Germanium-based Nanomaterials:Synthesis And Application As Anode Material Of Lithium-ion Batteries

In recent years, the nanostructures have been a successful strategy for rechargeable lithium ion batteries ?LIBs?, which were used to improve the lithium storage capacities and cycle performances. Many anode materials with high capacities and good cycle have been proposed to replace commercial graphite. Germanium has been considered as a promising anode material for LIBs due to its high theoretical specific capacity and fast Li diffusivity. Pure Ge anodes undergo large volume changes ?370%? during the lithium alloying and dealloying processes, leading to structural damage and rapid decays in the capacity. Researches show that, making the material to nanostructures, coating the surface of the material with carbon to build a buffer structure are all effective methods to prevent the aggregation of the Ge nanoparticles during the change/dischange process and relief materials volume expansion, thus improve the electrochemical properties. In this thesis, we synthesized urchin-like Ca₂Ge₇O₁₆ hollow spheres, and GeO₂/Ge/C, Ge/C. The structure and physical properties of synthetic materials were characterized by modern analytical techniques, such as SEM, XRD, TEM, TGA, et al. We explored the performance of the materials as anode LIBs. Our research work mainly is as these following aspects.In chapter 3, we synthesized urchin-like Ca₂Ge₇O₁₆ hollow spheres by using hydrothermal method assisted N-methylurea. The subunits of the as-obtained urchin-like hollow spheres could be changed through adjusting the amount of N-methylurea. We also researched the different alkalis and temperatures how to influence the morphology. We investigated the growth mechanism of urchin-like Ca₂Ge₇O₁₆ hollow sphere by time-dependent morphological evolution experiments. As lithium ion battery anode, Ca₂Ge₇O₁₆ showed excellent cycling performance and rate capability and displayed a high discharge capacity of 603 mAh g^-1 after 200 cycles at a current density of 1000 mA g^-1. When cycling at high current densities of 4 A g^-1, the capacity can keep 420 mAh g^-1.In chapter 4, we synthesized GeO₂/Ge/C, Ge/C by different ways. The cycling performances and rate capabilities of the two samples are better than commercial GeO₂.The as-obtained GeO₂/Ge/C displays a discharge capacity of 800 mAh g^-1 after 50 cycles at a current density of 100 mA g^-1, and it exhibited good rate performance The capacity of GeO₂/Ge/C is 480 mAh g^-1 under high current densities of 1800 mA g^-1. Ge/C demonstrated discharge capacity of 580 mAh g^-1 after 50 cycles at a current density of 100 mA g^-1 and the capacity can keep 440 mAh g^-1 under high current densities of 1800 mA g^-1. We investigated the reason why the capacities of the two samples increased. And we also studied the two different carbon-coating processes and their effects on the morphology and electrochemical performance of samples.