Metal-Organic-Frameworks Derivation Of Tin-Containing Composites And Their Lithium Storage Properties

Nowadays,lithium ion batteries have become one of the most promising energy storage systems,and have been widely applied in mobile electronic devices,military industry,and so on.However,to satisfy the urgent needs in electric vehicles and high-power batteries,the requirement for next-generation lithium ion batteries with large capacity and long-cycle stability is increasing.In this thesis,metal-organic frameworks are used as precursors to synthesize Sn@C composite,in which ultrasmall Sn nanodots are uniformly embedded in the nitrogen-doped porous carbon matrix?denoted as Sn@NPC?,as well as a unique microsphere of carbon coated Ni₃Sn₄/NiP₂ with deep-rooted carbon nanotubes?denoted as Ni-Sn-P@C-CNT?.Furthermore,the electrochemical properties of the two composites as lithium ion battery anodes are also explored.In the first section of this thesis,Sn-based MOF?Sn-MOF?is obtained from SnSO₄ and 1,4-BDC through a one-pot hydrothermal method,and then is calcined with dicyandiamide?DCDA?at high temperature to synthesize Sn@C composite,in which ultrasmall Sn nanodots are uniformly embedded in the nitrogen-doped porous carbon matrix?Sn@NPC?.Sn@NPC exhibits porous and sheet-like nanostructure,while a large number of ultrasmall Sn nanodots are observed to be homogeneously dispersed within the thin carbon nanosheets.The particle size of Sn is only 2-3 nm,which is substantially smaller than previous Sn@C composites in the literature.The doped N content from DCDA in Sn@NPC is as high as 10.23 wt%.The more N atoms doped at the edges of the carbon,the more defects may be generated,indicating more active sites for electrochemical reactions.Owing to the delicate size control and confined volume change within carbon matrix,the Sn@NPC composite can exhibit reversible capacities of 575 mAh g^-1(Sn contribution:1091 mAh g^-1)after 500 cycles at 0.2 A g^-1 and 507 mAh g^-1(Sn contribution:1077 mAh g^-1)after1500 cycles at 1 A g^-1.The excellent long-life electrochemical stability of the Sn@NPC anode has been mainly attributed to the uniform distribution of ultrasmall Sn nanodots and the highly-conductive and flexible N-doped carbon matrix,which can effectively facilitate lithium ion/electron diffusion,buffer the large volume change and improve the structure stability of the electrode during repetitive cycling with lithium ions.In the second section of this thesis,Ni-Sn bimetal-organic frameworks?Ni-Sn-BTC?are formed in thecation-exchange process by two-step uniform microwave-assisted irradiation reactions from Ni?NO₃?₂·6H₂O,SnCl₂ and H₃BTC.After calcined under C₂H₂/Ar mixture atmosphere and phosphorization,a unique ball-cactus-like microsphere of carbon coated Ni₃Sn₄/NiP₂ with deep-rooted carbon nanotubes?Ni-Sn-P@C-CNT?is successfully synthesized.The Ni-Sn@C-CNT consists of numerous nanoparticles with¹0-20 nm in size,and displays uniform microsphere morphology with average particle size of 1-1.5?m.The small CNTs are in-situ grown by the catalysis of Ni and Sn nanoparticles,with 20-30 nm in diameter and 100-200 nm in length.The presence of CNTs is beneficial for high ionic and electronic conductivity,resulting in facilitated transport kinetics.When evaluated as lithium ion battery anode,the Ni-Sn-P@C-CNT exhibits a large reversible capacity of 704 mA h g^-1 after 200 cycles at 100 mA g^-1 and excellent high-rate cycling performance(a stable capacity of 504 m A h g^-1 retained after 800cycles at 1 A g^-1).These good electrochemical properties are mainly ascribed to the unique three dimensional mesoporous structure design along with dual active components showing synergistic electrochemical activity within different voltage windows.