Structure Design Of Phosphorus And Boron Phosphide And Their Application Study On Lithium Storage And Water Splitting

Phosphorus is a relatively abundant nonmetallic element in the earth's crust.Phosphorus-based materials,including red phosphorus,black phosphorus and phosphide,have a great application potential in new energy fields such as secondary batteries and catalytic water splitting.With the increasing demand for energy,the structural design,synthesis and extended application of phosphorus-based materials are required.For red phosphorus,in addition to the conventional vapor deposition,in-situ reduction and high-energy ball milling processes,it is necessary to develop an universal and convenient nanocrystallization process.As a new two-dimensional semiconductor material,black phosphorus is expensive due to its complex preparation process,which seriously limits its application in the field of new energy.In addition,the design of nonmetallic phosphates also has great application prospects in the field of total water splitting.In this thesis,the controlled synthesis and crystallization mechanism of nano-sized red phosphorus and black phosphorus were studied.The structure and electrochemical properties of red phosphorus and black phosphorus and their composites were systematically studied by combining theoretical calculation and experimental research.A new spherical boron phosphate material was designed based on theoretical calculation,and the synergistic mechanism of phosphorus and boron atoms in total water splitting was revealed.Red phosphorus with uniform size and particle size of about 200 nm was successfully prepared by a liquid phase method.In order to enhance the electrical conductivity of red phosphorus composites and restrain the volume change of the material during the cycle process,the idea of"nanocrystallization&bonding cooperation"was adopted to prepare the sulfide polyacrylonitrile-red phosphorus(SPAN-RP)composites.Based on density functional theory(DFT)calculation,and analysis of molecular structure,charge distribution,DOS,FMO and SESP,the effects of sulfur on the molecular structure of PAN were systematically studied.It was found that the introduction of sulfur reconstruct the PAN molecular configuration and improve the electron and lithium ion conduction.At the same time,the bond cooperation between SPAN and red phosphorus P₄ molecule leading the composite good electron transport ability and structural stability.The specific capacities of first discharging and recharging(reversible)of SPAN-RP composites reach 2765 and 1214 mAh g^-1 at 0.2 C current density.The reversible capacity of the composite can still reach 863 mAh g^-1 after 200cycles.Black phosphorus on a gram scale was prepared from red phosphorus and ethylenediamine by one-step liquid phase method.The liquid phase method greatly reduces the preparation cost of black phosphorus and provides theoretical and technical basis for the large-scale preparation of black phosphorus materials.The FTIR,XPS and DFT calculation were used to analyze the synthesis mechanism of the material systematacially,and a complete crystallization mechanism of the transformation from red phosphorus phase to black phosphorus was proposed.The basic unit of red phosphorus P₄ molecule is first activated by ethylenediamine molecule,and the activated P₄ molecule symmetrical couples into a stable P₈ basic unit.After the complete phospholene plane formes by the crystal growth and expansion template of P₈basic unit.The stable black phosphorus layered structure was formed by van der Waals force between layers.The prepared black phosphorus was used as anode material for lithium-ion battery,and its first specific discharge capacity was 1912 mAh g^-1 at 0.5 C,and the specific charge capacity was 829 mAh g^-1.The reversible capacity was 440mAh g^-1 after 100 cycles.The material can still achieve the reversible specific capacity of 352 mAh g^-1 at 2 C.A spherical boron phosphide(B₈P₁₂) molecule was constructed as a model for based on theoretical calculation.The reaction process and underlying mechanism of B₈P₁₂ as a catalyst for total water splitting were studied.B₈P₁₂ molecule is a highly symmetric,concave-convex ordered dodecahedron structure composed of 8 boron atoms and 12 phosphorus atoms.Each side of the five-memed ring contains 2 boron atoms and 3 phosphorus atoms.By simulating IR,Raman and NMR spectra of B₈P₁₂,the molecular structure and vibration characteristics of B₈P₁₂ were studied.Under the synergistic effect of phosphorus and boron atoms,the H-O bond in water molecules can be broken.The decomposed oxygen-containing group(-OH,-O)is attached to the phosphorus sites,while the proton group(-H)is attached to the B sites.The total water splitting process consists of five transition states and four intermediates,among which the step of breaking of O-H bond shows the highest energy barrier(2.92 e V).The new boron phosphide molecule maintains good structural stability throughout the whole water splitting process.