Preparation And Potassium Storage Properties Of Bi Anode Materials For Potassium Ion Batteries

The secondary batteries have attracted the close attention of a large number of researchers because of the environmentally friendly,cheaper and reusable features.Due to the high cost of lithium and the crustal abundance is low（0.0017 wt%in the crust）,the further large-scale commercial application of LIBs is restricted.In recent years,potassium-ion batteries（PIBs）have attracted widespread attention due to their rich resources（1.5 wt%in the earth’s crust）,low cost,environmental friendness and similar electrochemical properties with lithium.However,due to the large radius of potassium ion,the structure of the anode material is easy to pulverize in the process of repeated charge and discharge.Therefore,it is particularly critical to develop the anode material suitable for the retraceable release of potassium ions.In this thesis,the potassium-storage properties of bismuth（Bi）metal-based anode materials were studied.Bi is a very stable nitrogen group element with abundant crustal content,non-toxic,large lattice spacing,high theoretical capacity(385 mAh g^-1)and suitable voltage platform.Recently,Bi has been reported as a promising anode material for PIBs.In this thesis,a new type of ultra-thin carbon film@carbon nanorods@bismuth nanoparticles（UCF@CNs@BiNs）was prepared by hydrothermal combined with high temperature annealing method and used as PIBs anode material.At the same time,a series of morphology characterization,cycling performance and potassium storage mechanism were tested.The main research contents are as follows:（1）A new type of rod-like composite UCF@CNs@BiNs was prepared by carbonization using hydrothermal Bi-MOF as precursor.The microstructure of the novel UCF@CNs@BiNs material by means of transmission electron microscopy（TEM）etc.It was found that the material was a rod-like structure,and Bi nanoparticles were uniformly attached to the carbon rods,and the whole structure was coated with a carbon film with a thickness of about 6nm.This special structure provide enough space for volume expansion in the potassium process.This is the fundamental guarantee of its excellent electrochemical performance.Then the material was used as anode to make button-type PIBs,and its cycling performance was studied.The test display that as the current density is 100 mA g^-1 for more than 600 cycles,the specific capacity can be425 mAh g^-1,and the attenuation of a single cycle is 0.038%.Even at high current density(1000 mA g^-1),the single cycle capacity degradation over 700 cycles is 0.036%,which is very low,this demonstrating the excellent electrochemical performance of Bi-based materials.（2）At the same time,we studied the potassium storage machanisim of UCF@CNs@BiNs materials by in-situ XRD.The initial two cycles of operando XRD test of UCF@CNs@BiNs during the current density was 0.2C.The results at the phase transition from original Bi to KBi₂,K₃Bi₂,K₃Bi was highly reversible,the process of phase transition was Bi（?）KBi₂（?）K₃Bi₂（?）K₃Bi.It is because of the highly reversible phase transition,obtained the UCF@CNs@BiNs material excellent electrochemical performance.The cycle after the electrode materials were characterized at the same time,through the end state of charge and discharge are further prove reversible phase transition.Through the measurement,the thickness of the stable passivation layer formed on the surface of the outmost ultra-thin carbon film during the first cycle was about 5.5 nm,the stable passivation layer has a certain protective effect on the structure of the electrode materials and improve the performance of the material.