以正負極材料最佳化提升鋰離子二次電池之電性表現

Improving the Electrochemical Performance of Li-ion Secondary Batteries by Optimizing the Cathode and Anode Materials

方婷(Ting Fang)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/博士(2007年)

摘要

為提升鋰離子二次電池之電容量及循環壽命，本研究將表面改質與複合材料的概念分別引入正極與負極材料之開發中，期能藉由電極材料本質之最佳化改善鋰離子電池之電化學表徵。
一般提高截止電壓雖可提高商用鋰鈷氧化物正極材料之電容量，其循環性卻將因此變差。以鍍覆金屬氧化物進行鋰鈷氧化物之表面改質可成功降低鋰離子電池在高於4.2V之截止電壓下的急速電容衰退。本研究經由濕式化學製程，在商用鋰鈷氧化物正極材料表面鍍覆氧化鋅，並針對氧化鋅之鍍覆量及製程中的煆燒溫度對鋰鈷氧化物電化學行為之影響進行探討。此外，亦將以鋰鈷氧化物表面特性及結構變化之觀點，探究氧化鋅表面改質可改善其循環特性之原因。
由穿透式電子顯微鏡照片及選區繞射圖譜，可知以氧化鋅進行表面改質後的鋰鈷氧化物，經450oC煆燒後在表面披覆一層約10 nm厚度之氧化鋅薄膜。除了氧化鋅之外，鋰鈷氧化物之表面區域亦有一鋰鋅氧化物存在，可知鋅離子已擴散進入鋰鈷氧化物表面。除了相鑑定結果外，以場發射電子微探儀所得之元素分佈結果亦可證實鋅離子之擴散。鋰鈷氧化物在表面改質製程中，經650oC煆燒後，鋅元素將擴散至粉體內部，呈均勻分佈之狀態。由X-ray繞射圖譜所得之結晶特性可知，若有適量之鋅離子擴散進入內部，鋰鈷氧化物將具有較佳之層狀結構特性。若是鍍覆過量的鋅離子於表面（大於0.2 wt.%），抑或在過高溫度煆燒（高於650oC），鋰鈷氧化物粉體內部將添入過多鋅離子，而導致鋰鈷氧化物中的陽離子錯位情形較為嚴重。
在電化學特性分面，以氧化鋅進行表面改質確可有效提升鋰鈷氧化物在高截止電壓下的循環壽命。除了高壓循環特性之外，氧化鋅表面改質亦可緩和助導劑種類及充放電速率對電池循環性的影響。在鋰鈷氧化物之系統中，鍍覆0.2 wt.%之鋅離子、於650oC進行煆燒所得之鋰鈷氧化物具有最佳電性。在3.0至4.5V之間充放電30圈後，原本的商用鋰鈷氧化物僅能保持55%之電容量，而表面改質後之鋰鈷氧化物仍可維持高達93%之電容量。本研究以陽離子錯位程度、表面特性及結構變化之觀點，探討鍍覆氧化鋅可改善循環性之原因。
本研究在負極方面將引入錫基化合物以提升負極材料電容量。為克服錫基負極材料電容量衰退迅速之問題，以改良式無電鍍製程，將錫基負極材料鍍覆於碳質基材，使碳材吸收充放電過程中鋰錫合金體積大幅變化所產生的應力，藉以延長此負極材料之循環壽命。利用此無電鍍製程可在碳材上同時鍍覆金屬錫、錫磷化物及磷酸錫。此多相鍍覆之複合負極材料將比單相錫基化合物具有更好的電化學性質。在0.001至1.5V之間充放電時，此材料具有比傳統碳負極材料更高的電容量，且有不錯的循環性。由充放電曲線及動電位極化曲線可知，此複合負極材料之電容衰退主因為生成LiP及錫凝團之產生。將高截止電壓由1.5V降至0.9V可避免鋰磷合金產生，此時此負極材料具有相當優異的循環性，在充放電45次後仍可保持96%之電容量。
此外，利用此無電鍍製程亦可在石墨上鍍覆非晶質之錫基化合物。此複合負極材料在0.001至1.5V之間充放電20次後可維持高達89%之電容量，幾乎等同於單純的石墨負極，循環特性非常良好。高解析穿透式電子顯微鏡影像顯示，此複合負極材料中的錫團簇在充放電數十次後仍保持在奈米尺寸狀態且均勻分散。可知鍍覆其中之磷及氧元素在充放電過程中確實提供了有效的緩衝作用，此為此複合負極材料具有優異之循環穩定性之主因。

