| Rechargeable lithium ion batteries(LIBs)have attracted significant interest due to their broad application,including Electric Vehicles(EV)and Hybrid Electric Vehicle(HEV)etc.The lithium ion batteries with higher energy and power densities,longer cycle stability and better ratio performance should be researched and developed to meet the current needs.Traditional commercial graphite anode materials with poor theoretical specific capacity,cannot meet the development of next-generation lithium ion battery needs.Therefore,it is badly in need of research and development a new lithium-ion battery electrode material.Besides to the commercial graphite,carbon based materials with excellent electrical properties and cycle stability have been used,but they possess the poor theoretical specific capacity.Similarly,metal oxide anode materials with high theoretical specific capacity,rich resources and environmental friendliness,cannot used in commercial lithium ion battery,due to their large volume change leading to the structural damage after lithium ion embedded and out,and the poor electrical conductivity.Carbon/metal oxide composite materials are considered to be the promising lithium ion battery cathode material,due to inherit metal oxide's higher reversible specific capacity,and carbon material's excellent cycle performance.However,carbon/metal oxide composite materials are still hard to commercial application,and main deficiencies can be described as following:a)the poor conductivity;b)the large volume changes;c)the relative low coulomb efficiency;d)the poor electrochemical performance at low temperature.Based on the status quo,in view of the above deficiencies,in this dissertation,we studied the following several aspects:a)Improving the conductivity.The composites were synthesized with high conductivity of single-walled carbon nanotubes or nano-metal,such as FeOOH/SWNT,ZnO/Fe@SWNT and Cu/V2O5 composite materials,which can improve the electrochemical performance of lithium ion battery;b)Easing the material volume change.Carbon-material coating can effectively ease the volume change,and enhancing the battery performance,such as SnOx NCNF and Fe3C/Fe3O4@C composites;c)Using the nano metal alloy as catalyst.In order to increase coulomb efficiency and cycle life,the nano metal alloy catalysts,such as ZnO/Fe@SWNT and Fe3C/Fe3O4@C composites,were used to promote the establishment of a uniform density of SEI,which enhance the charge-discharge process of SEI reversible reaction;d)Mixing nano metal alloys with good electrical conductivity at low temperature.Adding Ag,Cu and Fe3C nano metal alloys in the electrode materials can effectively improve electrical conductivity at low temperature the main contents in this paper are described as following:(1)Nano-crystalline FeOOH particles(5-10 nm)have been uniformly mixed with electric matrix of single-walled carbon nanotubes(SWNTs)for forming FeOOH/SWNT composite via a facile ultra-sonication method.FeOOH/SWNT composites as lithium ion battery cathode material enhances kinetics of the Li+ insertion/extraction processes,thereby effectively improving reversible capacity and cycle performance,which shows excellent electrochemical performance than pure FeOOH electrode materials,under a high current density of 400 mA/g,after 180 charge and discharge cycles,the reversible specific capacity is still as high as 758 mAh/g.The good electrochemical performance should be ascribed to the small particle size and nano-crystalline of FeOOH,as well as the good electronic conductivity of SWNT matrix.(2)Synthesized carbon nanofiber composite materials by electrospinning method,get the utmost out of CNF materials with good conductivity and stability of the frame structure,to improve the electrochemical performance of electrode materials,a)Free-standing paper of N-doped carbon nanofibers(NCNFs)containing SnO,was prepared by electrospinning,and directly as lithium ion battery anode materials.These Free-standing membrane electrodes exhibited excellent performance and good cycle stability.The anodes of SnOx NCNFs show high capacity of 520 mAh/g tested at a 200 mA/g after 100 cycles.More importantly,at a high current density of 500 mA/g,a large reversible capacity of 430 mAh/g after 100 cycles can still be obtained.The good electrochemical performance should be attributed to the good electronic conductivity from the NCNFs and the synergistic effects from NCNFs and SnOx materials,b)Composites of Ag-incorporated carbon nanofibers(CNFs)confined with Fe2O3 nanoparticles(Ag-Fe2O3/CNFs)have been synthesized through an electrospinning method and evaluated as anodes for lithium ion batteries.The Ag-Fe2O3/CNF electrode displays high capacity,superior cyclic stability and excellent rate performances at both room temperature and low temperature of-5 ?.Tested at 800 mA/g,the electrodes afforded high capacity of?630 mAh/g after 150 cycles at room temperature,and tested at 600 mA/g,high reversible capacity of?560 mAh/g after 65 cycles at a relatively low temperature of-5 ?C.Benefit from a synergistic effect of the CNF matrix and the highly conducting Ag incorporating.This unique configuration not only facilitates electron conduction especially at a relative temperature,but also maintains the structural integrity of active materials.It is important to transmit that the simple preparation method is suitable for use to improving the electrochemical performance of lithium ion battery electrode materials.(3)Introducing Fe/Fe3C catalyst to electrode materials to enhance the electrochemical performance of lithium ion batteries via wet chemical and thermal decomposition method.a)Develop and fabricate a novel composite of yolk-shell structured Fe3O4@C mixed with Fe3C catalysts directly used as LIB anodes.The anode of Fe3O4 nanospheres protected by the carbon shell and added with the Fe3C catalysts,can deliver obviously enhanced LIB performance.Tested at 1000 mA/g,the electrodes afforded high capacity of?600 mAh/g after 300 cycles at room temperature and high reversible capacity of?380 mAh/g even after 700 cycles at a relatively low temperature of 0 ?.The enhanced LIB performance could be associated with the contribution of carbon shell and Fe3C adding.First,the improved performance can be contributed from the Fe3C adding,which can promote the part of SEI film reversible;second,the carbon shell with enough space as a buffer layer to prevent the expansion of Fe3O4 particles in the charge and discharge process;third,Fe3C with excellent conductivity in low temperature can improve LIB performance at low temperature.b)By colloid chemistry method,Fe nanoparticles introduced to ZnO/SWNT composites.Synthesized ZnO/Fe@SWNT composites were used as lithium ion battery cathode.ZnO/Fe@SWNT electrodes show higher reversible specific capacity,better cycle stability and rate performance than ZnO/SWNT electrodes.The enhanced LIB performance could be associated with the contribution of Fe nanoparticles.Fe nanoparticles are conducive to the electrode in the charge-discharge process of SEi establishment,and a portion of Li+insertion/extraction reversible in the SEI.Fe nanoparticle with excellent conductivity can enhance the conductance of electrode material,reducing the electrode polarization in the cycle process,thus effectively improving the performances of lithium ion batteries.(4)In order to overcome the intrinsic drawbacks of V2O5,including the intrinsically low electrical conductivity and slow electrochemical kinetics,V2O5 nanospheres are uniformly mixed with the electric Cu nanoparticles for forming Cu/V2O5 composites.As used as a cathode material for LIBs,the Cu/V2O5 composite demonstrated obviously improved electrochemical performance including high reversible specific capacity,superior rate capability and outstanding cycling stability.The Cu/V2O5 cathodes can afford a high reversible capacity of 186 mAh/g after 70 cycles under a current density of 300 mA/g and good rate performance.Even at a high current density of 5 A/g,a high reversible capacity of 101 mAh/g after 350 cycles can still remain.The improved performance can be contributed from the decorated Cu nanoparticles,which can result in a good contact in active materials and facilitate transportation of the electron into the inner region of the electrode. |
PDF Full Text Request