鋰離子電池高容量富鋰鎳錳氧正極粉體之製備與分析

Abstract

鋰離子電池材料的研發需朝向降低成本與增加電極能量密度兩大方向進行，目前所開發的正極材料中，層狀富鋰鎳錳氧化物，xLi2MnO3•(1 - x)Li(Mn,M)O2,具有>250 mAh/g 的電容量及平均將近4V的氧化還原平台電壓，因此具~1000 WH/kg 的理論比能量潛力，較LiMn2O4、LiNi0.33Co0.33Mn0.33O2 及LiFePO4系統高出甚多。在本論文中，首次利用穿透式X射線顯微術探討層狀富鋰鎳錳氧化物在高溫合成過程中的微結構變化，然而，加熱層狀富鋰鎳錳氧化物的中間過程所產生的輻射狀分布的孔隙結構、少量的孔隙曲折以及較小的晶粒大小具有較小的電子轉移阻抗與增加的鋰離子固態擴散能力而有助於高速充放電的性能。 此外，結果顯示層狀富鋰鎳錳氧化物顆粒表面的過度金屬組成對於微結構與電化學性能有極大的影響。不論是富含錳或鎳的顆粒表面均會造成電容量的損失，而富含鎳的顆粒表面則因較高的電子轉移阻抗及鎳、鋰離子錯位導致其具有較低的電容量與較差的高速充放電能力，不過在循環充放電過程中，富含鎳的表層可以抑制層狀結構轉換成尖晶石的相變化以及伴隨的電壓衰退。此結果提供了一個新的方式，從層狀富鋰鎳錳氧化物顆粒表面過度金屬組成的觀點開發先進的層狀富鋰鎳錳氧化物正極材料。 另一方面，本論文提出兩個方式提升高電極密度(2.5 g/cm3)的鋰離子電池層狀富鋰鎳錳氧化物正極材料的第一圈的庫倫效率、高速充放電的性能以及循環充放電壽命。首先，藉由添加奈米片狀石墨烯在層狀富鋰鎳錳氧電極中，可以減少電極的極化現象且增加比電容量與高速充放電能力，但也促進正極材料與電解液之間鈍化層的生成。此研究是第一次發現石墨烯添加劑造成的負面效應，結果顯示，約100 ppm石墨烯是最適合同時增加高速充放電及循環穩定性的添加劑含量。 另一個改質方法是經由溶液混合且低溫的方式，將導電性高分子聚3,4-二氧乙基噻吩:聚苯乙烯磺酸 (PEDOT:PSS)進行富鋰鎳錳氧化物的表面包覆，經過2 wt.% PEDOT:PSS的包覆後，富鋰鎳錳氧化物的粉體阻抗大量地減少了4個級數，而PEDOT:PSS的包覆含量大於2 wt.%後，粉體阻抗的改善有限。對於電化學測試，顆粒表面的PEDOT:PSS薄膜可以驅使快速的表面電子傳移並提供電極內部高效率的傳導網絡，進而達到快速充放電與電容量的改善與提升；再者，PEDOT:PSS的表面包覆可抑制高電位下SEI的生成，使得富鋰鎳錳氧化物具有較少的第一圈電容量損失以及較高的循環穩定性。

The future development of active materials for lithium ion battery is expected to proceed toward two major directions, namely reducing material cost and increasing electrode energy density. Among the reported cathode materials so far, layered lithium-rich manganese-transition metal oxide composite cathode (abbreviated as LrMOs), xLi2MnO3•(1-x)Li(Mn, M)O2 (M= Mn, Ni, Co), possesses specific capacity of >250mAh/g with an average redox potential near 4V and therefore potential energy density nearly ~1000 WH/kg, which is much higher than those of Li2MnO4, LiNi0.33Co0.33Mn0.33O2 and LiFePO4. In this dissertation, this is the first time that the micro-structural evolution of the resulting LrMOs during calcinations was investigated mainly by transmission X-ray microscopy (TXM). Thus a radially distributed pore pattern, less pore tortuosity and small grain size produced by intermediate heating history, favor the rate performance of the composite oxide cathode due to reduced charge-transfer resistance and enhanced Li ion solid-state diffusivity. Moreover, the microstructures and electrochemical performance are shown to strongly depend on the transition metal composition on the surface of LrMOs particles. the electrodes containing either Mn- or Ni-rich surface cathode lead to capacity loss, while Ni-rich cathode exhibits much lower discharge capacity and poorer rate capability than the one with Mn-rich because of its high charge-transfer resistance and high degree of Ni/Li cations mixing. Nevertheless, the presence of Ni-rich out-layer suppressed the phase transformation from layer to spinel and associated voltage fading during cycling. The results provide a new strategy to develop advanced lithium-rich layered cathode materials from the viewpoints of metal composition on the surface of LrMOs particles. In the other hand, two methods in this dissertation are proposed to enhance the initial coulombic efficiency, rate capability and cycling stability of a highly packed (2.5 g/cm3) LrMOs cathode for lithium ion batteries. First, graphene nanosheets (GNSs) additive into the LrMOs electrodes can reduce electrode polarization and enhance specific capacity and rate performance, and also promote the formation of solid-electrolyte interface (SEI). This study is the first to identify that such an adverse effect is caused by a graphene additive. the results showed that a GNSs additive content of approximately 100 ppm is optimal for achieving both rate and cycle-life enhancements. In the other modification, a conductive polymer, Poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (abbreviated as PEDOT:PSS), was coated on the LrMOs particles by a liquid mixing and low temperature process. It revealed that the resistance of LrMOs powder can be substantially reduced by four orders through 2 wt.% PEDOT:PSS coating, while it have limited improvement of powder resistance when the amount of PEDOT:PSS is above 2 wt.%. For electrochemical test, the presence of PEDOT:PSS film can drive fast electron transport through the surface of the oxide particle and provide efficient conducting network inside the electrode, leading to enhanced rate performance and increased specific capacity for highly packed LrMOs microsphere electrode. Furthermore, the PEDOT:PSS-coated LrMOs exhibited less initial capacity loss and good cycling stability due to inhibited SEI formation under high potentials.