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

As the most widely used electrochemical energy storage device in today’s society,lithium-ion battery provides great convenience to all aspects of our life,especially at the current node of rapid development of new energy vehicles,its importance is even more self-evident.Therefore,in order to meet the huge demand of today’s society,the development of a new generation of lithium-ion batteries with higher energy density,better cycling stability and lower production cost has become more and more important.Among them,lithium-sulfur batteries have received high attention due to their high theoretical specific capacity（1675 m Ah/g）and specific energy（2600 Wh/kg）,and the cathode material for lithium-sulfur batteries,elemental sulfur,is abundant and low-cost.However,the poor conductivity of elemental sulfur as an active material,the large volume expansion effect during charging and discharging,and especially the shuttle effect of the formed intermediate products leads to rapid capacity decay,all of which hinder the practical application of lithium-sulfur batteries.In order to suppress the shuttle effect,improve the electrical conductivity and reduce the volume expansion of lithium-sulfur batteries,three different lithium-sulfur cathode composites were designed and prepared in this paper.First,a carbon aerogel was prepared by sol-gel and combined with freeze-drying method,followed by solution permeation impregnation and high-temperature carbonization to obtain a gadolinium oxide doped carbon aerogel composite（CA@Gd₂O₃）,after that,the final cathode material is obtained by high temperature sulfur melting and sulfur loading.The doping level of gadolinium oxide can be adjusted by changing the impregnation concentration of gadolinium nitrate,and the electrochemical test results show that the initial discharge specific capacity of CA@Gd₂O₃@S composites at 0.1 C multiplier was as high as 1168.4 m Ah/g,and the reversible specific capacity is maintained at 814.4 m Ah/g after 100 charge/discharge cycles,and the capacity retention rate was as high as 86.6%after 200 charge/discharge cycles at 1 C.The carbon aerogel doped with gadolinium oxide can not only maintain the high specific surface area and large pore capacity of carbon aerogel to alleviate the poor electrical conductivity and volume expansion effect of sulfur cathode material,but also make use of the strong polar adsorption of polysulfide by the abundant O^2-ions on the surface of gadolinium oxide,which can inhibit the shuttle effect of polysulfide and jointly improve the electrochemical performance of the composite material.Secondly,phenolic wet gels were synthesized in situ on the surface of titanium dioxide nanoparticles by sol-gel method and combined with freeze-drying method,followed by high temperature carbonization to obtain titanium dioxide in situ doped carbon aerogel composites（CA@Ti O₂）,after that,the final cathode material is obtained by high temperature sulfur melting and sulfur loading.The titanium dioxide doping level can be adjusted by changing the titanium dioxide dispersion concentration,and the electrochemical test results show that the initial discharge specific capacity of CA@Ti O₂@S composites at 0.1 C multiplier was 1242.8 m Ah/g,and the reversible specific capacity is maintained at 746.6 m Ah/g after 100 charge/discharge cycles.The carbon aerogels doped with nano-Ti O₂ in situ have more regular spherical structure and smaller pore structure,which is conducive to sulfur loading and physical confinement of polysulfide.At the same time,the abundant active sites on titanium dioxide have a strong polar adsorption effect on polysulfides,which can inhibit the shuttle effect of polysulfides and have a better catalytic effect on the redox reaction process of active substances,further enhancing the electrochemical properties of the composites.Finally,Ni-doped Prussian blue analogs were prepared by solution precipitation method,and Ni-Fe PBA with porous cluster structure was obtained by high-temperature pyrolysis,and polyaniline coating was performed after melting sulfur to form Ni-Fe PBA@S@PANI composites.The polyaniline and the pyrolyzed Ni-Fe PBA have excellent electrical conductivity,and both of them can provide abundant active sites for polar adsorption of polysulfides to suppress the shuttle effect of polysulfides,and the flexible polyaniline coating also has a certain mitigation effect on the volume expansion effect.The initial discharge capacity of Ni-Fe PBA@S@PANI at 0.1 C multiplier was1163.6 m Ah/g,and the discharge specific capacity is maintained at 802.6 m Ah/g after150 charge/discharge cycles,with an average decay rate of only 0.21%per cycle.The electrochemical performance results show that Ni-Fe PBA can enhance the overall structural stability of the composites after polyaniline coating,at the same time,it can synergistically adsorb polysulfides,which can improve the electrochemical performance very obviously.