鋰離子二次電池正極材料LiNi0.5Mn1.5O4合成及電化學性質研究

隨著手持電子裝置以及電動車市場等等各種不同電子產品的興起，鋰離子電池的使用更加頻繁性能需求亦大幅提升，現在的鋰離子電池除了成本低廉之外必須具備高功率密度、優異的循環穩定性以及高度的安全性。新型的LiNi0.5Mn1.5O4具備高達4.7伏特的工作電壓平台與穩定的尖晶石結構等等優點，為下一代鋰離子二次電池正極材料的選擇之一，本研究利用醋酸系的三元共晶製程LiNi0.5Mn1.5O4尖晶石結構做為鋰離子二次電池中正極材料的開發，在不同煆燒溫度參數以及過量的鋰前驅物導入製備微米級與奈米級的純相LiNi0.5Mn1.5O4尖晶石結構，提高結構中的鋰離子含量同時避免氧化鎳的生成，進而得到循環穩定性佳的尖晶石相，在初始電容量方面過量參雜4%莫耳比的鋰離子粉體具有較高的電容量，本實驗比較不同煆燒參數對於粒徑尺寸的影響；實驗結果指出當第一階段煆燒溫度由350◦C上升至500◦C後可以得到較純之尖晶石相；而粒徑大小方面以700◦C進行二階段合成可以得到粒徑大小約500nm的顆粒，以900◦C進行煆燒則粒徑提升為大約8μm，透過X光繞射與拉曼光譜分析得之鋰鎳錳氧為不規則排列的Fd3m結構，說明煆燒溫度足夠使結構中的氧散失，氧空缺使得價電不平衡故形成非計量比的結構；實驗第二階段利用不同比例的鉻離子摻雜使鋰離子在充放電時的循環穩定性提升，實驗指出摻雜0.04莫耳比的鉻亦保持Fd3m非計量比之尖晶石結構，電學性質方面初始電容量下降大約3%但循環穩定性大幅提升，於室溫(25◦C)下循環一百次之後仍有92%以上的初始電容量。在表面改質方面本實驗以球磨(ball milling)的方式成功將多壁奈米碳管(Multiwall carbon nanotube)纏繞到尖晶石結構的鋰鎳錳氧顆粒上面，實驗指出加入大約5%重量比的奈米碳管具有最佳的電學性質，本研究認為電容量的提升導因於奈米碳管的高導電性進而減少了因為極化效應所造成的電性損失，除了奈米碳管的高導電性提升了整體的充放電表現之外，本研究亦提出一個新觀點，認為透過奈米碳管優異的機械性質可以形成一個框架使受到碳管纏繞的粉體可以保持其結構，可以減緩充放電過程中鋰離子在進出正極材料時所造成的結構破壞，進而提升整體電性表現，實驗指出於高速充放電速率下纏繞奈米碳管的粉體具有較優異的循環穩定性。

關鍵字

醋酸系三元共晶；鋰鎳錳氧；尖晶石結構；離子摻雜；奈米碳管

並列摘要

Eutectic in the acetate-based ternary system of the LiNi0.5Mn1.5O4 spinel structure was used when developing of cathode materials for lithium-ion batteries. In order to avoid the formation of nickel oxide while obtaining a good cycle stability of the spinel phase, the excessive lithium - precursor was introduced to prepare pure LiNi0.5Mn1.5O4 spinel structure in micron and nanometer sizes from variants of sintering conditions. Effects of particle sizes and temperatures on the cycled performance were studied with a 25 % excess of the lithium precursor. Structure of NiO secondary phase was hardly found in the initial sintering stage. Pure phase of spinel was obtained at temperature heated from 350 ◦ C to 500 ◦ C. The particle size of spinel around 500 nm and 8 microns were found during the second stage of sintering at 700 and 900 ◦ C respectively. The surface modification treatment was made by coating MWCNT (Multiwall carbon nanotube) to the sintered specimens for a further enhancement of the battery stability. The X-ray diffraction pattern was used to confirm that the surface modification has no effect on the spinel structure. A 100 cycles of charge-discharge cycle tests at temperature of 25◦C were performed to evaluate the most stable performance of battery under the processes of multistage sintering in this study. In addition, good capacity retention is exhibited from mixing different ratios of sintered specimens under the same conditions of cycled tests. MWCNT were used as surface coating for the LiNi0.5Mn1.5O4 spinel. The rate capability of the material is improved when MWCNT is coated on the particles by ball milling. This results were contributed to the formation of MWCNT skeleton. After high C-rate cycles the capacity was higher than the bare sample. We coated MWCNT by ball milling with 1~9wt.%. The best electric property is 5wt.% especially in high C-rate.

並列關鍵字

acetate-based ternary ； spinel structure ； LiNi0.5Mn1.5O4 ； Ion doping MWCNT (multiwall carbon nanotube)

參考文獻

[1] Bruno Scrosati et al, “Lithium Battery: Status, Prospects and future” , J. Power Source ,95(2010) pp.2419-2430

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鋰離子二次電池正極材料LiNi0.5Mn1.5O4合成及電化學性質研究

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