Preparation Of Metal Phosphide M_xP_y (M=Sn, Co, Mo) And Its Lithium/sodium Storage Properties

Metal phosphides have high theoretical specific capacity and low redox potential,which have attracted more and more attention as anode materials for lithium ion batteries and sodium ion batteries.Researchers have adopted the hydrothermal method,template method to prepare cobalt phosphide,iron phosphide,zinc phosphide and other metal phosphides.However,the complex synthesis process of phosphides,poor conductivity in charge and discharge process,large volume change,resulting in low electrochemical activity,rapid attenuation of capacity,poor rate performance,limited the development of their applications.To solve the above problems,the researchers have made some progress by means of structural design and carbon recombination.In order to further improve the material properties,a series of nano-sized metal phosphide particles embedded in carbon matrix composites were prepared by solid-state reaction method in this paper.The synergistic effect between the outer carbon materials and the internal active particles greatly reduces the volume effect in the energy storage process,effectively inhibits the structural pulverization,provides a faster reaction kinetics,and shows excellent electrochemical performance.The main research contents and results are as follows:（1）We chose graphene oxide（GO）as carbon source,sodium hypophosphite as a source of phosphorus,polyethylene glycol（PEG200）as surface active agent,through the solid state reaction at room temperature,we obtained the tin oxide precursors loaded on 2D flake graphene,followed by low temperature phosphating,tin phosphides particles evenly distributed in two-dimensional graphene sheets（denoted as Sn₄P₃/GO-PEG200）,and applied as anode materials for sodium ion batteries.Due to the synergistic effect of GO carbon matrix with high specific surface area and PEG200surfactant,the material showed great electrochemical performance:the reversible charge specific capacity could still reach 419 m A h g^-1 after 100 cycles at the current density of 0.1A g^-1;the average charge capacities were 443,428,378,325,267 and 187m A h g^-1 at the current density of 0.1,0.2,0.5,1,2 and 5 A g^-1,respectively.（2）We selected ethylenediamine tetramethylene phosphonic acid（EDTMPA）as a chelate,which was rich in nitrogen,phosphorus,carbon elements and could chelating metal ion tightly.Because EDTMPA itself can be used as a source of phosphorus and carbon,no extra materials need to be added,mixing it with cobalt acetate,and under Ar/H₂（5%）mixed gas atmosphere,high-temperature solid-state reaction was carried out to successfully prepare Co P particles embedded in irregular N,P co-doped carbon sheets（denoted as Co P/NPC）.The doping of heteroatoms further improved the electronic conductivity of the materials and provides more active sites for lithium ion storage.The Co P/NPC composites exhibited excellent cycle performance and remarkable rate performance:the reversible charge specific capacity could still reach563 m A h g^-1at the current density of 0.1 A g^-1 after 200 cycles,with the initial coulombic efficiency exceeded 80%;the average charge specific capacities were 703,496,444,408,353 and 270 m A h g^-1at 0.1,0.2,0.5,1,2 and 5 A g^-1,respectively.The reversible capacity could maintained at about 170 m A h g^-1 even after 3000 cycles at a high current density of 5 A g^-1.（3）We learned from the successful preparation experience of cobalt phosphide,tried to mix ammonium molybdate with EDTMPA for expanding the application of this method and successfully fabricated Mo P particles embedded in irregular N,P doped carbon matrix（denoted as Mo P/NPC）.As the anode materials for lithium ion batteries,the composites also showed outstanding electrochemical performance:the reversible charge specific capacity could still reach 406 m A h g^-1after 100 cycles at the current density of 0.1 A g^-1,with the initial Coulombic efficiency was 66%.The average charge specific capacities were 466,404,329,236,143 and 53 m A h g^-1at 0.1,0.2,0.5,1,2and 5 A g^-1,respectively.