Preparation And Electrochemical Performance Of MXene-based Cathode Materials For Lithium Sulfur Batteries

Lithium–sulfur?Li-S?batteries,based on the redox reaction between elemental sulfur and lithium metal,have attracted considerable attention due to their inherent high theoretical energy density in the field of energy storage.However,despite the advantages of environmental friendliness,abundant resources and low cost,the practical processes of Li-S batteries are still greatly hindered by some intrinsic obstacles of sulfur cathode,including fast capacity fading,poor rate performance and low actual energy density.In order to overcome these shortcomings,based on the key issues such as insulating nature of sulfur and solid discharge-end products,notorious shuttle effect derived from dissolution of intermediate products,and serious volumetric changes during charge-discharge cycling,the novel sulfur cathode materials were constructed from the idea of structure design and chemical regulation to synthetically improve the electrochemical performances of Li-S batteries.The main research contents and results were summarized as follows:?1?The self-supporting Ti₃C₂T_x foam,as a novel sulfur host,was synthesized via a direct stacking of Ti₃C₂T_x MXene flakes into film followed by a hydrothermal-induced foaming.The self-supporting Ti₃C₂T_x foam structure can effectively prevent the restacking of Ti₃C₂T_x flakes,which not only increased the specific surface area and pore volume,but also significantly enhance the exposure of inherent Lewis acidic sites.Therefore,the advantages of self-functional surface of Ti₃C₂T_x flakes were fully utilized,thus effectively strengthening the capability of physical and chemical co-adsorption for polysulfides under a high sulfur content?71.1 wt%?.Combining with the favorable electrolyte wettability and extraordinary structural stability,the resultant self-supporting Ti₃C₂T_x foam/S cathodes achieved high initial discharge capacity and advanced rate performance.Remarkably,the self-supporting structure without conductive agents,binders and current collectors conferred a significantly elevated gravimetric energy density of 1297.8 Wh kg^-1 at 0.2 C,exhibiting more than a 246.4%improvement over that of the non-self-supporting Ti3C2Tx/S cathode.?2?MXene-based Co,N-codoped porous carbon nanosheets?MCoNPCNSs?as sulfur host were developed by in-situ self-assembly of bimetallic zeolite imidazole framework?Co/Zn-ZIF?on two-dimensional Ti₃C₂T_x MXene nanosheets followed by carbonization and etching treatments.As manifested by characterizations and measurements,the resultant MCoNPCNSs presented large specific surface area(726.6m² g^-1)and pore volume(1.6 cm³ g^-1),providing enough spacings for sulfur loading and alleviating the volumetric expansion of cathode.In addition,the synergistic effects of Co,N codoping not only strengthened the adsorption capacity of MCoNPCNSs to polysulfides,but also significantly promoted the redox reactions kinetics of cathode.A cathode-separator-integrated MCoNPCNSs/S-M-PP electrode structure was successfully obtained through coating MCoNPCNSs/S composite on the MXene-modified PP?M-PP?separator.This design further inhibited the shuttle effect of polysulfides and prominently improved the battery performances:initial discharge capacity of 1340.2 mAh g^-1 was obtained under the current density of 0.2 C.Besides,benefited from the integrated structure design,a significantly elevated energy density was actualized.