Construction And Controllable Preparation Of Vermiculite Based Two-dimensional Silicon Nanomaterials And Their Lithium Storage Properties

The development of green and efficient lithium-ion battery as energy storage technology is a strategic demand for improving the ecological environment,alleviating the energy crisis,and achieving the goal of“carbon peak and carbon neutralization”of China.Silicon materials with high capacity and rich raw materials are considered as the most promising anode materials for lithium-ion battery.Considering the high preparation cost and poor cycle stability of silicon materials,two-dimensional porous silicon nanosheets were prepared by a low-temperature（350 ^oC）aluminothermic reduction using vermiculite with layer structure and abundant silicon elements as raw material.Furthermore,based on the excellent cation exchange capacity of vermiculite,two-dimensional silicon/carbon composite nanosheets with a coating structure were fabricated by using chitosan,a natural polycationic polysaccharide,as carbon source.The obtained vermiculite-based two-dimensional silicon nanomaterials effectively overcame the defects of silicon anode,such as volume expansion,poor electrical conductivity and unstable cycling performance.The results provide basic data and technical support for the research and development of high-performance anode materials and the high-value utilization of vermiculite resources.The mineralogy characteristics of vermiculite purchased from Weili,Xinjiang province were thoroughly analysed.The further investigation focused on the transformation of thermal expansion and chemical expansion treatment on the vermiculite’s crystal structure,chemical composition,micro morphology and so forth.The research results showed that the raw vermiculite mainly contained vermiculite minerals,phlogopite-vermiculite interstratified mineral（hydrophlogopite）and some phlogopite and augite impurities.The volume expansion rate of vermiculite modified by chemical expansion is 11.22 times,which is obviously better than thermal expansion（7.58 times）.The tightly stacked three-dimensional structure of vermiculite were exfoliated into two-dimensional nanolayer by chemical expansion modification,and vermiculite’s original crystal structure,good flexibility and high cation exchange capacity were preserved.However,the crystal structure was damaged severely,layer brittleness was large,and the cation exchange capacity was reduced significantly when vermiculite was modified by thermal expansion.Based on the layer structure and abundant silicon elements of chemical expansion vermiculite,two-dimensional porous silicon nanosheets were prepared by low-temperature（350 ^oC）aluminothermic reduction in a Al Cl₃/Na Al Cl₄eutectic molten salt system.The obtained porous silicon nanosheets exhibited the characteristics of high crystallinity,ultra-thin（<50 nm）,hierarchical porous（i.e.meso-and macroporous）and lager specific surface area(71.1 m² g^-1).As anode material for lithium-ion batteries,porous silicon nanosheets showed high capacity(Initial discharge/charge capacity of2829/2229 m Ah g^-1),excellent rate performance(Specific capacity of 1314 m Ah g^-1 at4.0 A g^-1),and good cycle stability(Capacity retention rate is 73%after 300 cycles at 1.0A g^-1).Based on the excellent cation exchange capacity of chemical expansion vermiculite,two-dimensional silicon/carbon composite nanosheets with a coating structure were fabricated by using chitosan,a natural polycationic polysaccharide,as carbon source.The synthesis process composed of ion-exchange intercalation,in-situ carbonization,and low-temperature aluminothermic reduction.The obtained silicon/carbon composite nanosheets showed few layer structure（<10 layers）,large specific surface area(208.2m² g^-1),abundant nanoporous channels（i.e.,micro-and mesoporous）and high carbon content（13 wt%）.The carbon layer coated on the surface of silicon nanosheets not only acted as a physical buffer layer to buffer the volume expansion of silicon,but also played a role of conductive medium during electrochemical reaction.As a result,the cyclic stability of silicon/carbon composite nanosheets obviously improved.At 0.1 A g^-1,the specific capacity remained at 1745 m Ah g^-1 after 100 cycles,and the corresponding capacity retention rate was 89%.At 1.0 A g^-1,the capacity remained 985 m A h g^-1 after300 cycles,and the corresponding capacity retention rate was 88%.