Preparation And Electrochemical Properties Of High-Performance Lithium Sulfide Based Composites

The low energy density of commercial lithium-ion batteries using embedded lithium compounds as cathode materials is no longer sufficient to meet the needs of the rapidly developing electronics and energy storage sectors.Therefore,the search for new,high energy density energy storage devices is urgent.Lithium-sulfur（Li-S）batteries are regarded as a high energy density battery system with great potential for development due to their high energy density and low cost.The cathode materials for Li-S batteries are commonly sulfur（S）and lithium sulfide（Li₂S）.Among them,Li₂S cathodes have received a lot of attention because Li₂S cathodes can be matched with lithium-free cathodes（e.g.silicon,carbon-based materials,etc.）to fundamentally avoid the safety hazards associated with lithium dendrites.Li₂S,as the final embedded lithium state of S,not only has good compatibility with lithium-free anodes,but also has high safety and structural stability.However,the Li₂S cathode suffers from its own low electronic and ionic conductivity,the shuttle effect of the intermediate lithium polysulfides（LiPSs）during the reaction,a very high kinetic activation potential and slow reaction kinetics.The above problems seriously hinder the application of Li₂S in Li-S batteries.Currently,in order to solve these problems,researchers have focused on the modification of Li₂S cathode materials in terms of improving the electrical conductivity,adsorption and catalytic ability of the composites.In this regard,nanoparticles of noble and transition metals have extremely strong adsorption capacity of LiPSs and excellent electrocatalytic properties.Moreover,nanoparticles of noble and transition metals offer significant advantages in improving the electrical conductivity of composites,inhibiting the shuttling of LiPSs,enhancing the kinetics of electrochemical reactions and catalyzing the conversion of LiPSs to S or Li₂S.However,the currently reported methods for the synthesis of metal nanoparticles suffer from complex steps,harsh conditions,and high costs,which cannot meet the needs of practical commercialization.In this paper,two ideas are proposed to address the above problems.The main research contents are as follows:（1）Li₂S-Mo nanocomposites were synthesized in situ using a simple,feasible and energy-efficient mechanical ball milling approach.Subsequently,fig-like Li₂S-Mo@C nanocomposites with a core-shell structure were successfully prepared using a polystyrene（PS）cladding and carbonization process.The microstructural characterization revealed that the fig-like Li₂S-Mo@C nanocomposite has amorphous carbon as the rind of the fig,Li₂S nanodots as the pulp of the fig and metallic Mo nanodots of 5-15 nm in size as the seeds of the fig uniformly dispersed in the nucleus.The carbon shell and the unique structure not only improve the electronic and ionic conductivity of the composites,but also enable further inhibition of LiPSs shuttling.After a series of adsorption and catalytic performance tests and combined with theoretical calculations,it was found that the metal Mo nanodots were not only able to produce extremely strong chemisorption of LiPSs,but also had the ability to accelerate the conversion of LiPSs to S and Li₂S.Electrochemical tests show that the fig-like Li₂S-Mo@C nanocomposites exhibit a low initial activation potential（2.51 V）,a very high first-discharge capacity(1086 m Ah g^-1),a very small charge transfer resistance（18.8Ω）and a long cycle life（500 cycles）.（2）A simple method for the preparation of three-dimensional porous carbon skeletons（Pd@PC-CNT）modified with palladium（Pd）quantum dots is proposed.Specifically,the ZIF-CNT material was first synthesized in situ by hydrothermal method,followed by high temperature carbonization to prepare porous carbon-CNT（PC-CNT）composites,and finally,the Pd@PC-CNT nanocomposites were successfully prepared by uniformly dispersing the precious metal Pd quantum dots in the PC-CNT framework.The composite with Li₂S was successfully prepared by rapid evaporation of an alcohol solution containing Li₂S,which allowed the nucleation and generation of Li₂S nanocomposites in the Pd@PC-CNT composite.The unique morphology,high specific surface area and rich pore characteristics of the Pd@PC-CNT/Li₂S nanocomposites were further investigated by SEM,TEM and BET characterization,and the intrinsic correlation with the electrochemical properties and electrochemical reaction kinetics.It is shown that the unique three-dimensional porous structure not only accommodates more active material,but also promotes electrolyte imbibition and inhibits LiPSs shuttling,which in turn improves the electrochemical capacity performance.In addition,the homogeneous distribution of Pd quantum dots in the framework can provide more adsorption sites and catalytic active sites,thus improving the multiplicative performance,long cycle performance and reaction kinetics of the composites.