Layer Structure Regulation Of 1:1 Type Clay Minerals And Application In Lithium-sulfur Battery

Lithium-sulfur batteries with high discharge capacity,low cost and environmental friendliness are considered as one of the next generation portable energy storage devices.However,the polysulfide shuttle effect during the discharge process seriously affect the cycling performance of the battery.Thanks to the adsorption properties of natural clay minerals on polysulfides,the use of clay minerals to modify battery separators to suppress the polysulfide shuttle effect has received extensive attention in recent years.Appropriate layer structure regulation of natural clay minerals can change the crystal structure and morphological characteristics of the minerals as well as enhance their adsorption performance,which can be applied to lithium-sulfur battery separator to further improve the comprehensive performance of the separator and enhance the electrochemical performance of the battery.In this paper,three kaolinites with different crystallinity（HI=1.30,1.02,and 0.78）were chosen as raw materials to prepare expanded kaolinites by calcination of urea intercalated kaolinite mixed with urea/potassium chlorate system.The experimental results show that the higher the crystallinity of kaolinite,the larger the intercalation rate of its intercalation complex and the more urea was intercalated into the interlayer.Under the same expansion energy conditions,the more NH₃ and CO₂ produced by the decomposition of interlayer urea,resulting in a more pronounced expansion effect of kaolinite layer.The expanded kaolinite prepared exhibited the lowestζ-potential（-34.5 m V）and the highest methylene blue adsorption capacity(32.44 mg g^-1).The same method can be used for layer expansion of dickite,which is also a kaolinite group mineral,and the expansion degree of dickite can be adjusted by changing the mass ratio of potassium chlorate to the dickite-urea intercalation complex.When the potassium chlorate/intercalation complex exceeds 0.375,the（002）diffraction peak of the expanded dickite disappears almost completely,and the layers are fully expanded,showing an expanded accordion-like porous morphology.Its specific surface area and total pore volume were 3.7and 2.8 times higher than those of the raw dickite,respectively,and exhibited higher absoluteζ-potential values（-36.3 m V）than that of the raw dickite.The adsorption efficiency of the obtained expanded dickite could reach over 90%,which was a new reusable and efficient mineral adsorption material.The difference in the distribution and orientation of the hydroxyl groups between dickite and kaolinite resulted in a higher intercalation rate of dickite-urea intercalation complex（97.4%）than kaolinite-urea intercalation complex（85.9%）under the same preparation.The energy required for the expansion of dickite layer is only 1/3 of that required for the expansion of kaolinite layers.The silicon oxygen functional groups in dickite have polarity and have a certain adsorption effect on polysulfides.The unique and rich-OH groups exhibit Lewis acid characteristics,which can be further anchored to polysulfides through S-O and Li-O bonding.After the expansion of the dickite layer,more surface hydroxyl groups of the layer were exposed,which provided more adsorption active site for the expanded dickite to inhibit the shuttle effect of polysulfides.At the same time,the porous morphology and polar groups of expanded dickite are beneficial for improveing the wettability between the separator and the electrolyte,reducing the ion migration resistance,and further enhancing the battery performance.The ionic conductivity of the separator co-modified by expanded dickite and acetylene black is 1.58 m S cm^-1,which is twice as high as that of the Celgard separator.The composite separator was applied to lithium-sulfur battery with an initial discharge specific capacity of 1311 m Ah g^-1 at the current density of 0.1 C.The coulomb efficiency of more than96%was maintained in the rate test of 0.1→2.0 C.After 100 cycles at a current density of 0.5C,the cell still has the capacity of 862.5 mAhg^-1,with a capacity retention rate of 77%.