Investigation On Fabrication And Lithium Storage Properties Of Silicon-based And Germanium-based Anode Materials

Since the beginning of the 21st century,the large-scale use of chemical raw materials has led to increasingly serious problems of environmental pollution and energy crisis.Researchers are beginning to turn their attention to the development of renewable energy sources such as solar and wind energy.But these energy sources are cyclical and require energy storage.Lithium-ion batteries have become the most promising energy storage technology at present due to their high energy density,high operating voltage,long cycle life,no memory effect,and environmental protection,and are widely used in various fields.The current commercial anode materials are mainly carbon materials,but their capacity is too low to meet the current commercial demand for high energy density.The elements of group IVA,such as Si and Ge,have ultra-high theoretical specific capacity and are considered to be ideal materials to replace commercial graphite.However,both Si and Ge have obvious volume effects during the discharging-charging process,and the drastic volume changes make the electrode materials pulverize and fall off,which seriously affects their electrochemical performance.At present,the common solutions are nanoscale,porosity,alloying,carbon coating,etc.In view of the above problems,the precursor materials were prepared by rapid solidification and gas atomization techniques.Then different types of high-performance siliconbased and germanium-based anode materials were prepared by selective corrosion and dealloying methods.The characterization of the microscopic morphology and structure,the electrochemical performance test and the explore of the lithium storage mechanism were carried out.The main contents of the thesis are as follows:1.Formation,lithium storage properties and mechanism of nanoporous germanium fabricated by dealloying.Firstly,Al-Si-Ge precursor ribbons were prepared by simple rapid cooling method.Hierarchically nanoporous Ge(np-Ge)was fabricated by the combination of selective phase corrosion with chemical dealloying.As an anode for LIBs,the np-Ge electrode shows good cycling stability with capacity retentions of 1060.0 mAh g-1 at 0.2 A g-1 and 767.1 mAh g-1 at 1.0 A g-1 after 100 cycles.Moreover,the electrode shows excellent rate capability with capacity retention of 844.2 mAh g-1 at 5 A g-1.Notably,the(de)lithiation mechanism of np-Ge and p-Si6Ge4 was revealed by operando X-ray diffraction(XRD).2.Large-scale fabrication and electrochemical properties of Si-based and Ge-based anode materials.In order to solve the large-scale fabrication problem of precursors,the gas atomization(GA)was used to the large-scale preparation of the Al-Si-Ge precursor powders.Afterwards,GA-Si6Ge4 and GA-Ge samples were prepared by the same etching process as before.The specific discharge capacities of GA-Si6Ge4/GA-Ge electrodes can reach 1330.0/1158.8 mAh g-1 after 100 cycles at 0.2 A g-1,respectively,demonstrating excellent cycling stability.Among them,the GA-Ge electrode still has a capacity retention rate of 51.4%even at a high current of 5.0 A-g-1,showing excellent rate performance.In addition,the cyclic voltammogram(CV)method,galvanostatic intermittent titration technique(GITT)method and electrochemical impedance spectroscopy(EIS)test results show that the GA-Ge electrode has superior lithium-ion transport property.Finally,we explored the lithium storage mechanism of GA-Si6Ge4 electrode by operando XRD technique.3.Large-scale fabrication and lithium storage properties of carbon-coated Si(Si@C)anode materials.We prepared Al-Si precursor powder by gas atomization.Porous Si samples were obtained by the dealloying method and coated with different masses of glucose.Then Si@C-1,Si@C-2 and Si@C-3 samples with different carbon contents were obtained by high temperature pyrolysis,and the effects of carbon content on their electrochemical performance were investigated.This method not only reduces the cost but also improves the conductivity of Si.The results show that the Si@C-3 sample with the highest carbon content delivers a capacity retention rate of 40.3%after 250 cycles at 0.2 A g-1.At a high current of 1.0 A g-1,there is still a specific capacity of 915.8 mAh g-1.When the current density returns to 0.2 A g-1,the capacity retention rate reaches 77.8%,indicating excellent cycling stability and rate performance of Si@C-3.In addition,the lithium-ion transport property of the Si@C-3 sample was also improved.Finally,we used differential electrochemical mass spectrometry(DEMS)to analyze the gas generation during the first cycle of the Si@C-3 electrode,and used operando XRD to reveal its lithium storage mechanism,confirming the amorphization of Si during the first cycle.