Microstructure Regulation And Lithium Storage Properties Of Iron-based Transition Metal Compounds

With the depletion of fossil fuels such as coal and oil,carbon dioxide emissions increasing sharply,the energy crisis and environmental pollution problems have become more and more serious.Under the background of carbon peaking and carbon neutrality proposed by the state,the development of new energy system is the country’s energy strategic demand.Lithium-ion batteries have been widely used in portable electronic devices,medical devices and hybrid vehicles due to their high energy density,high power density,no memory effect and low selfdischarge.However,at present,commercial graphite anodes can no longer meet the growing energy demand due to their low specific capacity(372 mAh g-1)and safety concerns.The scientists are seeking a new generation of promising anode materials that promise to replace graphite.Among them,iron-based transition metal compounds have attracted the attention of scientists due to their high specific capacity,suitable lithium insertion potential,low price,environmental friendliness and abundant reserves.However,there are also some key problems,such as:poor intrinsic conductivity will lead to slower ion and electron diffusion rates,resulting in poor electrochemical kinetics and electrochemical performance;the serious volume expansion during discharge/charge can result in stress concentration of the material,structural collapse,severe electrode pulverization,and the material falling off from the current collector,causing poor cycle performance.In order to solve the above faced problems,a series composites of iron-based transition metal compound were prepared by hydrothermal method,water bathing method,freeze-drying method,high-temperature heat treatment and other processes.And the above shortcomings were improved through various strategies,their structure and properties were deeply studied,and their electrochemical reaction mechanism was revealed,and their excellent electrochemical performance was further revealed through density functional theory(DFT).1.The heterogeneous interface and hollow structure jointly improve the electrochemical performance of H-Fe3O4/FeP@C composites.H-Fe3O4/FeP@C composites were synthesized by simple solvothermal solvothermal treatment,dopamine coating,high temperature treatment,acid etching and low temperature phosphating processes.When used as an anode material for lithium-ion batteries,H-Fe3O4/FeP@C nanospheres can maintain a specific capacity of 630.2 mAh g-1 after 1000 cycles at a current density of 2.0 A g-1.The heterogeneous interfaces were formed by the(311)crystal plane of Fe3O4 with the(0 0 2)crystal plane of FeP,and DFT theoretical calculation reveals that it has higher differential charge density and good metal properties at the heterogeneous interface,which improves the conductivity of the material.Higher adsorption energy and lower lithium ion diffusion barrier were obtained at the heterogeneous interface,which provided more active adsorption sites and facilitated the transport of lithium ions and electrons.At the same time,the hollow structure can not only alleviate the volume expansion of Fe3O4 and FeP,but also effectively shorten the diffusion distance of lithium ions.2.Vacancy engineering and structural engineering were constructed together to improve the electrochemical performance of V-FeP nanorods.Through the guidance of DFT theoretical calculation,it is confirmed that the existence of phosphorus vacancies can improve the metal characteristics of FeP.Therefore,metal-organic framework(MOF)-derived porous FeP nanorods(V-FeP)with phosphorus-vacancy-rich were designed and prepared for the first time by the simple method,which was applied into the anode of lithium-ion batteries.Through the test,the V-FeP nanorods maintained a high specific capacity of 1228.3 mAh g-1 after 120 cycles at a current of 0.1 A g-1.The electron paramagnetic resonance and synchrotron radiation confirmed the presence of phosphorus vacancies.The V-FeP nanorods obtained excellent electrochemical performance,attributed to abundant phosphorus vacancies and FeP nanoparticles dispersed in the conductive carbon network,which enhanced their conductivity,provided more reactive sites,shortened the diffusion distance of lithium ions,alleviated the change of volume.Finally,the V-FeP nanorods obtained good lithium storage performance.3.Ni doping and carbon shell coating jointly optimize the electrochemical performance of NiFeP@C.Carbon-coated FeP with Ni-doping(Ni-FeP@C)composites were prepared by twostep solvothermal method and low-temperature phosphating process.Ni doping changed the morphology of the material,improved the conductivity of the material,provided more lithium adsorption sites,which was conducive to improving the electrochemical reaction rate.Secondly,carbon coating could buffer the volume change of FeP and maintain the structural stability of the material.After the assembled battery test,Ni-FeP@C delivered capacity of 855.7 mAh g-1 after 100 cycles under the current density of 0.1A g-1.4.Phosphorus doping and sulfur vacancies strategy synergistically promote the lithium storage performance of Py-FeS2-x microflowers.Phosphorus-doped FeS2 micronflowers with in-situ induced sulfur vacancies were reasonably designed and prepared as anode materials of lithiumion batteries.As an anode material,P1.0-FeS2-x microflowers achieved the good rate performance with the specific capacity of 642.6 mAh g-1 at a high current density of 5.0 A g-1.Through characterization,phosphorus doping increased the lattice spacing of FeS2,which could promote the intercalation reaction of lithium ions.After characterization,the oxidation state and coordination number of Fe in FeS2 decreased after phosphorus doping,indicating that there existed sulfur vacancies in the material.DFT theoretical calculation also explained that phosphorus doping could improve the conductivity of the material,increase the adsorption energy of lithium,and accelerate the electrochemical reaction.5.The hierarchical heterogeneous interface and fluorine doping strategy synergistically regulate the lithium storage performance of F-FeS2@CoS@C nanorods.The FeS2@CoS@C nanorods with in-situ induced hierarchical heterostructures by F-doping were prepared by using simple hydrothermal methods,ion exchange,and vulcanization processes.The strong hierarchical coupling effect enabled ions and electrons to penetrate the entire material and accelerate the diffusion of ionic electrons.F doping can increase the conductivity of the material and provide more active sites for lithium-ion storage.Next,the in-situ induced uniform carbon shell could buffer the volume expansion of FeS2 and CoS during discharge/charge process,and can maintain the structural stability of the material.By electrochemical tests,the F-FeS2@CoS@C electrode retained the specific capacity of 597.1 mAh g-1 after 800 cycles with the current density of 2.0 A g-1,showing good cycling performance.