can adsorb Li PSs species through the super polarity of Ce O₂.In addition,self-redox Ce O₂ can catalyze the conversion of polysulfide to solid Li₂S₂/Li₂S and increase the surface oxygen defects,thus improving the reaction kinetics.The initial discharge capacity of the Li₂S-CGA/Ce O₂ anode at 0.05 C is 1014.24 m Ah g^-1.After200 cycles at 1 C,the cycle retention rate is 88.93%,with an average attenuation of 0.055% per cycle.