Studies On The Preparation And Application Of Carbon And ZnO/C Porous Microspheres As Anode Materials Of Li-ion Batteries

Li-ion battery is a new generation environment-friendly green battery which entered the market in 1990s. It has been extensively used in portable electric appliance and battery-operated automobile due to its unique performance including high voltage, low discharge rate, small size and non-memory effect. The anode material of Li-ion battery is a key factor to control its overall performance. At early stage, the dominant anode material of Li-ion battery is mainly carbon/ graphite. However, this kind of material has its drawbacks. A passivation membrane is going to be formed on the surface of carbon electrode at its first charging/discharging, causing loss to the capacitance of the battery. Furthermore, lithium is going to precipitate on the surface of carbon electrode, forming dendritic crystals which may cause safety issues as well as low capacitance problems. Therefore, it's urgent to develop a new material for high-power Li-ion battery.Hard carbon is a type of carbon material which refuses to graphitize even at very high temperature. It's a sort of pyrogenation carbon composed of macromolecular compounds. There are a number of raw materials for producing hard carbon, such as glucose, Polyvinylidene fluoride (PVDF), Novolac epoxy resin, Polyvinyl fluoride (PVF), Polyfurfuran resin (PFR), cotton cloth and so on. As a new anode material for producing of Li-ion battery, hard carbon has a set of remarkable advantages:1) Its charging/discharging curve has a slope, which is different from graphite's flat charging/discharging curve. Thereafter, the capacitance of the battery can be calculated from the voltage of charging/discharging curve. This is very useful for the management of batteries, especially driving batteries.2) It has a stable molecular structure with an excellent cycle nature.3) It has a good compatibility with PC electrolyte so that it can work at low temperature.4) It's easy to obtain raw materials to produce hard carbon at a low cost.Hard carbon materials have drawn more and more attention from technicians and researchers for manufacturing high-power Li-ion battery.This research studies the preparation of porous hard carbon spheres with hydrothermal method, analyzes their structure and shape, and finally tests their electrochemistry performance in Li-ion battery. In addition, composite ZnO hard carbon spheres have been prepared based on the features of ZnO in Li-ion battery, and the comparison of hard carbon spheres' and composite ZnO hard carbon spheres' electrochemistry performance have also been presented in the study.Chapter 2 introduces the preparation of hard carbon spheres with hydrothermal method using glucose as raw materials. It also characterizes and analyzes the crystal phase, component and shape of hard carbon spheres with XRD, SEM, TEM and BET method respectively. In this chapter, it's interesting to discover that hard carbon anode has a good cycle performance in the electrochemistry test. In the test, the hard carbon anode displays slow decay rate, good reversibility nature. However, it also shows shortcomings like low capacitance mass ratio.Chapter 3 firstly carries out the preparation of ZnO 1-D ultrafine material with hydrothermal method and preliminarily investigates the lithium-embedding capacity of this material in electrochemistry tests. It's found in the tests that ZnO anode has an initial charging/discharging capacitance mass ratio as high as 1090 mAh·g^-1 . However, it also displays a poor cycle performance, in which the charging/discharging capacitance mass ratio decreases to about 200 mAh·g^-1 after 10 cycles. Consequently, glucose and Zn(CH₃COO)₂ are used to prepare composite ZnO hard carbon spheres with hydrothermal method to perform electrochemistry tests. It's found that the charging/discharging capacitance mass ratio of the composite anode remains at a value of 350 mAh·g^-1 after 6 cycles, higher than that of hard carbon anode material. After 6 cycles, the composite anode shows a comparatively slower decay rate of the capacitance mass ratio and a markedly better cycle performance compared with pure ZnO.In conclusion, hydrothermal method is employed in this research to prepare porous hard carbon spheres, ZnO 1-D ultrafine materials and composite ZnO hard carbon spheres as anode materials of Li-ion battery. Hydrothermal method is a simple, cheap and fast method to prepare anode materials under amiable reaction conditions with user-friendly controllability. As the material combines the merits of hard carbon (high stability) and zinc oxide (high capacitance mass ratio), composite ZnO hard carbon spheres can improve the comprehensive performance of Li-ion battery via control of oxide dilation.