The Design And Synthesis Of Silicates-based Composite Material For Lithium Storage Performance

The renewable energy sources,such as solar, winder,and geothermal,has been large-scalely developed to mitigate the oil shortage and air pollution which are becoming more and more urgent. The technology of large-scale energy storage is the key factor to the development of renewable energy sources. Among the energy storage devices, lithium-ion battery has attracted more attention due to its high energy conversion efficiency, long cycle life and low self-discharge. As an important part of secondary batteries,the electrochemical performance of cathode materials largely determines the total performance of the lithium-ion batteries. The exploration and improvement of new lithium storage materials have never been stopped from 1980s.Silicates-based materials with abundant resource, low cost, stable tetrahedron structure consisting of the Si-O bonds and high theoretical specific capacity, have attracted considerable attention as a new type of cathode materials. However, these materials, have intrinsically low ion mobility and electrical conductivity, which restrict their actual performance.The nano-sized structure of materials can shorten the transmission path of lithium ions. Besides, it is necessary to build good conductive network to provide high-speed ways for electronic. The materials with synergistic effect between nanoparticle and conductive network were designed to solve the above problems effectively. The silicates-based materials have been explored in following aspects in this paper:?1? A series different sizes of Li₂FeSiO₄/C nanoparticles were controlled and synthesized to modify the Li-ion battery performance. Silicon dioxides with different sizes were used as sources and templates to prepare nano Li₂FeSiO₄/C materials with controllable particle sizes. The size of prepared particles is strongly related to the performance of materials by conducting electrochemical tests. Li₂FeSiO₄/C using 20 nm SiO₂ as precursor material exhibited the best cycling and rate performance. The discharge capacities of this Li₂FeSiO₄/C were 178.2 mAh g^-1 in the first cycle and 164.8 mAh g^-1 after 70 cycles at 0.1 C. This material still exhibited a discharge capacity of 78 mAh g^-1 even at a high current rate of 2 C. Smaller particle size is beneficial for shortening the path way of Li-ion. Moreover, uniformly coated carbon reduced the charge transfer resistance during electrochemical processes. These modified conditions of Li₂FeSiO₄ resulted in excellent improvement of rate performance.?2? Fe₂SiO₄/SiO₂/C composites with different degrees of crystallinity and particle sizes were synthesized by solvothermal method and introduced to Li-ion batteries as cathode materials. The electrochemical tests demonstrated that amorphous Fe₂SiO₄/SiO₂/C with the smallest particle size ?20 nm? exhibited the best discharge capacity. At a current rate of 16 mA g^-1, the specific discharge capacity of this material could reach up to 314 mAh g^-1. Furthermore, ex situ X-ray diffraction ?XRD? and X-photoelectron spectroscopy ?XPS? were employed to study the mechanism in charge and discharge processes.?3? Fe₂SiO₄/rGO with a conductive interlinked three-dimensional network were designed and constructed, which was favorable for fast electronic transportation to enhance the electrochemical activity. The three-dimensional structural SiO₂/GO was synthesized via in-situ growth method. After introducing Fe source, 3D-Fe₂SiO₄/rGO composite was obtained. The experimental results showed that the mass ratio of graphene had great effect on the performance of Fe₂SiO₄/rGO. The charge transfer resistance of material decreased with the increasing content of graphene, which resulted in the superior capacity and better rate performance. Fe₂SiO₄/rGO with 120 mg graphene delivers 111.1 mAh g^-1 at the rate 0.5 C in initial discharge progress.