Construction Of Mo-MXene-based Interlayer Materials For Lithium-sulfur Batteries And Study Of Sulfur Fixation Mechanism

With the continuous growth of energy demand worldwide,sustainable development of clean energy has become increasingly important.Therefore,lithium-sulfur（Li-S）batteries,as a clean energy technology with high energy density,high theoretical capacity,and low cost,have attracted the attention of researchers worldwide.However,there are still some problems in its commercialization,such as extremely low conductivity of positive electrode sulfur and discharge end products（Li₂S）,dissolution of soluble polysulfides,passivation and deactivation of lithium negative electrodes,volume expansion,and lithium dendrites.To address these issues,based on Mo-based MXene,three derived materials have been designed and synthesized using the ideas of exogenous introduction,endogenous bridging,and vacancy defects for the interlayer modification of Li-S batteries.The main research is as follows:（1）A cobalt-filled one-dimensional nitrogen-doped carbon nanotubes（Co@N-CNTs）was constructed on a MXene derived two-dimensional nitrogen-doped nanosheet（N-Mo_xC）,and a multi-dimensional hybrid nanostructure（Co@N-CNTs/N-Mo_xC）was synthesized,and applied to the interlayer modification to improve the electrochemical performance of Li-S batteries.The prepared Co@N-CNTs/N-Mo_xC modified interlayer not only exert strong physical blocking and chemisorption of polysulfides,but also provide a large number of active sites to catalyze their reversible conversion.The Li-S batteries assembled with Co@N-CNTs/N-Mo_xC modified interlayer has an enhanced capacity of 610.0 m Ah g^-1 after 500 cycles at 1 C,with an ultra-low-capacity attenuation of 0.09%per cycle,which is better than the N-Mo_xC nanosheet or Co@N-CNTs modified interlayer.Furthermore,Co@N-CNTs/N-Mo_xC modified interlayer also has good cycle stability at high sulfur load(3.1～4.4 mg cm^-2),with a capacity of 612m Ah g^-1 at 2 C.（2）Mo S₂@Mo₂CT_x endogenous heterostructural materials were synthesized from Mo₂CT_x MXene as raw material through pre-oxidation and hydrothermal vulcanization.Mo₂CT_x MXene acts as an active conductive substrate to facilitate electron transfer.The derived Mo S₂ nanoflowers provides a large number of polysulfide adsorption and catalytic active sites,and the endogenous heterostructure of Mo S₂@Mo₂CT_x effectively coordinates the electronic structure of Mo S₂ to further improve the intrinsic activity of the catalyst,thus inhibiting the shuttling effect.Li-S batteries assembled using the Mo S₂@Mo₂CT_x modified interlayer have an initial discharge-to-discharge specific capacity of up to 1510 m Ah g^-1 at 0.1 C.The initial specific capacity was 1034.8 m Ah g^-1 and remained 581.6 m Ah g^-1 after 500 cycles at 1 C,with an average decay rate of 0.11%per cycle.At low liquid sulfur ratio(5μL mg^-1)and high sulfur load(5.34 mg cm^-2),the initial specific capacity is 622.6 m Ah g^-1 at 0.2 C,maintaining the specific capacity of 58.66%after 230 cycles.（3）Gas phase etching i-MAX phase(Mo_2/3Y_1/3)₂Al C preparation(Mo_2/3Y_1/3)₂CT_xbimetallic material.(Mo_2/3Y_1/3)₂CT_x has an accordion-like structure similar to MXene,which can effectively relieve the volume expansion during the charge and discharge process.With good electrical conductivity(Mo_2/3Y_1/3)₂CT_x also has good adsorption and catalytic conversion ability for polysulfides,which can effectively inhibit the shuttling effect of polysulfides.Li-S batteries assembled with(Mo_2/3Y_1/3)₂CT_x modified interlayer have 1405.3 m Ah g^-1 at 0.1 C and maintain a reversible capacity of 548.5 m Ah g^-1 after 500 cycles at 1 C with an average capacity attenuation rate per lap of only 0.10%.It has good cycle stability at high sulfur load(5.36 mg cm^-2)and low liquid sulfur ratio(5.60μL mg^-1),an initial discharge capacity of 660.0 m Ah g^-1at 0.2 C,and a specific capacity of 514.2 m Ah g^-1 after 170 cycles,with a capacity retention rate of 77.9%.