Synthesis And Properties Of Germanium Based Composite As Anode Materials For Lithium Ion Batteries

Lithium-ion batteries?LIBs?have been extensively used in portable electronic devices?e.g.smart phones,laptops and smart glasses etc.?and electric vehicles due to the high energy density,long cycle life,no memory effect and environmentally friendly.However,the graphite anode for commercial lithium-ion batteries has a low specific capacity of only 372 mAh g^-1,which cannot meet the requirements of the rapidly developing electric transportation industry and the urgent need for high energy and power density.Therefore,the development of new high-performance anode materials for LIBs is urgently needed.As a potential alternative material of graphite anode,germanium?Ge?has received wide attention in the past few years due to its high theoretical capacity(ca.1600 mAh g^-1),good electronic conductivity and Li⁺diffusivity.However,the germanium-based anode is often subjected to a large volume change?more than 230%?during charge–discharge process,resulting in severe pulverization and flaking of the active material,and so a rapid capacity fading.In this thesis,low-cost germanium oxide?GeO₂?is used as germanium source,and carbon materials such as reduced graphene oxide?RGO?are introduced to improve the conductivity and specific surface area of Ge-based anodes.Several new germanium-based composites are prepared by simple heating reflux,solid-phase reaction and thermal reduction methods.The as-synthesized products are systematically characterized by using XRD,SEM,TEM,Raman spectroscopy,XPS and BET measurements etc,and the formation mechanism of the materials is discussed.The electrochemical Li-storage properties of as-prepared samples are studied with cyclic voltammogram?CV?,galvanostatic charge-discharge,and electrochemical impedance spectroscopy?EIS?etc.The effect of the size,morphology,structure and RGO content of as-synthesized composites on the electrochemical performance is revealed,and the Li-storage mechanism is also proposed.The main points are as follows:1.With GeO₂ as the germanium source,highly uniform Ge/RGO composite are synthesized by an effective poly?diallyldimethylammonium chloride??PDDA?assisted method.Different Ge/RGO composites are prepared by changing the loading content of RGO,and their lithium storage properties are studied.Due to the uniform distribution of Ge nanoparticles?about 5 nm?and the high conductivity of RGO nanosheets,compared with pure germanium,the Ge/RGO composite anode exhibits remarkably enhanced electrochemical performance in terms of capacity,rate performance and cycle stability.The Ge/RGO composite has a high reversible capacity of 960 mAh g^-1 after 100 cycles at 0.2 A g^-1 and an excellent rate performance(631 mAh g^-1 at a high current density of 5 A g^-1).More impressively,by coupling with LiFePO₄ cathode,the full battery also shows a good electrochemical performance in terms of specific capacity and cycling stability.2.The hollow Zn₂GeO₄/RGO composites are synthesized by simple heating reflux followed by thermal treatment.The Zn₂GeO₄ rods with hollow structure are in-situ and uniformly loaded on RGO sheets.By adjusting the pH value of the reaction solution,Zn₂GeO₄ microrods/reduced graphene oxide?m-Zn₂GeO₄/RGO?and Zn₂GeO₄ nanorods/reduced graphene oxide?Zn₂GeO₄/RGO?composites can be achieved.The electrochemical Li-storage properties of the as-prepared composites are detailly investigated.Compared with m-Zn₂GeO₄/RGO,Zn₂GeO₄/RGO composite exhibits better electrochemical performance with a high discharge specific capacity(1005 mAh g^-1)after 110 cycles at 500 mA g^-1,a good capacity retention of661 mAh g^-11 after 500 cycles at 1 A g^-1,and excellent rate capability of 515 mAh g^-1at 5 A g^-1.The superior electrochemical performance of the Zn₂GeO₄/RGO composite mainly benefits from the Zn₂GeO₄ nanorods with hollow structure,high conductivity RGO sheets with large specific surface area,and the synergistic effect between hollow Zn₂GeO₄ nanorods and RGO sheets.The Zn₂GeO₄/RGO composite is a promising anode material for next-generation high-performance LIBs.3.Two different Ge/C/RGO composites are prepared by solid-state reaction and heat treatment by using 2-methylimidazole and 1,4-benzenedicarboxylic acid?H₂BDC?as organic ligands,respectively.In the composites,the Ge/C particles are in-situ deposited on the RGO sheets.Compared with the Ge/C/RGO composites prepared by using H₂BDC as organic ligand,the Ge/C/RGO composite prepared by using 2-methylimidazole as organic ligand exhibits a higher initial coulombic efficiency?80.9%?,better long cycle stability(capacity retention of 600 mAh g^-1after 500 cycles at 200 mAh g^-1),and good rate performance(429 mAh g^-1 at a high current density of 3 A g^-1),which is mianly attributed to the relatively high Ge content,excellent RGO matrix with high conductivity and large specific surface area,and ZIF-8-derived nitrogen-rich porous carbon layers.Moreover,the carbon component with good electrochemical stability in organic electrolyte is favorable to forming uniform SEI film,thereby improving the initial coulombic efficiency of the electrode material and enhancing the electrochemical performance.