| Lithium-ion batteries are widely used in portable electronic devices,power tools and hybrid/electric vehicles due to their high energy density and power density.If the majority of gasoline powered transportation are replaced by electric vehicles(EVs),which will greatly reduce greenhouse gas emissions.In addition,the high energy efficiency of Li-ion batteries also makes them widely used in wind energy,solar energy,geothermal energy and some secondary auxiliary energy field,which will be benefit to their application and promotion further.As many advantages of Li-ion batteries,they obtain strong interests from industry,government funding agencies and corresponding research field.Yet looking to the future,lithium-ion battery still remains many deficiencies.Graphite,as the most successfully commercialized anodic material,has proper working voltage vs.Li+ and long cyclic stability.However,its imperfect capacity(372 mAh g-1)and low Li+ diffusivity(10-9-10-10 cm2 s-1)retain its restrictions for high-energy and high-power requirements.In order to promote the energy density of Li-ion batteries,various active materials have been studied.Tin and Fe-based compounds,as the promising electrode candidates,have received increasing attention because of their high theoretical capacity and suitable potential vs.lithium.However,tin and Fe-based compounds are subject to active materials losing and drastic volume expansion during the cycling process,which leads to rapidly decrease in electrochemical performance of electrodes.It is one of the most important problems of tin and Fe-based compounds that cannot be used as a commercial Li-ion battery electrode material.Designing the reasonable phase composition and microstructure can effectively inhibit the pulverization of the electrode caused by the active materials volume changed during the long-term charge/discharge process.One of possible practical solutions has been attempted by introducing an inactive element to alloy with Sn and compositing Fe-based compounds with carbon materials,which effectively improve the capacity and cycling stability via constructing complex phase and complex nanostructure respectively.Different from the other chemical preparation methods,in this paper,DC arc discharge method,as the main way,combined with following chemical modification for preparing Sn-based alloy nanoparticles and carbon-coated Fe-based compounds nanopowders,which were designed to solve the low energy density and poor stability of Li-ion battery electrodes by building the material phases and nanostructure respectively.The main results are shown as following:1.Construction complex phase by DC arc-discharge method and their electrochemical propertiesConstructing the phase of Sn-based alloy nanoparticles and their electrochemical behaviors(1)A direct current arc-discharge method was applied to prepare the Sn-Fe,Sn-Al,Sn-Ni,Sn-Cu and Sn-Mg bi-alloy nanoparticles in a mix gas of H2 and Ar.All the particles are in tori-spherical shape with average size in the range of 40-200 nm.The XPS measurements were carried out to qualitatively analyze the shell oxides of the Sn-Fe,Sn-Al,Sn-Ni nanoparticles and thermodynamic is introduced to analyze the energy circumstances for the formation of the nanoparticles during the physical condensation process.The intermetallic compounds in Sn-Fe,Sn-Cu,Sn-Mg nanoparticles have been characterized and thermodynamics analysis is introduced to elucidate the energy balance of their formation mechanism.(2)In the Sn-Fe nanoparticles electrode,the active FeSn2 and inert FeSn both make a synergic contribution to improve the performance of the electrode.The Sn-Fe nanoparticles electrode showed excellent electrochemical Li-ion storage properties with high initial charge/discharge capacity(256.3 mA h g-1/338.4 mA h g-1 at a rate of 100 mA g-1)and a 227 mA h g-1 reversible capacity maintaining after 20 cycles.Sn-A1 nanoparticle electrode sharply decays to 44.8 mA h g-1 after 20 cycles,which is attributed to the accelerated pulverization of the active materials and the eventual ohmic disconnection with the electric collector during cycling process.The Sn-Ni,Sn-Cu,Sn-Mg nanoparticles electrodes a similar weak electrochemical reaction with Li+ due to the electrochemical activity of their intermetallic compounds(Ni3Sn4,Cu3Sn,Mg2Sn)is suppressed in the normal test conditions.Based on the above results and following electrochemical impedance testing,an actual anode material needs to demand the following features:1)an inert element is required to introduce alloy with the active component to relieve the volume expansion of active element during lithiation/delithiation process;2)the intermetallic compound has high active to Li+ during cycling process.2.Construction complex nanostructure by DC arc-discharge method and their electrochemical properties(1)Construction of carbon-coated Fe3N nanostructures and their characteristics as Li-ion battery anode1)Fe@C nanoparticle precursors(Fe@C)are prepared by a direct current arc-discharge method in a mix of CH4,H2 and Ar.Then the Fe@C precursors are served as the starting material heat-treated in NH3 environment at various temperatures.A unique nanostructure of carbon-constraint Fe3N(Fe3N@C)is finally acquired in such an easy and feasible way.2)The as-prepared Fe3N@C nanoparticles electrode showed excellent electrochemical Li-ion storage properties with high initial charge/discharge capacity(545/675.8 mAh g-1 at a rate of 100 mA g-1 and a superior cycle stability(a specific reversible capacity of 358 mAh g-1 maintained after 500 cycles).The first principle calculation and following XRD results showing the Fe3N discharge products are Fe and Li3N.Double capacity lithium storage mechanism on the surface carbon of our sample is obtained by electrochemical impedance spectroscopy.The superior cyclic performance and high reversible capacity are attributed to the carbon coatings,the rigid structure and strong mechanical strength of which play a significant role in buffering the volume variation and stabilizing the electrode reactions as well.(2)Construction of carbon-coated FeS2 nanostructures and their electrochemical behavior as Li-S battery cathode1)Fe@C nanoparticle precursors(Fe@C)are prepared by a direct current arc-discharge method in a mix of CH4,H2 and Ar.Then the Fe@C precursors and sulfur are grounded together with a mass ration of 1:1,and sealed in a reactor under the moisture and oxygen both less than 0.1 ppm.After heating treatment,the FeS2@C-S core-shell nanoparticles are finally obtained with S-doped carbon shell and FeS2 core.2)FeS2@C-S nanoparticle electrodes exhibit excellent cycling stability and rate performance.The initial discharge/charge capacity of the FeS2@C-S nanoparticles electrode is 1040/1370 mAh g-1,Coulombic efficiency is about 76%.The capacity maintains at the level of about 862 mAh g-1 and exhibits satisfactory cyclic stability during 300 cycles.The electrode delivers discharge capacities of around 983 mAh g-1 at 0.1 A g-1,which then slowly reduces to 887 mAh g-1 at 0.2 A g-1,735 mAh g-1 at 0.5 A g-1,595 mAh g-1 at 1.0 A g-1,370 mAh g-1 at 3.0 A g-1 and 276 mAh g-1 at 5.0 A g-1,significantly,after the high rate charge-discharge cycles,an discharge specific capacity as high as 894 mAh g-1 can be still maintained when the current density was reduced to 0.1 A g-1.An outstanding high rate response can be achieved when the current density rising to as high as 10 A g-1,it is shown that the composite electrode still gives a reversible capacity of about 230 mAh g-1 after 300 cycles,which is still higher than that of conventional Li-ion battery.Both the basically unchanged in capacity and the high Coulombic efficiency are attributed to the conductive carbon nanocage,as a strongly spatial shield,aids in constraining the polysulfides in a nanoscale electrochemical reaction vessel and provides essential electrical contact to both of almost insulated FeS2 and its discharge products.The inner FeS2 core provides a strong Fe-S chemical bond that further retards diffusion of the polysulfides out of the cathode region. |
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