磷酸亞鐵鋰(LiFePO4)因具結構穩定、原材料價格低廉、環境危害低、優良的安全性能及循環壽命等優點，近年來已成為熱門的鋰離子電池陰極材料；然而此材料不易量產、導電度及鋰離子擴散速率不佳，為目前所遭遇最大的瓶頸。在本研究中，利用高溫固態法之製程，分別加入不同結構之有機酸作為碳源，製備LiFePO4/C複合陰極材料，期能改善材料之導電度，進而提升電池性能。 本製程所製備之LiFePO4/C複合材料，經X光粉末繞射分析結果顯示皆為純相。各種有機酸添加量中，以添加60 wt.％丙二酸有最佳之電池性能，在充放電截止電壓為4.0~2.8 V及0.2 C的測試條下有150 mAh g-1左右之電容量，藉由總有機碳分析儀(TOC)進行碳含量之分析，最佳樣品碳含量為1.9 wt.％。另外，吾人亦使用拉曼光譜儀分析樣品中碳的成份及組成，並藉由特徵峰強度之比值(ID/IG)估算碳的石墨化程度多寡，添加各種有機酸碳源於60 wt.％添加量下之樣品，其ID/IG值介於0.907~0.938之間，顯示使用本製程能獲得含有較多石墨化碳比例之LiFePO4/C複合陰極材料。 在製備LiFePO4之過程中添加有機酸作碳源，可以抑制LiFePO4晶粒成長，同時達到縮小粒徑及提升電池性能之效果，並可經由FE-SEM觀察材料之表面型態可知，碳源添加量過少，則抑制晶粒成長之能力亦降低，但若添加量過多，則易造成材料團聚，同時發生因電活性物質之比例降低，致使電容量下降的情形；另外，為了解材料中碳之塗佈情形，吾人以TEM/SAED/EDS進行分析，其中半透明網狀之碳膜包覆於灰黑色LiFePO4材料表面，經SAED分析後發現樣品中同時具有結晶型及不定型結構之碳，並可與拉曼光譜之測試結果相互佐證；綜觀來說，添加各種有機酸碳源進行改質後，可提升材料導電度至10-4 S cm-1左右，但導電度亦受碳含量影響，適當之碳含量可於導電度、材料粒徑及電活性物質的比例中取得平衡，進而使材料能展現最佳之性能。 In order to enhance the capability of LiFePO4 materials, we attempted to coat carbon by incorporating various organic carboxylic acids as carbon sources. These acids include (a) mono-acid containing a ring structure, (b) straight-chain di-acids, and (c) tri-acids. The LiFePO4/C composite was synthesized using lithium carbonate, iron (II) oxalate dihydrate, and ammonium dihydrogen phosphate in a stoichiometric molar ratio (1.03:1:1) by a high temperature solid-state method. The purity of LiFePO4 was confirmed by XRD analysis. Galvanostatic cycling and conductivity measurements were used to evaluate the material’s electrochemical performance. A galvanostatic charge-discharge study was carried out between 2.8 and 4.0 V. The best cell performance was delivered by the sample coated with 60 wt.% malonic acid. Its first-cycle discharge capacity was 149 mAhg-1 at a 0.2 C rate or 155 mAhg-1 at a 0.1 C rate. The presence of carbon in the composite was verified by total organic carbon (TOC) and Raman spectral analysis. The actual carbon content of LiFePO4 was 1.90 wt.％ with the addition of 60 wt.% malonic acid. The LiFePO4/C samples sintered with 60 wt.％ various carboxylic acids were measured by Raman spectral analysis. The intense broad bands at 1350 and 1580 cm-1 are assigned to the D and G bands of residual carbon in LiFePO4/C composites, respectively. The peak intensity (ID/IG) ratio of the synthesized powders is from 0.907 to 0.935. Carbon coatings LiFePO4 with low ID/IG ratios can be produced by incorporating carboxylic acid additives before the final sintering process. The product morphology was analyzed by SEM and TEM/SAED/EDS. The particle size of the product decreased as the added amount of malonic acid increased. However, adding too much malonic acid caused a dramatic increase in particle growth. The EDS carbon map shows a uniform distribution of carbon in the sample on the surface of the composite particles. The TEM micrograph consists of two parts: a dark region and a grayish region which surrounds the dark region. It is interesting to know that both SAED patterns indicate that the materials of interest were in a crystalline phase. The TEM/EDS results unambiguously show that the particles in the dark region are LiFePO4 with a trace of carbon and those in the grayish region are carbon only. To produce LiFePO4 with carboxylic acid as a carbon source not only increases the overall conductivity (~ 10-4 Scm-1) of the material, but also prevents particle growth during the final sintering process.
