Li-Fe-Si-H_2O体系的热力学分析

Thermodynamic analysis of Li-Fe-Si-H_2O system

根据已有热力学数据,绘制25℃时Li-Fe-Si-H2O体系各溶解组分的lgc-pH图、Li-Fe-Si-H2O体系主要物种的优势区图和强碱性区域Li-Fe-Si-H2O体系各沉淀的优势区图。利用这些平衡图对液相制备硅酸铁锂的工艺条件进行热力学分析。研究结果表明:控制溶液的碱度是制备磷酸铁锂前驱体Li2H2SiO4·Fe(OH)2的关键因素,若要利用Li+,Fe2+和H2SiO24-在液相形成Li2H2SiO4·Fe(OH)2,则要维持溶液较高的碱度,使得Li+能与H2SiO42-生成Li2H2SiO4沉淀,并且抑制FeH2SiO4的生成,使Fe2+以Fe(OH)2的形式沉淀;溶液体系的碱性越强,Li2H2SiO4·Fe(OH)2的优势区域越宽,更有利于Li2H2SiO4·Fe(OH)2的合成;采用液相法制备硅酸铁锂时,由于pH值大于15,体系的碱度较高,溶液中Fe2+极容易被氧化,故制备过程难以实现;而利用固相法制备硅酸铁锂,其保护性气氛容易控制,工艺简单,流程短,便于实现。

Based on thermodynamic data, lgc-pH diagrams of various species and predominance diagram of main compositions and various precipitations in alkalescence region were presented in Li-Fe-Si-H2O system at 25℃. The thermodynamic equilibrium diagram of Li-Fe-Si-H2O system was used to investigate the process conditions of preparing Li2FeSiO4 by liquid phase method. The results show that controlling alkalinity of solution is the key factor of preparing the precursor of LizH2SiO4·Fe（OH）2; the precursor of Li2H2SiO4·Fe（OH）2 can be prepared by controlling the concentration of H2SiO4^2-, Fe^2＋ and Li^＋ in alkalescence system. Under this condition, Li^＋ can react with H2SiO4^2- to form Li2H2SiO4 precipitation and the formation of FeH2SiO4 can be restrained, and Fe^2＋ is deposited in the form of Fe（OH）2. The stronger the alkaline of system, the wider the predominance diagram of Li2H2SiO4·Fe（OH）2, which is beneficial to the synthesis of Li2H2SiO4·Fe（OH）2 precursor. When the pH value is more than 15, Fe^2＋ in the Li2FeSiO4 solution is very easily oxidized with liquid phase method, and it is difficult to achieve the preparing process. But it is easy to prepare Li2FeSiO4 with solid state method, and is easy to control its protective atmosphere.