Preparation And Properties Of Magnesium Boride Based Materials For Lithium Sulfur Batteries

At present,the development of new energy vehicles has put forward high requirements for energy storage equipment including batteries.Lithium-sulfur batteries have the advantages of high theoretical mass specific capacity（1675 m Ah/g）,high energy density（2600 Wh/kg）,low cost and environmental friendly,showing a good application prospect.However,the“shuttle effect”and sluggish reaction kinetics of the intermediate lithium polysulfide produced by the charge and discharge of the sulfur cathode have greatly hindered the development of lithium-sulfur batteries.Based on this,this thesis selected MgB₂,which has excellent adsorption and catalytic ability to Li PSs,as the research object.Two kinds of MgB₂-based composites with different structures were constructed through morphology control and composite strategy and applied to lithium-sulfur batteries.Firstly,MgB₂nanosheets were prepared by ultrasonic-assisted exfoliation technology,which was suction-filtered onto the separator and used as a separator modification layer for lithium-sulfur batteries.The ultrasonically exfoliated MgB₂nanosheets are introduced into a microporous structure,which increases the specific surface area and pore volume,and has more adsorption sites,which can fully exert the surface adsorption/catalytic Li PSs performance of MgB₂while accommodating volume-expanded Li₂S.Therefore,the battery with MgB₂@PP（polypropylene）modified separator significantly improves the sulfur utilization and cycling stability in Li-S batteries.The MgB₂@PP modified separator has an initial discharge specific capacity of 1184 m Ah/g at 0.1 C current density,and still has a high specific capacity of 732 m Ah/g at 2 C high current density.It still has a specific discharge capacity of497 m Ah/g after 500 cycles at 2 C current density.Secondly,in order to further improve the sulfur-carrying capacity of the cathode support material,based on the above-mentioned ultrasonic-assisted exfoliation technique,Mo S₂@MgB₂composites were prepared by van der Waals liquid phase self-assembly of MgB₂nanosheets and Mo S₂quantum dots.The stacking of MgB₂nanosheets is inhibited by the isolation of Mo S₂quantum dots,which provides sufficient pore structure,which is beneficial to the infiltration of electrolyte and the diffusion of Li⁺in the composite cathode material.At the same time,the anchoring of Mo S₂quantum dots by MgB₂nanosheets is used to inhibit their agglomeration,thereby providing sufficient surface adsorption sites and giving full play to their sulfur fixation/catalysis capabilities.Mo S₂@MgB₂composites exhibited strong sulfur fixation and catalytic conversion for Li PSs redox reaction as a cathode carrier for Li-S batteries,exhibiting high specific capacity（1154 m Ah/g at 0.2 C rate）,Excellent rate performance（833 m Ah/g at 2 C rate）and stable long-cycle performance.