Preparation Of Molybdenum Carbide Composites For Cithium-Sulfur Batteries And Their Electrochemical Properties

Lithium-sulfur secondary batteries are thought to be the most likely alternative for lithium-sulfur secondary battery technology due to their high theoretical capacity(1675m Ah g^-1)and high energy density(2600 Wh kg^-1).Lithium-sulfur batteries are still far from being widely used due to issues including low sulfur usage,poor cycle stability,and low coulomb efficiency.The weak"shuttling"effect,the delayed conversion of polysulfides,and the poor conductivity of elemental sulfur are the underlying reasons of these issues.This work suggests creating two novel anode carriers with outstanding electrochemical performance,faulty molybdenum carbide（Mo_xC@NC）and molybdenum carbide-based heterojunction（Mo₂C-Mo P/NC）,by modifying carbon-based transition metals in order to address the aforementioned issues.The primary research is described as follows:（1）The synthesized defect-graded nano-hollow microsphere structure（Mo_xC@NC）is a surface-grown nanosheet-like and hollow structure inside,which is conducive to electron transport and inhibits the shuttle of polysulfides.Higher specific surface area due to the presence of surface sheet structure,and the internal hollow structure can accommodate more nanosulfur,inhibit volume expansion,and firmly fix nanosulfur at the positive electrode.At the same time,due to the high-temperature carbonization treatment,the extraction of some metal compounds forms metal compound defects,and these exposed defective active sites have strong binding ability to polysulfides,and reduce the reaction potential energy by bonding with polysulfides to achieve adsorption-catalytic synergy.When electrochemical reactions are performed,polysulfides cannot escape,inhibit the shuttle of polysulfides,accelerate the conversion of polysulfides,and provide an initial discharge capacity of up to1152 m Ah g^-1 through charge-discharge testing.In addition,it maintained a high reversible capacity of 982 m Ah g^-1 after 400 cycles.（2）Carbon-based heteronodule shell structure microspheres（Mo2C-Mo P/NC）are created using a straightforward in situ polymerization procedure and carbon heat treatment.Using characterisation techniques like SEM and TEM,this structure was verified.The microspheres of carbon-based heterogeneous nodule shell structure were found to be able to enhance the electrochemical performance mechanism of the cathode of lithium-sulfur batteries.Nanosulfur entered the conductive carbon shell through nanopores through heating and melting,according to the electrochemical analysis of the material after sulfur loading.Since polysulfides cannot pass through the carbon shell during electrochemical processes,the composite’s electrochemical performance is improved.The carbon shell’s conductive nature encourages electron flow and speeds up polysulfide conversion.The carbon shell and matrix include a large number of heterojunction nanocrystals and uniformly dispersed heteroatoms that offer a wealth of highly active catalytic sites that significantly speed up the conversion of polysulfides.Electrochemical tests show that S@Mo₂C-Mo P/NC combines the advantages of high ionic conductivity of metal carbides and the strong adsorption catalytic conversion of metal phosphides to polysulfides to form a synergistic effect of adsorption-catalysis-conversion,and the average coulomb efficiency of composite materials under 100 cycles of cyclic charge and discharge is as high as 98.1%,and the high reversible capacity of 812 m Ah g^-1 is maintained after 100 cycles,effectively improving the cycle performance of lithium-sulfur batteries.