"溶膠凝膠法製備Li(Mn,Fe)PO4正極材料與特性分析"

Abstract

本文使用溶膠凝膠法並以洋菜粉作為螯合劑和碳源，成功地合成橄欖石結構的LiMn0.5Fe0.5PO4正極材料。當前驅物不含洋菜其經過高溫煅燒後無法合成LiMn0.5Fe0.5PO4單相，產物含有Li3Fe2(PO4)3相。推測其原因為三價鐵無法有效地被還原為二價鐵。當前驅物加入洋菜後，經400oC、8h煅燒即可合成LiMn0.5Fe0.5PO4單相。前驅物熱分析顯示在360oC時洋菜粉裂解形成的碳源，經由碳熱還原反應將三價鐵還原為二價鐵。以洋菜粉為碳源，發現當煅燒溫度提高時，其晶粒大小與粒徑亦隨之增加。當溫度由400oC提升至700oC時其晶粒大小由20nm增加到42nm，且粒徑則由1um增加到3um。經700oC、8h煅燒後之LiMn0.5Fe0.5PO4具有高結晶性。再經Rieveld分析確定其材料具有橄欖石結構，屬於正交晶系且為Pbnm空間群。顯示在合適的煅燒溫度和時間下，藉由添加洋菜粉可製備良好的LiMn0.5Fe0.5PO4粉體。

LiMn0.5Fe0.5PO4 cathode materials with olivine structure were successfully synthesized via the sol-gel method. The agar powder was used as the chelating agent for synthesizing the cathode materials as well as the carbon sources. The pure-phased LiMn0.5Fe0.5PO4 could not be synthesized even after the calcination of the precursors at high temperatures. The product contained an impurity of Li3Fe2 (PO4)3 phase, owing to the inefficacy of the reduction of Fe3+ to Fe2+. When the agar was added to the precursors, LiMn0.5Fe0.5PO4 pure phase was synthesized at 400oC for 8h. The thermal analysis of the precursors showed that the carbon sources formed after the pyrolysis of the agar, and reduced Fe3+ to Fe2+ via carbothermal reduction at 360oC. It was observed that when the calcination temperature was increased, the grain sizes and particle sizes were also increased. When the temperature was raised from 400oC to 700oC, the grain sizes increased from 20 nm to 42 nm, and the particle size increased from 1 um to 3 um.The high crystalline characteristics were observed for the synthesized LiMn0.5Fe0.5PO4 materials, after the precursors were calcined at 700oC for 8h. The Rieveld analysis revealed the structure of the obtained materials as an orthorhombic structure having the space group as Pbnm. This research indicated that at appropriate calcination conditions, pure LiMn0.5Fe0.5PO4 powder synthesis was facilitated via the addition of agar.