以熔融鹽電解法製備含鋰之輕金屬合金及應用:(1)以鎂‒鋰‒鋁‒鋅合金作為鎂電池陽極材料之研究;(2)鋁鋰介金屬化合物用於合成鋰‒鋁層狀雙氫氧化物之研究

Abstract

In this thesis, four parts (Chapter 1 to Chapter 4) of studies involving the preparation and applications of Mg–Li–Al–Zn alloy strips and AlLi intermetallic compound (IMC) by molten salt electrolysis were included. Chapter 1 studies the fabrication of Mg–12Li–2.6Al–0.72Zn (wt.%) alloy strip, Mg–11.9Li–8.5Al–0.57Zn (wt.%) alloy strip and AlLi IMC (19±0.5 wt.% of Li and the rest balanced by Al) in air by electrolysis from LiCl–KCl molten salt at 500 °C. Electrolytic deposition of Li atoms on Mg–Al–Zn alloys and aluminium alloy, and then diffusion of the Li atoms to form the bcc Mg–Li–Al–Zn alloys with ~12 wt.% Li and the AlLi IMC, respectively. Low K concentration (~0.264 wt.% K) in the bcc Mg–11.9Li–8.5Al–0.57Zn alloy strip after the electrolysis process was resulted from 47% atomic size misfit between K and Mg atoms and low solubility of K in Mg matrix. 64% atomic size misfit between K and Al atoms and low solubility of K in Al matrix led to the low K concentration in the AlLi IMC. Chapter 2 investigates the electrochemical and corrosion performance of Mg–Li–Al–Zn anodes with Al compositions of ~3 wt.% and ~9 wt.%. Mg–Li–Al–Zn alloy with ~9 wt.% Al had a relatively negative open-circuit potential and a high discharge voltage in MgCl2 electrolyte, owing to the distribution of numerous AlLi particles in the matrix of the alloy. AlLi particles were believed to transform to Al particles during the corrosion of the Mg–Li–Al–Zn anode. The high-Al anode material exhibited good corrosion performance since a dense and continuous Mg(OH)2/Al composite layer covered the surface of the high-Al anode. Experimentally, increasing the Li+ concentration in the electrolyte improved the corrosion performance of the Mg anode. Chapter 3 describes a novel method based on a metal salt-free system to synthesize a Li–Al–CO3 layered double hydroxide (LDH) by simply adding powdered AlLi IMC to a stirred water bath at ambient temperature and atmosphere. The AlLi powder in the water bath hydrolyzed strongly, subsequently forming an alkaline Al3+- and Li+-containing aqueous solution of pH 12.3. Additionally, dissolution of CO2 gas from the ambient atmosphere into the stirred alkaline solution produced the anion, CO32–, in the solution to form Li–Al–CO3 LDH. Washing in water was unnecessary because metal salts were not used to synthesize the LDH. Notably, transmission electron microscopy (TEM) results indicate that nano-sized Al particles were incorporated on the surface of Li–Al–CO3 LDH flakes. Chemical analysis data suggest that the chemical formula of Li–Al–CO3 LDH was Li0.33Al0.67(CO3)0.17(OH)2. This work also investigates the removal of the fluoride ions from an aqueous solution by using Li–Al–CO3 LDH. Experimentally, residual fluoride concentration decreased to 0.4 ppm with an initial concentration of 20 ppm, which satisfies World Health Organization (WHO) guidelines for drinking water quality. Chapter 4 studies a rapid synthesis method to prepare Li–Al–CO3 layered double hydroxide (LDH) by directly introducing CO2 gas into an alkaline aqueous solution containing Al3+ and Li+. Synthesis time of some minutes was enough to prepare sufficient amount of Li–Al–CO3 LDH. The CO2 gas that dissolved in the alkaline aqueous solution produced CO32- and H+, and thus the pH declining rate of the alkaline solution increased with a higher flow rate of the CO2 gas injecting in the solution. Experimentally, a slow pH declining of the Al3+- and Li+-containing solution favored the production of Li–Al–CO3 LDH. For instance, in the alkaline aqueous (12.34) containing 1070 ppm Al3+ and 594 ppm Li+, applying a CO2 gas with flow rate of 70 ml/min to alter the solution’s pH gradually from 12.34 to 6.75 in 180 s could produce Li–Al–CO3 LDH. However, when higher CO2 flow rate (120 ml/min) being used to change the solution’s pH from 12.34 to 6.75 in 60 s, the final product was amorphous aluminium hydroxycarbonate. The crystalline Li–Al–CO3 LDH was in situ grown from the alkaline solution prepared herein, therefore saving significant time by bypassing the process of e.g., hydrothermal treatment.