關鍵字

鋰離子電池 ； 陰極材料 ； 鋰鈷氧化物 ； 表面改質 ； 陽極材料 ； 錫

並列摘要

For developing lithium-ion secondary batteries with higher capacity and longer cycle life, the employment of surface modification and composite materials were introduced to explore positive and negative electrode materials, respectively, in this study. It was expected that electrochemical performance of lithium-ion batteries could be improved by optimizing the intrinsic characteristics in electrode materials.
Raising the upper cut-off voltage can increase the capacity of commercial LiCoO2 cathodes, yet degrade the cycleability. Surface modification by metal-oxide coating was verified to be an effective way to retard the abrupt capacity fading of LiCoO2 cycled at cut-off voltage higher than 4.2 V. In this study, ZnO was coated on the surface of LiCoO2 particles via a wet-chemical process. The influences of the amount of coated ZnO and calcination temperatures on electrochemical performance of LiCoO2 were discussed. Furthermore, how ZnO modification improved the cycling behavior was also probed from the view points of surface and structural properties of LiCoO2.
The bright-field transmission electron microscopy (TEM) images and selected area diffraction (SAD) patterns of as-coated LiCoO2 particles calcined at 450oC revealed the existence of continuous ZnO films with a 10-nm thickness deposited on LiCoO2 surfaces. This showed that a Li-Zn-O phase would be formed on the surface region besides ZnO. This also indicated that Zn2+ ion diffused into the surface region of LiCoO2. In addition to the phase identification results, uniform distribution of Zn atoms in the ZnO-modified LiCoO2 after calcination at 650oC revealed by X-ray color mapping with field-emission electron probe microanalyzer (FE-EPMA) also confirmed the diffusion of Zn2+ ions into LiCoO2 particles during the surface modification process. Crystallography data derived from XRD analysis demonstrated that appropriate amounts of diffused Zn2+ ions was beneficial to the layer property of LiCoO2, while unsuitable Zn doping derived from excess amount of deposited Zn2+ ions (>0.2wt.%) and calcinations at excessively high temperatures (>650oC) would aggravate the cation mixing of LiCoO2.
The cycle life of LiCoO2 cathodes cycled at a high cutoff voltage was greatly improved by surface modification with ZnO coating. In addition to high-voltage cycleability, cycle-life degradation caused by inappropriate conductive carbon could also be moderated by ZnO coating. Furthermore, the rate capability at high current density was also notably improved. In this material system, the LiCoO2 cathode coated with 0.2 wt.% Zn and calcined at 650°C exhibited the most superior electrochemical performance. As tested between 3.0 and 4.5 V for 30 cycles, the capacity retention of the optimal ZnO-modified LiCoO2 was as high as 93%, which was much higher than that of the pristine LiCoO2 (55%). The mechanism of the cycleability improvement was proposed with respect to degree of cation mixing, the surface conditions, and structural evolution during cycling.
As for the negative electrodes, raising the specific capacity of anodes by introducing tin-based compounds was the task in this work. To overcome the rapid capacity fading of tin-based anodes, carbonaceous materials were used as the matrix to accomodate the stress induced by the large volume changes of Li-Sn alloys during cycling. A modified electroless plating technique was adopted to prepare the composite electrodes of Sn compounds/carbonaceous material. Multiphased Sn compounds were deposited on mesophase graphite powders (MGP) by this process. The multiphase compounds containing metallic Sn, SnP3 and SnP2O7 were expected to provide a higher spectator to Sn ratio for improved cycleability. During cycling between 0.001 and 1.5V, the charge capacity was significantly enhanced without appreciable fading. From the voltage profiles and cyclic voltammetry (CV) curves, it was revealed that the capacity fading was caused by either the formation of insulated LiP in the early stage or by aggregation of metallic Sn after prolonged cycling. As the cut-off voltage was lowered from 1.5 to 0.9V, the capacity retention was improved to be as high as 96% of the highest capacity after 45 cycles.
In addition, amorphous tin compounds were deposited on graphite by the modified electroless plating process. The charge capacity retention of the amorphous Sn-P-O/graphite composite anode tested between 0.001 and 1.5V was 89% after 20 cycles. This was nearly identical to that of the pristine graphite and indicated very good cycleability. High-resolution transmission electron microscopy (HR-TEM) images demonstrated that Sn clusters in the composite anode were nano-sized and well separated even after prolonged cycling. This clearly revealed that deposited P and O provided effective buffer effect during cycling, which was the major reason for the improved cycling stability of the Sn-P-O/graphite composite anode.

並列關鍵字

Lithium-ion batteries ； cathode materials ； LiCoO2 ； surface modification ； anode materials ； Sn