鋰離子電池LiFePO4正極材料之合成及其性質研究

Synthesis and Properties of LiFePO4 cathode Materials for Lithium-Ion Battery

李宜庭(Yi-Ting Li)

指導教授 ： 洪逸明

元智大學/工程學院/化學工程與材料科學學系/碩士(2011年)

https://doi.org/10.6838/YZU.2011.00162

摘要

本實驗利用兩種不同的製程製備LiFePO4正極材料， Citrate-EDTA 及水熱法。Citrate-EDTA技術具有同時螯合多種金屬離子且其成本低之特性。在空氣中不同煆燒溫度分別為400、500、600、700oC，探討溫度對LiFePO4粉末電化學性質之影響。再將經不同煆燒溫度過後的粉末，經過還原氣氛下煆燒探討，還原氣氛對LiFePO4粉末電化學性質之影響。第二種製程利用水熱法在高溫高壓下進行快速且劇烈的化學反應，合成出高純度、顆粒小且結晶性佳之LiFePO4粉末。藉由調整不同pH值觀察其表面形態且在不同氣氛下煆燒，觀察其對其電化學性質之影響。利用X光繞射分析(XRD)鑑定其結構、場發射電子掃描顯微鏡分析(FE-SEM)煆燒過後粉末顆粒大小及型貌、雷射拉曼散射光譜儀(Raman)分析粉末表面鍵結、光電子能譜儀(XPS)進行元素價數探討、能量分散光譜儀-元素分析(EDS-Mapping)觀察元素分佈。
由放電平台得知利用Citrate-EDTA方式在不同溫度下製備正極材料會在2.9V及2.7V出現兩個放電平台與LiFePO4原本3.4V的放電平台不同，由XRD分析得知利用Citrate-EDTA合成法在不同煆燒溫度下製備之粉末皆為Li3Fe2(PO4)3結構。經還原處理過後則轉變為單相LiFePO4。在不同pH值下利用水熱法合成之粉末為單一相斜方晶系的橄欖石結構之LiFePO4；Citrate-EDTA法隨著前處理溫度升高LiFePO4粉末晶粒會隨之成長且會有團聚燒結的現象發生。相較之下水熱法可以合成出小晶粒的粉末，調整pH值有助於表面形態由桿狀轉變為球型，但其晶粒大小也會隨成長且出現團聚現象；Citrate-EDTA法合成之粉末晶粒較大，大晶粒會增加鋰離子在材料內部擴散的距離致其電化學性質不佳，電容量僅為 45 mAh/g；水熱法合成小尺寸的粉末有助於縮短鋰離子在材料內部的擴散距離，且表面有導電碳層的分布使其有較佳的電化學性質。
利用水熱法經氬氣氣氛下煆燒之LiFePO4粉末在0.1C及1C下進行放電測試，其慢速充放電電容量較在氫氣氣氛下煆燒之LiFePO4電化學佳。在pH=5 所合成並在氬氣氣氛下煆燒之LiFePO4粉末，在0.1C下放電電容量為160 mAh/g與理論值相近。經氫氣還原後之LiFePO4粉末則有較佳之快速充放電性質，由EIS結果顯示經過氫氣煆燒過後較經氬氣煆燒之LiFePO4具有較小之介面轉移電阻，其結果對應充放電測試可以發現電阻值越小其極化效應越小，具有較好的快速充放電性質。

關鍵字

鋰離子電池 ； 陰極 ； 磷酸鋰鐵 ； 水熱法

並列摘要

In this study, LiFePO4 powders were prepared by two different methods. In the first method, the LiFePO4 powder was prepared by Citrate-EDTA complexing method. This technology has chelate metal ion and low cost. The effect of calcinations temperature in the range of 400?700oC on the LiFePO4 powder were investigated.. The electrochemical properties of LiFePO4 powders calcined in H2 atmosphere more also discussed In Second method, high temperature and pressure conditions were used in hydrothermal method for the rapid and violent reaction to synthesize high purity, small particle size and good crystalline LiFePO4 powders. The surface morphology of LiFePO4 powder prepared from different pH value was observed The crystalline structure, particle morphology, surface bonding, elements valence and elements distribution were characterized by XRD, FE-SEM, Raman spectroscopy, XPS and EDS-mapping, respectively.
The discharge voltage plate shows that the use of Citrate-EDTA complexing method, prepared at different temperature, has two plate at 2.9 and 2.7V which differs from the theoretical voltage plate of 3.4V. The XRD pattern shows Li3Fe2(PO4)3 structure in the LiFePO4 powder prepared by Citrate-EDTA complexing method at different temperature in the air. But LiFePO4 structure was observed after calcined at H2 atmosphere. LiFePO4 powders were prepared by hydrothermal method form different pH value solution and calcined at 700oC. The structure of all LiFePO4 sample is orthorhombic olivine type structure. The calcined temperature in Citrate-EDTA complexing method has an important effect on the LiFePO4 particle size growth and particle aggregation. In hydrothermal method when the pH value increased from 5 to 9, the morphology of the particle changed from rod to spherical-like with small sized powder. The powder size prepared in Citrate-EDTA complexing method is larger than the hydrothermal method. A large aggregated particle size will lead a long diffusion distance for lithium-ion intercalate and deintercalate in LiFePO4 structure. Therefore, they did not exhibit good electrochemical properties and the capacity was about 45 mAh/g. On the contrary, the hydrothermal method can synthesize small particle size, which exhibit a better electrochemical performance due to its short diffusion distance and has a uniform carbon layer on the particle surface.
LiFePO4 powders prepared by hydrothermal method calcined in Air with discharge rates of 0.1C and 1C, are better than calcined in H2 atmosphere. The capacity of the LiFePO4 powder prepared in Air atmosphere at pH-5 is 160 mAh/g, which is close to the theoretical capacity. LiFePO4 powder prepared by hydrothermal method and calcined in H2 atmosphere, has a better capacity at the high discharge rate. EIS shows the result of the LiFePO4 calcined in H2 atmosphere has small charge transfer resistance than calcined in Air atmosphere. From the charge-discharge curve, the small charge transfer resistance has small polarization phenomenon with better discharge capacity at high discharge rate.

並列關鍵字

Li-ion Battery ； cathode ； LiFePO4 ； Hydrothermal

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