Preparation And Electrochemical Performance Of Modified Silicon Anode Composites By Metal Organic Framework

Anode material is an indispensable part of lithium ion battery,which plays a decisive role in energy density,cycle life and safety performance of the battery.With the increasing energy density of lithium ion batteries,the traditional graphite material(theoretical specific capacity of it is only 372 mAh g^-1)is not fully satisfied with the demand.Research on high energy density anode materials is extremely urgent.Due to its abundant reserves,high theoretical capacity(Li₁₅Si₄,3579 mAh g^-1)and low lithium intercalation potential?<0.4 V vs Li/Li⁺?,silicon-based materials have become hot spots for researchers.However,there is a huge volume change occur during the process of lithiation/delithiation of silicon,repeatedly charging and discharging process makes it difficult to adjust the stress caused by the volume change,resulting in the pulverization or break of silicon-based materials,which causes the active substance to flake off and the electronic contact to fail,resulting in battery capacity decline rapidly.In addition,the conductivity of silicon is poor.To address the above problems,in this paper,a three-dimensional hierarchical porous nitrogen-doped carbon coated Si composite was prepared by high-voltage sputtering and heat treatment;Silicon/metal organic frame?ZIF-8?/polyacrylonitrile?PAN?was prepared by electrospinning technology,and Si/C-ZIF-8/CNFs nanofiber composites were obtained by heat treatment;Silicon@phenolic resin based carbon@metal organic framework based carbon composites were prepared by sol-gel and self-template methods combined with heat treatment.The contents of the study are as follows:?1?Three-dimensional hierarchical porous nitrogen doped carbon coated silicon microspheres?NPC@Si?were prepared by high-voltage sputtering and heat treatment with PAN and ZIF-8 as carbon sources.Compared the differences of electrochemical properties with pure silicon and the carbon coated silicon microspheres?NC@Si??without added ZIF-8?.The results show that the cyclic stability of NPC@Si and NC@Si are obviously improved compared with pure silicon.In addition,the NPC@Si with ZIF-8 has better cycling stability than the NC@Si without ZIF-8.Under the current density of 0.5 A g^-1,the discharge capacity of NPC@Si is still maintained at1565 mAh g^-1 after 100 cycles.The excellent cyclic stability is mainly attributed to the high silicon-loading and the ZIF-8 pore-making effect,which makes the NPC@Si composite has a large void space,the space can effectively buffer the volume expansion of the silicon,while nitrogen-doped carbon helps to improve the conductivity of the composite.?2?The silicon/metal organic framework?ZIF-8?/polyacrylonitrile?PAN?fiber composite?Si/ZIF-8/PAN?was prepared by electrospinning technology,and Si/C-ZIF-8/CNFs composites were obtained by heat treatment.Compared the differences of electrochemical properties with Si/C-ZIF-8.The results show that Si/C-ZIF-8/CNFs has more stable cycle performance than Si/C-ZIF-8.The reversible capacity of Si/C-ZIF-8/CNFs maintained at 795 mAh g^-1 under the current density of0.2 A g^-1 after 100 cycles.The good electrochemical performance is mainly caused by the ZIF-8 decomposition,which can leave some space for the expansion of Si.In addition,large specific surface area and mesoporous structure shorten the transmission path of lithium ion and electron,and provide more lithium ion storage sites.At the same time,carbon nanofibers coating layer can further buffer the volume expansion of silicon.?3?Resorcinol-formaldehyde resin layer was coated on the surface of silicon nanoparticles?Si@RF?by sol-gel and self-template method with resorcinol and formaldehyde as the precursor of phenolic resin,then disperse Si@RF in a methanol solution containing cobalt nitrate and mixed with a methanol solution containing imidazole,the silicon@phenolic resin@metal organic frameworks composite?Si@RF@ZIF-67?was prepared.Cobalt and nitrogen co-doped double carbon coated multi-core yolk-shell like mesoporous composites?Si@C@ZIF-67-800N?were obtained after heat treatment and compared the differences of electrochemical properties with phenolic resin based carbon coated silicon?Si@C?.The results show that Si@C@ZIF-67-800N has more excellent cyclic stability than Si@C composite.Under the current density of 0.5 A g^-1,the reversible capacity is maintained at 1107mAh g^-1 and 887 mAh g^-1 after 100 cycles,and under higher rates,Si@C@ZIF-67-800N composites show higher discharge specific capacity.The excellent electrochemical performance is attributed to the cobalt and nitrogen co-doped double carbon coated multi-core yolk-shell like mesoporous structure can improve the conductivity and prevent the electrolyte from contacting directly with silicon.Moreover,the carbon matrix derived from ZIF-67 has a large number of empty space,which can effectively regulate the volume change of silicon.