Polymer-Derived Porous Carbon Supported Nanocrystals As Anode Materials For Lithium-Ion Batteries

As a new type of electrochemical energy storage device,lithium-ion batteries（LIBs）have aroused keen interest in both academic and industrial area,by virtue of light weight,high energy,durability and environmental benignity.Nevertheless,it becomes a struggle for current LIBs based on commercial graphite anode(theoretical capacity of 372 m Ah g^-1)to meet the burgeoning demand of emerging applications such as electric vehicles in terms of energy density.Developing large-capacity anode materials is regarded as a highly efficacious way to elevate the energy density of LIBs.Metal-based materials such as tin(994 m Ah g^-1),germanium(1625 m Ah g^-1),and cobalt phosphide(894 m Ah g^-1)have demonstrated great potential as alternative anode for LIBs because of their appreciably high theoretical capacity.However,they frequently suffer from rapid capacity fading,due to their huge volume changes upon lithiation/delithiation,which leads to the pulverization and exfoliation of active material in the later cycling.In addition,several drawbacks such as poor conductivity and low ion diffusion coefficient also severely limit their rate performance.In this thesis,I have elaborately designed tin,germanium,and cobalt phosphide nanocrystals supported on hierarchical porous carbon by using polymers as structure-directing agents.The continuous and interconnected nanoporous structure can effectively alleviate the volume expansion of active materials,restrain the agglomeration of nanoparticles,and greatly shorten the diffusion pathway of lithium ions.Plus the synergetic effect of highly conductive carbon network,the as-obtained composites materias show excellent electrochemical performance when used as anode material for LIBs.The main content of this thesis is as follows:（1）Tin-doped cross-linked porous polymer was obtained through the solvothermal copolymerization of divinylbenzene（DVB）and 1-vinylimidazole（VI）monomers with the addition of tin tetrachloride,which was then converted intoconver a tin/hierarchical porous carbon composite（Sn_x/HPC）after pyrolysis.By controlling the initial molar ratio of Sn/VI（1:1,2:1,3:1）,I have prepared a series of tin/carbon composites（Sn1/HPC,Sn2/HPC and Sn3/HPC）.The results show that the Sn2/HPC anode has the best comprehensive lithium storage performance:As the current density is increased by 10 times from 0.2 A g^-1,the reversible capacity of Sn2/HPC can maintain about 60%of the initial value;when the density is restored to 0.2 A g^-1,its reversible capacity is as high as 885 m Ah g^-1 after continuous 400 cycles;in terms of cyclic stability,it can be repeatedly charged and discharged for 1000 times at a high rate of 2 A g^-1and retains a reversible capacity of 561 m Ah g^-1.（2）A germanium/porous carbon composite electrode material（Ge/MPC）was fabricated by a self-assembly-thermosetting-carbonization strategy with the assistance of the block copolymer Pluronic P123,by using melamine formaldehyde resin（MF）as main carbon source and carboxyethyl sesquigermanium oxide（Ge132）as germanium source.Meanwhile,only P123 or MF prepolymer was added to react with Ge132 to synthesize another two batches of germanium/carbon composite electrodes（Ge/PC,Ge/MC）for comparative study.The results reveal the best electrochemical performance of Ge/MPC anode:at a current density of 0.2 A g^-1,the reversible capacity of Ge/MPC reaches 1290 m Ah g^-1;when the current density is increased by 10 times,the reversible capacity still retains 487 m Ah g^-1;after 1000 cycles at a current density of 2 A g^-1,the reversible capacity can still be maintained at～634 m Ah g^-1.The superb cycling stability and rate performance is related to the continuous pore structure of the Ge/MPC electrode material.（3）Cobalt-containing composite micelles was obtained through the evaporation-induced self-assembly of block copolymer PS-b-PEO,resorcinol formaldehyde resin prepolymer,and cobalt nitrate,which was then transformed into ordered mesoporous carbon supported cobalt phosphide composite（Co_xP/OMC）by post carbonization-phosphorization treatment.As a control,disordered porous carbon-supported cobalt phosphide samples were synthesized without the addition of PS-b-PEO.Electrochemical tests show that the Co_xP/OMC composite with ordered mesoporous structure not only has the highest reversible lithium storage capacity,but also exhibits the best cycling stability and rate performance.After 480 cycles at a current density of 0.2 A g^-1,the reversible capacity of the Co_xP/OMC composite electrode is as high as878 m Ah g^-1,and the capacity still maintains an upward trend.After 1000 cycles at a higher rate(2 A g^-1),its reversible capacity can still be maintained at～373 m Ah g^-1.In the rate performance test,when the current density is gradually increased from 0.1 A g^-1 to 20 times,its reversible capacity can maintain 42.1%of the initial value.The solid ordered mesoporous carbon framework not only effectively mitigate the volume variation of cobalt phosphide particles during the electrochemical reaction,but also provides fast lithium ion transport capability and enhances the overall conductivity of the material,thus endows Co_xP/OM superior rate performance and cycle durability of composites.