Synthesis And Energy Storage Mechanism Investigation Of Phosphides/Selenides-Carbon Composites For Alkali Metal Batteries

The increasing market demands call for the lithium ion batteries（LIBs）has raised great concerns about reserves and cost of lithium resource.Owing to the wider availability and lower cost of sodium/potassium resource,sodium ion batteries（SIBs）and potassium ion batteries（PIBs）have drawn considerable attention from researchers.However,the theoretical capacities of the traditional graphite anode in LIBs,SIBs and PIBs are only 372 m Ah/g,31m Ah/g and 279 m Ah/g,respectively.Therefore,seeking for the advanced materials suitably for LIBs/SIBs/PIBs seems to be urgent.Phosphides and selenides have attracted much attention due to their high theoretical specific capacity and semiconductor properties,respectively.However,compared with traditional carbon materials,its disadvantages are lower intrinsic conductivity and easy to produce crystal structure deformation during the cycle.In this paper,the electronic conductivity and structural stability of phosphides/selenides are improved by strategies such as carbon modification,structural design,scale regulation,and introduction of heterointerfaces and vacancies.In addition,The physical and chemical properties of the material were predicted from the elements and crystal structure of the compound by first-principles density functional calculation method.The main research contents are as follows:1.A hybrid of nickel phosphide（Ni₂P）nanoparticle assembled submicrospheres coated with glucose-derived carbon shell（Ni₂P@C）is synthesized through a coating route and subsequent calcination-phosphatization approach.The theoretical specific capacity of Ni₂P with space group P62m can reach 542 m Ah/g,as a typical conversion material,Ni₂P can undergo abundant redox reactions.In addition,the glucose derived carbon layer can suppress the structural deformation of Ni₂P during the storage process by enhancing the deformation resistance under mechanical stress,improve the utilization rate of the electrode material,and also give the composite electrode more excellent conductivity to a certain extent.Therefore,Ni₂P and glucose carbon coating formed a heterogeneous structure and played a good complementary role.When used as anode material for lithium/sodium ion batteries,compared with Ni₂P,Ni₂P@C exhibits better cycling performance,rate performance,ion migration dynamics and structural stability.2.Based on the above research of phosphide,Co P（894 m Ah/g）has a higher theoretical specific capacity than Ni₂P,is chosen as the host material,and use graphene as the basic conductive structure.Firstly,Co P was prepared by hydrothermal method and phosphating method,and then the urchin-type Co P is encapsulated into graphene framework（Co P@GF）by vacuum filtration method and calcination reduction method.In addition,graphene can accelerate the reaction kinetics,enhance the anchoring and transformation process through theπ-πstacking interaction.Meanwhile,its coupled conjugated molecules as active sites,can improve the volume effect of Co P in cycling.Therefore,Co P@GF has better Li⁺/Na⁺/K⁺storage capacity than Co P/GF anode,and the electrochemical properties of the electrode in different alkali metal ion batteries can be ordered as LIBs>SIBs>PIBs.To shed light on the phenomenon,we calculate the adsorption energy of alkali metal ions on Co P surface using first principles.The results indicate that more electrons at the Co P surfaces transfer to Li atom than Na and K,and the surface reactivity thus can be ordered as Li-Co P>Na-Co P>K-Co P.3.In the research content of 1 and 2 above,the synthesis process of materials requires four steps,which is slightly complicated.In order to explore a simpler and more feasible experimental scheme,in this section,a porous Mn Se/Fe Se₂@C adhered/inserted with interlaced carbon nanotubes（CNTs）have been successfully prepared via a simple chemical precipitation approach and a subsequent one-step carbonization-selenization process（Mn-Fe-Se@C/CNTs）.The exploration found the existence of a heterogeneous interface formed by cubicα-Mn Se and orthorhombic Fe Se₂in the composite,which can generate a built-in electric field and promote the transfer of electrons within the crystal.The double-carbon structure composed of CNTs and PBA-derived carbon layers has a positive effect on the electrical conductivity and the volume expansion.In addition,the porous structure of the composite can further alleviate the volume expansion problem during cycling.Therefore,the diffusion coefficient of Na⁺in the material can reach 5.5×10^-9cm²/s.The diffusion coefficient of K⁺in the material can reach 0.7×10^-9cm²/s.In addition,the results of ex-situ XRD and quantitative calculations of capacitance/diffusion behaviors further illustrate the modification mechanism for the enhanced energy storage properties.4.Based on the experimental ideas in the research content 3,a fascinating Fe Se₂-Co Se₂/C（FCSe@C）containing heterostructure and special selenium vacancies is confined within the void space inside carbon sphere by using a combination strategy of melting diffusion and selenization（FCSe@C@void@C）.In this part,the crystal structure,work function difference,electronic density of states and differential charge density of different materials（Fe Se₂,Fe Se₂@C,Co Se₂-Fe Se₂）were deeply explore via the first-principles calculations.Combined with the experimental results,the relationship between the energy storage properties of the material and the structural characteristics is well explained.The theoretical calculation results show that the density of states of Fe Se₂@C and Co Se₂-Fe Se₂increases near the Fermi level compared with single Fe Se₂,indicating that the electronic conductivity of the material is enhanced.Finally,according to the galvanostatic intermit tenttitration techniques test and Fick’s law,the diffusion coefficient of Na⁺in the material can reach the order of 10^-10to 10^-8.5cm²/s.The diffusion coefficient of K⁺in the material can reach the order of 10^-11to10^-9.5cm²/s.5.In the research contents 1 and 2,the host material is a phosphide with high specific capacity,and in the research contents 3 and 4,the host material is a selenide with semiconductor properties.Here,a double-anionic Co PSe nanoparticles is encapsulated in carbon nanofibers（Co PSe/CNFs）by using a combination strategy of electrospinning and calcination.Because the electronegativity of Se is greater than that of P,there will be more anion vacancies in Co PSe.The rearrangement of extra electrons around the particular metal atoms will be occurred by the introduction of vacancies,which can be considered as negatively charged activation regions to attract alkali metal ion and enhance the adsorption energy for Na⁺/K⁺.The structure with vacancies may generate new ion channels and active sites in perpendicular directions of crystal,allowing the alkali-ion transfer to the interlaminar space more quickly.In addition,the introduction of double anions in the ternary single-phase material leads to the splitting of the energy level of the d orbital in the valence electron layer of the metal phase.Eventually,antibonding orbitals are exhausted,resulting in less repulsion between the metal phase and the ligand,and thus better chemical structural stability.Furthermore,the introduction of carbon nanofiber film can not only improve the conductivity of electrode materials,but also effectively avoid the cumbersome process of preparing electrodes with adhesives and conductive agents.Electrochemical measurements show that the double-anionic Co PSe/CNFs possesses reversible potassium-ion storage with excellent rate performance and prominent cycling stability.