Modification And Electrochemical Properties Of Carbon-based Cathode Materials For Lithium-selenium Batteries

In recent years,owing to the environmental pollution and climate change caused by the using of fossil fuels,the development of green and sustainable energy has become very important.Electrochemical batteries are considered as optimal energy storage devices due to their high capacity and low energy loss during conversion.In various battery types,rechargeable lithium metal batteries with selenium（Se）cathodes have attracted the interest of many researchers in the past few decades.Se cathode has a high electronic conductivity,a moderate output voltage,environmental-friendly characteristics and a high volume energy density,which is comparable to sulfur.Nevertheless,the application of lithium-selenium（Li-Se）batteries is hindered by the side reaction between selenium anion and carbonate electrolyte,the volume expansion of selenium and the sluggish electrochemical reaction rate during the charge-discharge process.In view of the above major problems,this paper concentrates on the design and synthesis of the carbon hosts in Se cathodes,combining the physical limit domain of porous carbon materials and the introduction of electrochemical active sites,such as heterojunctions and single atoms.The effective anchorage of selenium is realized,which can avoid the capacity decay resulted by the side reaction between Se and electrolyte.The diffusion rate of lithium ions and the conversion reaction rate of selenium are accelerated to improve the electrochemical performance of Li-Se batteries.The main research results of this paper are as follows:First,Zn Se/Co Se heterojunction modified hollow carbon nanorod was obtained by one-step selenization-carbonization by using bimetallic Zn/Co-ZIF nanorod as the precursor.In the high-resolution TEM images,the Zn Se/Co Se heterointerface is clearly observed.X-ray photoelectron spectroscopy results suggest that interaction exists at the heterointerfaces of Zn Se and Co Se,which introduces more electrochemical active sites.When the carbon nanorod modified by heterostructures is combined with selenium as cathodes in Li-Se batteries,it shows good electrochemical performance in the tests,with an average capacity decay of 0.022%per cycle in the 2400 cycles.Subsequently,to further improve the performance of the carbon/selenium cathode,the hollow carbon host modified by three-phase heterostructure Ni Se₂/Ni₂Co/Co Se₂was obtained by the single-dispersed Ni Co-acetates,followed by the coating of polydopamine（PDA）,the corrosion of tannic acid（TA）,carbonization and incomplete gas selenization.The addition of Ni₂Co alloy phase increases the strain stress among the heterointerfaces,which is attributed to the Jahn-Teller effect between transitional metal selenides and this phenomenon can produce more lattice distortion and extra electrochemical active sites.XPS results demonstrate the generation of built-in electric field at the heterointerfaces,which accelerates the transmission of electrons and ions.As cathodes in Li-Se batteries,the Li⁺diffusion coefficient of the carbon/selenium composite modified by three-phase heterojunctions was calculated according to cyclic voltammetry tests and achieves improvement.Based on the physical confinement of hollow carbon host and the three-phase heterostructure active sites,in the charge-discharge tests,the cycling performance at high current rates of this carbon/selenium cathode is enhanced,which can maintain 2200 cycles at 12 C.Due to the high density of selenides as active sites for accelerating battery reactions in the previous two parts,in order to reduce the mass ratio of catalytic sites in batteries and further improve the performance and energy density of lithium-selenium batteries,at last,the single-atom Fe and Co doped porous carbon host APPC was prepared.After loading selenium,the material was coated with polydopamine as carbon/selenium composite cathode.The high-resolution TEM image intuitively demonstrates the uniform dispersion of single atoms while the synchrotron X-ray absorption spectra suggests the coordination environment of single atoms.The theoretical calculation indicates that the synergistic effect of single atom sites Fe N₄ and Co₂N₃ strengthens the chemical absorption between the carbon host and Li₂Se.The ex-situ XPS results confirm the formation of C-Se-Li bonds,which is beneficial to enhance the performance of cycling.As a consequence of single-atom metal sites and polydopamine shell,the rate capabilities and cycling performance are further improved.In summary,the relevant research in this paper mainly focuses on the modification of carbon-based cathodes for Li-Se batteries and the improvement of electrochemical performance.A series of carbon hosts with various morphologies and porous structures were fabricated by different metal-organic coordination precursors,followed by the modification of electrochemical active sites.Stepwise improvements in the cycling performance at high rates of Li-Se batteries have been achieved in this paper,which provides referential value for the further research of Se cathode materials and moving towards practical application.