Modified Strategy And Sodium Storage Mechanism Of Nano Metal Selenides

With the fast development of portable electronic devices and electric vehicles,the demand for rechargeable batteries with high energy/powder densities and long cycle life is growing tremendously over the years.Sodium-ion batteries（SIBs）,which have a similar energy storage mechanism to lithium-ion batteries（LIBs）,are considered as a suitable replacement to LIBs due to the merits of rich natural resources,low cost and appropriate redox potential（-2.71 V vs SHE）of Na.However,the radius of Na⁺（～0.97?）is 55%larger than that of Li⁺,which would cause sluggish diffusion and large volume change during the electrochemical processes,resulting in an inferior sodium storage capability,thus limiting the practical application of SIBs.It is thus an urgent need to explore active materials to overcome the abovementioned shortcomings.In the past decade,various anode materials have been extensively investigated for SIBs,such as carbonaceous materials,alloys,metal oxides and dichalcogenides.As a promising anode material for SIBs,metal selenides have received much attention considering their high theoretical capacity and volumetric capacity.In addition,owing to the large atom radius of selenium,the metal-selenium bonds are weaker than the metal-oxygen and metal-sulfur counterparts,leading to an improved efficiency of the conversion reaction in the selenides.However,metal selenides still suffer from several drawbacks such as low intrinsic electronic conductivity,sluggish Na⁺diffusion process and severe volume variation during charge/discharge processes,leading to inferior rate performance and poor cycling stability.The goal of this dissertation is to obtain high-performance metal selenides electrodes for SIBs by heterostructure engineering and hetero-atom doping,and the mechanism behind each method for enhanced sodium storage properties were investigated by first-principle calculation.The specific research contents are as follows:（1）A highly durable yolk-shell SnSe₂ nanospheres withSe-doped carbon shell（SnSe₂@Se-C）was prepared by a multi-step templating method,which involved an in situ gas-phase selenization of the Sn O₂@C hollow nanospheres.By controlling the duration of selenization,Se could be introduced into the carbon shell with a tunable amount and formSe-C bonds.Density functional theory calculation results revealed that by having theSe-C bonds in the carbon shell,the charge transfer properties as well as the binding interaction between the SnSe₂ core and the carbon shell were greatly enhanced.As expected,it delivered reversible capacities of 441 m Ah g^-1 and 406 m Ah g^-1 after 2000 cycles at 2 A g^-1 and 5 A g^-1,respectively,for sodium storage.Such a performance was significantly better than the control sample without theSe-C bonding and also other metal selenide-based anodes,evidently showing the advantage of the heterostructured interface.（2）The hollow nanorods composed of In₂Se₃/Co In₂/CoSe₂ were fabricated by selenizing the MIL-68@ZIF-67 core-shell MOF-based composite precursor.By controlling the time for selenization,In₂Se₃ and CoSe₂ were formed with the emergence of the alloy phase Co In₂,giving rise to a heterostructure consisting of two interfaces,offering synergistically enhanced electrical conductivity,Na diffusion process and structural stability,in comparison to the single Co In₂-free interface with only two metal selenides.As expected,this nano construction delivered a high reversible capacity of297.5 m Ah g^-1and 205.5 m Ah g^-1 at 5 A g^-1and 10 A g^-1 after 2000 cycles,respectively,and a superior rate performance of 371.6 m Ah g^-1 at even 20 A g^-1.（3）A heterostructure consisting of non-layered CoSe₂ and layered VSe₂ was formed on the surface of N-doped carbon nanofibers via an electrpsinning method.The lattice mismatch between these two components created a large amount of lattice distortion at the phase boundaries,which would accelerate the Na⁺diffusion kinetics and enhance the electrical conductivity.Comparing with the single-phase counterparts,such a heterostructure displayed an enhanced electrochemical performance for sodium-ion battery and also superior potassium storage capabilities.Molecular dynamics calculation showed that the heterostructure of CoSe₂/VSe₂ could effectively prevent Na⁺from being trapped and forming‘dead sodium’,thus facilitating the migration of Na⁺.The mechanism for boosting Na storage of the layered/non-layered heterostructures was investigated by the first principle calculation,which confirmed the reduction of Na diffusion barrier at the interface and the enhancement of electrical conductivity by the electron transfer between the two phases.（4）FeSe_2-xS_x microspheres have been prepared via a self-doping solvothermal method and followed by selenization using NH₄Fe（SO₄）₂ as both the Fe and S source.The carbon-free nature of the FeSe_2-xS_x microspheres resulted in a high tap density and a high initial columbic efficiency of 85.6%.Compared with pure FeSe₂,such FeSe_2-xS_xdelivered a high reversible capacity of 373.6 m Ah g^-1 at a high current density of 5 A g^-1after 2000 cycles and an enhanced rate performance of 305.8 m Ah g^-1 at even 50 A g^-1.In addition,a FeSe_2-xS_x//hard carbon full-cell achieved high energy and power densities of 113.2 Wh kg^-1 and 164.0 W kg^-1,respectively.The density functional theory calculation results revealed that S doping could enhance the adsorption and lower the diffusion energy barrier of Na atoms at the S doping sites,and also improve the electronic conductivity of FeSe_2-xS_x.