Structural Design Of Anode Materials Based On Nickel Phosphides For Lithium-ion Batteries And Their Improved Electrochemical Performance

Transition metal phosphides (M-P, where M= Fe, Co, Ni) are the novel non-carbon anode electrode materials for lithium ion batteries. It has been reported that 3d transition metal phosphides exhibit the lowest polarization due to their metallic character and react with lithium over the lowest and narrowest potential range. The electrochemical conversion mechanism of 3d transition metal phosphides towards Li+ is to form metallic nanoparticles M and Li3P phase, which is similar to 3d transition metal oxides. The research carried out in this dissertation is based on Ni-P system, to develop several methods such as preparation of nanomaterial, nanocomposite and solid-state structural design to improve the Li+ storage property, aiming at exploring simple synthetic strategy, improving the initial Coulombic efficiency and the cyclic performance.(1) Core/shell-structured and hierarchical Ni2P spheres were facilely synthesized by a one-pot in situ organic-phase strategy by using Ni(acac)2 as a metal precursor, TOP as a phosphorus source, OAm as the capping agent, and 1-octadecene and TOA as solvent, respectively. The amorphous carbon on the surface of Ni2P spheres can buffer the volume expansion, increase the electrode stability during cycling, and offer a conductive network, which facilitates the electron transfers. Meanwhile, the carbon network can effectively control the SEI formation, resulting in the uniform metallic Ni nanoparitcles in the matrix without aggregation. The specific reversible capacities after 50 cycles for the M2P/C nanocomposite are 435 mAh g-1 at 0.1 C and 303 mAh g-1 at 0.5 C, and for the hierarchical Ni2P, they are 365 mAh g-1 at 0.5 C and 167 mAh g-1 at 3 C.(2) Ni2P nanowires were synthesized with the help of a syringe pump. The 1-D nanowire morphology facilitates the reactive kinetics because it exhibits very small size, resulting in the fast Li+diffusion. Also, the nanowire structure facilitates the reversible and continuous insertion process (around 1.5 V), thus improve the Li+ to transfer fast. The 1-D nanowire structure has a great relationship with the reversible capacity and rate performance. The specific reversible capacities after 50 cycles.for the Ni2P nanowire is 253 mAh g-1 at the current density as high as 5 C, with the initial Coulombic efficiency improved from 52.7% to 69.5%.(3) Based on the nanoscale Kirkendall effect, Ni2P/C nanotubes, porous Ni2P nanosheets and Ni2P thin films were fabricated by using metallic Ni nanowire, nanosheet and film templates, respectively. The nanotubes covered by the amorphous carbon exhibit superior high-rate capability and good cycling stability. There is still about 310 mAh g-1 retained after 100 cycles at 5 C, with the initial Coulombic efficiency of 69.8%. The tubular nanostructure of Ni2P is also preserved after prolonged cycling at a relatively high rate. Porous Ni2P nanosheets were synthesized by the etching effect of TOP on Ni nanosheets, and after that, the thickness of the Ni2P sheet is reduced. The porous and thin sheet structure would result in better contact between the active material and electrolyte. These nanosheets present a reversible discharge capacity of 379.8 mAh g-1 after 50 cycles and facilitate the uniform formation of SEI. Similarly, Ni-P films with different surface morphologies and thicknesses were synthesized through etching Ni films. After seriously etched by TOP, there exhibits a Ni2P phase with plenty of pores on the surface, which may facilitate the Li+ transportation. The Ni2P films delivered a large reversible discharge capacity around 398.5 mAh g-1 after 50 cycles, corresponding to 91.4% retention of the initial charge capacity.(4) A binary phase of Ni2P-Ni5P4 was obtained at different temperatures via the organic-phase strategy. A monophase Ni5P4/C composite with a thin uniform carbon shell was controllably synthesized through a solid-state reaction. It is suggested that the further diffusion of phosphorus atoms in carbon shell during the solid-state reaction can be responsible for the chemical transformation from binary phase of Ni5P4-Ni2P to monophase phosphorus-rich structure of Ni5P4. It is affirmed that there exists a two-step discharge of the Li+insertion process and the conversion process. During the entire discharge/charge process, metallic Ni nanograins and Li3P phase firstly form, and continuously an amorphous electrode appear after the cell is fully charged. The cyclic voltammetry peaks still can be observed during the anodic scanning, which indicates the homogenous existence of extraction/conversion reactions, resulting in the conversion reactions of nanoparticles occur within a small scaled range. And Li+ cannot be fully extracted due to the kinetic limit to form a ternary intermediate phase of LiεNi5P4. The Ni5P4/C composite exhibits superior high-rate capacibility and good cycling stability. About 76.6% of the initial charge capacity (644.1 mAh g-1) can be retained after 50 cycles at 0.1 C rate, with the initial Coulombic efficiency of 79.6%.(5) Hybriding 2-D Ni2P@graphene sheet composite was accomplished via a one-pot solvothermal method. The cyclic stability and rate capability of Ni2P are significantly improved after the incorporation of graphene due to the synergistic effect. The incorporation of graphene sheets can effectively decrease the voltage polarization, aggregation of nanoparticles, and the SEI formation. Ni2P/graphene hybrid with a 3-D architecture was fabricated in the hydrothermal experiment due to the different adding sequence of SDBS into graphene oxide solution. The 3-D porous architecture enhances the electrical conductivity over the 2-D nanostructure, thus exhibits a remarkable cyclic stability and rate performance.(6) Using in situ electrochemistry in a high-resolution transmission electron microscopy, we demonstrate the lithiation behavior of Ni2P nanostructure. The effect of the amorphous carbon layer on the growth of Li3P dendrite was investigated. The Li3P dendrite growth can be limited within small size by the carbon layer. However, the counterpart is in the range of big size without the limitation of carbon layer. It is also proved by the in situ observation that the amorphous carbon layer affects the structural stability of Ni2P nanostructure.