Controllable Nanostructured Design Of High-performance Metal Phosphide/Carbon For Sodium-storage

Sodium ion batteries are considered to be the next generation energy storage system due to their advantages such as rich resources,low cost,and environment friendly.In the current research,the anode material as a key material restricts the practical development of sodium ion batteries,and it is important to find a suitable anode material.Metal phosphides are one of the conversion reaction materials.It has attracted wide attention because of its high theoretical specific capacity and low storage potential.However,it also has the problems of poor conductivity and relatively large volume expansion rate during charge and discharge process.This problem will inevitably lead to its poor rate performance and cycle stability.In order to solve the above problems of the electrode materials,transition metals nickel and cobalt were selected to prepare metal phosphides in this thesis.The composite materials were carried out through carbon coating/compositing,nanocrystallization and element doping for enhancing the electrochemical performance.At the same time,the prepared metal phosphide/carbon composite material was used as the anode of sodium ion battery,and the relationship between the electrode material morphology and electrochemical performance was systematically explored,and among with its application in sodium ion full cell.The main research contents are as follows:（1）Three-dimensional graphene and nickel phosphide nanoflakes array composites（Ni₂P/3DG）are grown in situ on nickel foam by a simple hydrothermal and gas-solid phase reaction,and used as a self-supporting anode for sodium ion batteries.Due to the ultra-small size and uniformly distribution of Ni₂P crystal particles,Ni₂P/3DG can maintain 402.6 m Ah g^-1 after cycling100 cycles at a current density of 200 m A g^-1.The 3DG located in the nanoflakes array structure ensures the high-speed transmission of electrons,so that Ni₂P/3DG still has a high rate capacity of 273.3 m Ah g^-1 at a current density of 1000 m A g^-1.In addition,the array structure makes the Ni₂P/3DG composite material have a initial coulomb efficiency of 88.28%and volume expansion rate of 166.9%.（2）In order to further optimize the cycle performance of Ni₂P/3DG,the Ni₂P and 3DG composites are designed by controlling the reaction system.The research shows that the precursors grow orientately and form a hierarchical nanosheets finally.The optimal morphology and electrochemical properties are obtained when the precursor is prepared at p H=4.Ni₂P/3DG exhibites a specific capacity of 155.4 m Ah g^-1 at 1000 m A g^-1 over 1000 cycles and the volume expansion rate is 149.1%.The excellent cycle stability is attributed to the hierarchical structure,which can effectively buffer the volume change during charg-discharge process,and effectively increase the wettability between electrode and electrolyte.In addition,3DG,which is intimate contact with Ni₂P,provides fast and continuous electron transport.The full cell is assembled with Ni₂P/3DG and Na₃V₂（PO₄）₃.After 50 cycles of stable cycling,it can still be maintained at 143 m Ah g^-1.（3）Graphitized carbon-coated Ni₂P polyhedral particles（Ni₂P@C-N）are obtained by deriving from nickel-based MOF.Ni₂P@C-N has a large number of microporous and mesoporous structures,which can effectively endow adequate electrolyte penetration.The graphitized carbon layer structure provides a conductive substrate for ion and electron transmission,while further limiting the volume change during the sodiation/desodiation.Finally,the introduction of carbon fiber as a conductive current collector can effectively disperse the material and prevent its agglomeration（Ni₂P@C-N?CF）.The results show that Ni₂P@C-N?CF has a specific capacity of 196.8 m Ah g^-1 after 1000 cycles at1000 m A g^-1,and the average capacity decay rate per cycle is 0.04%,showing excellent cycle stability.Furthermore,the Ni₂P@C-N?CF is prepared into a flexible half-cell,the capacity retention rate was 81.7%after repeated mechanical bending 100 times.Additionally,the full cell exhibits an high energy density of 217.4 Wh kg^-1.（4）By introducing a second metal to control the structure of the material,a nickel-doped cobalt-based MOF precursor was prepared,and then a graphitized carbon-coated nickel-doped cobalt phosphide（Ni-Co P@C-N）was prepared via a gas-solid phase method.The experimental results show that the specific surface area of Ni-Co P@C-N is between their respective single metal-derived phosphides.Surprisingly,the electrochemical performance and ion diffusion kinetics of it are greater than their respective single metal-derived phosphides,manifesting a balance between specific surface area and electrochemical performance.Furthermore,it is supported on carbon fiber and exhibits a rate capacity of 270.0 m Ah g^-1 at a current density of 2000 m A g^-1.The excellent electrochemical energy storage is also attributed to the graphitized carbon layer,which increases the conductivity and restrains the volume expansion,the synergistic effect of nickel and cobalt,the microporous structure shortens the Na⁺diffusion path,and the appropriate specific surface area increases the wettability of the material while prevent too low initial coulomb efficiency.It is worth mentioning that the full cell test shows an energy density of 231.1 Wh kg^-1.