Research And Application Of MXene And Its Derivatives In Lithium-sulfur Batteries

With the development of green economy and the change of energy environment,the demand for high energy density energy storage batteries is becoming more and more urgent.Compared with other batteries,lithium-sulfur（Li-S）batteries have higher energy density and power density,and are one of the most promising battery systems to break through 400 Wh Kg^-1 energy density.The cathode of lithium-sulfur battery is elemental sulfur,and the anode is lithium metal（Li）.During discharge,sulfur is reduced to polysulfides（Li PSs）and finally transformed into lithium sulfide（Li2S）,and the charging process is the opposite.The problem of lithium-sulfur battery mainly focuses on the cathode side,firstly,both sulfur and lithium sulfide are insulators,and they produce about 79%volume change in the process of mutual transformation;secondly,the shuttle effect of Li PSs dissolved in ether electrolyte between cathode and anode electrodes.To address the above problems,this paper prepares some series of MXene based composite electrode materials from the construction of sulfur cathode materials,combining the excellent catalytic activity of excessive metal compounds and the conductivity of 3D MXene backbone,and systematically explores their electrochemical performance in lithium-sulfur batteries.The specific studies are as follows:（1）Garnet-like Ti N@MXene microspheres were prepared by combining the two-dimensional layered materials MXene and Ti N by spray drying method,and it was experimentally demonstrated that the microspheres have both adsorption ability and catalytic effect on polysulfides.The three-dimensional skeleton composed of MXene not only solves the problem of easy stacking of MXene,but also ensures the uniform distribution of sulfur,and the overall electrical conductivity of the cathode is improved.The Ti N hollow nanospheres are encapsulated by MXene sheets to strengthen the adsorption of the material on polysulfides and accelerate the conversion of higher-order polysulfides to lithium sulfide.（2）A Ti O2-Ti N/C heterojunction material was developed as an efficient sulfur host material using MXene as the nitrogen and carbon source by exploiting the easy oxidation property of MXene.The composite material combines the advantages of anchoring,surface diffusion and catalytic polysulfide conversion,which can accelerate the kinetics of redox reactions for the interconversion of sulfur species,regulate lithium sulfide nucleation and decomposition,and alleviate the shuttle effect of polysulfides.The bifunctional Ti O2-Ti N/C composites exhibit good electrochemical performance in lithium-sulfur batteries,including high reversible capacity(1432 m Ah g^-1)and stable coulombic efficiency（～100%）.（3）The MXene surface with a large number of anionic functional groups after chemical etching was cationically modified to prepare the MXene derived material Ti N-Fe3N-2,and it was applied to lithium-sulfur batteries as a sulfur host material.It has been experimentally demonstrated that Ti N-Fe3N-2 not only has a good chemisorption effect on polysulfides,but also can reduce the reaction energy barrier of polysulfide conversion,promote the reaction and improve the utilization rate of active substances.