Synthesis And Alkali Metal Ion Storage Mechanism Of Carbon-based Transition Metal Phosphide Anode

Lithium-ion batteries（LIB）,sodium-ion batteries（SIB）,and potassium-ion batteries（PIB）have been widely studied as basic energy storage devices.As one of the core materials of the battery system,the anode material determines the energy density of the battery to a large extent.Among them,transition metal phosphides have been studied as anode materials for alkali metal ion batteries（LIB,SIB,PIB）due to their high discharge capacity and stable voltage platform.However,transition metal phosphides have some problems such as poor electronic conductivity,low ion migration efficiency,and serious volume expansion,which limit their electrochemical performance.To solve the above problems,this thesis synthesized carbon sponge-based Co P nanosheets（Co P/CS）by adjusting the morphology of Co P and introducing carbon sponge（CS）with high conductivity.The synthesized Co P/CS was used as the anode material for alkali metal ion battery and its electrochemical performance was studied.In addition,based on cost considerations,Fe-based phosphide materials were prepared by using cheaper Fe instead of Co.A carbon fiber network structure combined with graphene was constructed,and its electrochemical performance in PIB was explored.The main research contents and results of this thesis are as follows:（1）The Co P/CS anode material was synthesized by using the strategy of high-temperature carbonization combined with in-situ growth.The synthesis process of precursors with different morphologies was explored by changing the hydrothermal temperatures.The advantages of in-situ N doping in carbon sponges were revealed,the combination of Co P and CS was explored,and the changes in the degree of graphitization and N content of CS at different carbonization temperatures were analyzed.（2）The electrochemical performance and alkali metal ion storage mechanism of Co P/CS at different carbonization temperatures were tested.Among them,the Co P/CS-800electrode prepared at 800℃has excellent capacity and cycling stability:in LIB,the capacity is 2.4 m Ah·cm^-2 after 70 cycles at a current density of 0.6 m A·cm^-2;In SIB,after 50 cycles under 0.2 m A·cm^-2,it is 0.834 m Ah·cm^-2.And its excellent cycle stability is verified in SIB(the capacity is 0.514 m Ah·cm^-2 after 900 cycles at 1 m A·cm^-2);in PIB,50 cycles are completed at 0.2 m A·cm^-2,it can still maintain a high discharge capacity of 0.32 m Ah·cm^-2.In addition,after 1000 cycles at a high current density of 2 m A·cm^-2,the high discharge specific capacity of 0.14 m Ah·cm^-2 can still be maintained.（3）The Fe-based metal-organic framework（MIL-88A）rod-shaped particles were sequentially coated by electrospinning to synthesize Fe₂P（Fe-P-O/NCFs）encapsulated by carbon fibers,and it was further combined with graphene to obtain Fe-P-O/NCFs/G composites.The introduction of carbon through a multi-step method improves the conductivity and ion transmission efficiency of the material.The In-situ doped N brings a wealth of surface defects and promotes ion adsorption and transfer.Fe-P-O/NCFs/G has excellent electrochemical performance as a PIB anode material:it has a high specific capacity of 575.2 m Ah·g^-1 after 100 cycles at a current density of 200 m A·g^-1,which is much higher than that without electrostatic spinning material(178.6 m Ah·g^-1).