應用於高功率鋰離子電池之LiNi0.5Mn1.5O4正極材料的合成與電化學性質之改良

Synthesis and Improvement of Electrochemical Characteristics in LiNi0.5Mn1.5O4 Cathode Material for High Power Lithium Ion Battery

金怡君(Yi-Chun Chun)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/碩士(2012年)

摘要

近年來，由於電動車市場與多功能電子產品的崛起，鋰離子電池之需求已由原先高能量密度轉變為高功率密度、高安全性與長圈數循環壽命。因此，本研究針對新型LiNi0.5Mn1.5O4正極材料進行深入研究，其具備高伏充放電平台與高結構穩定度之尖晶石結構等優勢。
在粉體製備方面，藉由改良式共沉澱法，使用草酸取代傳統之氨水作為共沉劑，並以金屬氯酸鹽類作為起始物，可有效製備出高結晶度且無雜相之LiNi0.5Mn1.5O4正極材料。另外，藉由高分子添加劑之輔助，還可進一步控制合成粉體之顆粒大小，並伴隨多孔性結構之形成。於電化學性質改良上，700 oC合成之LiNi0.5Mn1.5O4正極材料，於室溫(25oC)下之循環充放電表現優異，於200圈後還可維持90%以上的初始電容量，而高溫(55oC)下亦可維持於80%以上。另一方面，若使用奈米粒徑之LiNi0.5Mn1.5O4粉體，其快速充放電性質亦有優異表現，於7C高放電速率下，其電容量可由原先的40 mA/g大幅提升至100 mA/g。
經由調整煆燒溫度得出不同結晶結構之LiNi0.5Mn1.5O4材料，在700 oC擁有鎳錳規則排列之P4332結構，而在750 oC時完全轉變成鎳錳不規則排列之Fd3m結構。此一結構轉變與粉體內部三價錳的產生具有高度相關，可由X光繞射得出之晶格常數變化與紅外線光譜搭配循環伏安圖分析得知。錳離子的價態變化源自於LiNi0.5Mn1.5O4高溫燒結之氧散失現象，氧空缺造成粉體內部價電不平衡，最後導致非計量比的LiNi0.5Mn1.5O4-x與三價錳的產生。深入研究發現，存在適量的三價錳可有效提升LiNi0.5Mn1.5O4的快速充放電表現，在高放電速率7C下還可保有83%以上之初始電容量。此一優異表現源自於非計量比LiNi0.5Mn1.5O4-x之導電度的提升與快速的相變化轉換，而本研究亦提出另一新觀點，歸功於鎳錳離子在LiNi0.5Mn1.5O4-x晶格中的不規則排列導致電子躍遷速率之有效提升。

關鍵字

鋰鎳錳氧 ； 鋰離子電池 ； 高功率 ； 正極材料

並列摘要

Recently, there is a great demand for lithium ion battery to change from high energy density toward high power density owing to the rise of electronic vehicle market. LiNi0.5Mn1.5O4, an extended cathode material of spinel LiMn2O4 which possesses the advantages of high power density, safety and long cycle life, is intensively investigated for next generation high power supply.
For realistic applications, pristine LiNi0.5Mn1.5O4 still has some bottlenecks to overcome, including: (i) the degraded cyclability due to impurity contents, (ii) poor rate capability owing to a relatively low conductivity, (iii) severe capacity fading at high temperature caused by material dissolutions, and (iv) the delayed phase transition during charge-discharge processes. Therefore, a modified co-precipitation method was developed in this study. The impurity-free spinel LiNi0.5Mn1.5O4 cathode material with high-crystallinity was successfully fabricated. A significantly improved cyclability at both room temperature and elevated temperature was revealed. The best capacity retention of as-fabricated LiNi0.5Mn1.5O4 cathode at 25 oC and 55 oC are 97 % and 90% respectively after 100 cycles. The serious capacity decay associated with material dissolutions in conventional cathode was also suppressed by applying the modified co-precipitation method, which produces a highly-stoichiometric LiNi0.5Mn1.5O4 with a better structural stability.
In order to enhance the rate capability, the polymer-assisted method was incorporated to control the particle size of pristine LiNi0.5Mn1.5O4. The excellent capacity retention around 88 % was derived by nano-sized LiNi0.5Mn1.5O4 at a high cycling rate of 7 C, which significantly increased about 40 % as compared to micro-sized one. Besides, the rate capability of LiNi0.5Mn1.5O4 was also promoted by controlling Mn3+ contents during material calcinations. Through adjusting the calcination temperature, the oxygen non-stoichiometric LiNi0.5Mn1.5O4-x was derived above 700 oC, accompanying the formation of Mn3+ and crystallographic structure transformations. The superior capacity retention about 83 % was achieved in non-stoichiometric LiNi0.5Mn1.5O4-x (x=0.033) cathode with higher Mn3+ contents cycled at 7 C.

並列關鍵字

LiNi0.5Mn1.5O4 ； Lithium Ion Battery ； High Power ； Cathode Material

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