Design And Electrochemical Performance Of Halloysite And Its Derivatives As Cathode Materials For Lithium-sulfur Batteries

Lithium-sulfur batteries are considered as one of the most competitive candidates for the next-generation energy storage devices due to the high theoretical specific capacity and energy density,and the abundant resource reserves and environmental friendliness of the cathode material sulfur.However,the low electrical conductivity of sulfur,the volume expansion during cycling,slow reaction kinetic and the shuttle effect caused by the dissolution and migration of polysulfides restrict the practical application of lithium-sulfur batteries.In order to improve the performance of lithium-sulfur batteries,in this thesis,a series of halloysite and its derivatives as cathode materials for lithium-sulfur batteries were designed,and the electrochemical performance of the assembled batteries was tested.The main research results are as follows.（1）Acid-modified halloysite（AHNT）was prepared using sulfuric acid treatment and used in lithium-sulfur batteries.The results showed that sulfuric acid can destroy the aluminum-oxygen octahedra in the halloysite（HNT）structure,reducing the wall thickness and creating etched holes in the wall,thus enhancing the HNT’s ability to accommodate active sulfur and accelerating lithium-ion transport.After acid modification,the reaction kinetics and initial capacity of the battery were improved,but the capacity retention decreased instead due to the leaching of Al active sites,which reduced the chemisorption of polysulfides by HNT.Among them,the A5HNT/S cell had the best overall performance.The initial specific capacity of the A5HNT/S cell was 592.4 m Ah g^-1 at 0.5 C.The specific capacity was 406.9 m Ah g^-1 with a capacity retention rate of 69%after 150cycles.（2）Kaolinite nanoscroll（Kaol-NS）with halloysite-like structure was prepared by intercalation method using kaolinite as raw material.Compared with HNT,Kaol-NS had a more homogeneous morphology,larger lumen volume and thinner walls（～5 nm）,which was more favorable for Li⁺migration and diffusion,thus exhibiting faster reaction kinetics and higher sulfur utilization,as well as better rate performance and cycle stability.At a current density of 0.5 C,the initial discharge specific capacity of the Kaol-NS/S cell was 575.0 m Ah g^-1.After 500 cycles,the Kaol-NS/S cell still had a discharge specific capacity of 359.7 m Ah g^-1,with a capacity retention rate of 62.6%and a capacity decay rate of only 0.07%per cycle.（3）Using halloysite as template,glucose and polyaniline as carbon source,porous carbon materials PC1 and PC2 were prepared by template-etching method.PC1 was bulky and the surface was covered with gullies,while PC2 retained the original tubular structure of halloysite with a rich pore structure,which was more favorable for the loading of active sulfur and ion transport,and PC2 was a nitrogen-doped carbon material,therefore exhibited the best electrochemical performance.At a current density of 0.5C,the initial discharge specific capacity of PC2/S was 757.8 m Ah g^-1,and the capacity retention rate was 80%after 200 cycles.In addition,PC2-Co S₂composites were synthesized by hydrothermal method,and the performance of PC2-Co S₂/S cell was further enhanced due to the strong adsorption and catalytic effect of Co S₂ on polysulfides.The initial discharge capacity of PC2-Co S₂/S cell was as high as 1076.1 m Ah g^-1 at0.2 C,and still had a discharge capacity of 615.4 m Ah g^-1 after 200 cycles at 1.0 C,with a low capacity fading rate of 0.078%per cycle.