鋰離子電池複合正極材料的合成、改質 及其電化學特性研究

Abstract

近年來，隨著下世代3C整合產品（智慧型手機、平板電腦）及高功率系統產品（電動機車、電動車、智慧機器人）之電源需求，高能量密度及高安全性鋰電池的開發將是未來之重點。由於鋰電池是透過鋰離子之嵌入/釋出以進行充放電反應，因此鋰電池之電化學性能，高度依賴於正極材料之技術特性與品質。正極材料不僅作為電極材料參與電池中之電化學反應，亦為電池中鋰離子主要來源，其活性與鋰離子分布等指標是影響鋰電池性能表現之最大關鍵。故高容量鋰電池正極材料之開發研究乃刻不容緩。LiCoO2由於其結構穩定性佳、能量密度高，發展較久而製程品質穩定等優點，乃為目前商業化最普及之正極材料，但 LiCoO2熱穩定性較差、可逆電容量僅140 mAhg-1左右及不利深度充電等特性，因此並未列入車用鋰電池材料選用體系中。LiMn2O4¬正極材料屬於尖晶石結構，因價格優勢、無環保污染顧慮及無鈷酸鋰過充電之問題等優勢，被認為是安全性較高且可大量應用之正極材料，但必須克服其高溫下錳溶解所導致電容量衰退過快之問題。 高容量複合正極材料：xLi2MnO3•(1-x)LiMO2 (M = Ni, Co, Mn, Fe, Cr)為Li2MnO3 和LiMO2 依不同成分比例而形成之固溶體，目前雖未商品化，但其在低倍率放電下具有相當高之克電容量(> 270 mAhg-1)與優良之循環穩定性等優點，是未來5~10年間有機會商品應用之材料。更被眾多學者認為未來有機會取代鈷系、鎳系、錳系及多元系正極材料之首選正極材料，但其初次不可逆電容量過高及倍率放電特性差等缺點，仍須投入相當時間克服。 本論文首先將針對此高容量複合正極材料xLi[Li1/3Mn2/3]O2•(1-x)LiMO2 (M = Ni, Co, Mn, etc.)作一文獻回顧與剖析，接著並詳述此複合材料之實驗方法(共沈澱法)與製程參數(計量比、先驅物、燒結溫度及反應時間)對電化學性能之影響。最後，針對上述研究結果作一簡短之結論，以供未來研究此高容量複合正極材料開發之參考依據。

In recent years, there has been increasing interest to develop cathode materials for lithium-ion batteries with high energy density. Most of the commercial lithium batteries use layered LiCoO2 as the cathode material, but it has a limited practical capacity of 140 mAhg-1 because of chemical and structural instabilities at deep charge state. The spinel LiMn2O4 and olivine LiFePO4 have been proposed as an alternative to LiCoO2. However, their poor capacities and low conductivity of electron and/or lithium diffusivity hamper the possibility of commercial cell application. Solid solution cathode materials which can be denoted as a composite xLi[Li1/3Mn2/3]O2•(1-x)LiMO2 (M = Ni, Co, Mn, etc.), though it alternatively can be expressed as a layered form Li1+xM1-xO2, are of great potential as a generation of positive electrode materials for high energy density lithium-ion batteries. This series of materials can deliver around 270 mAhg-1 at a low charge-discharge current with good cycle performance. However, there are several problems such as the lower initial efficiency during the first charge-discharge process and the poor rate capability which should be overcome so as to put them into application as soon as possible. The goal of this dissertation was to study a literature review for the composite cathode material xLi [Li1 / 3Mn2 / 3] O2 • (1-x) LiMO2 (M = Ni, Co, Mn, etc.), and then detailing the experimental methods of this material (co-precipitation method). We also study the effect of the parameters including the calcination temperature、sintering time and stoichiometry on the electrochemical properties. The Cr-doped composite cathode delivered a first discharge capacity of 268 mAh/g at a 0.2 C-rate between 2.0-4.8 V, and the capacity remains 90 % after 200 cycles. X-ray diffraction and electron diffraction pattern investigations demonstrated that all the composite cathode products are a layer phase crystal. TEM micrographs show the prepared products are highly crystalline with an average particle size of 20-50 nm. Finally, these findings make a brief of conclusions for future research in this high-capacity composite cathode material development of reference.