| 摘要: | 本論文共分兩大部分，前半部探討以丙二酸為螯合劑之溶膠凝膠法製備混合金屬氧化物LixNi0.8Co0.2O2之最佳製程，利用不同的合成變因─煆燒溫度、煆燒時間、溶劑種類、溶液pH值及鋰計量，探討理想的合成條件，製備出擁有較佳電化學性質材料，並冀望藉由此製程所合成之材料，在電容量與循環穩定性方面優於商用陰極材料。 後半部則以改善商用層狀結構陰極材料之循環穩定性。吾人擬以金屬氧化物MgO、Al2O3與MgAl2O4分別塗佈於層狀結構LiCoO2及LiNi0.8Co0.2O2表面上，以期能提升材料之循環穩定性。一般層狀結構陰極材料之正常充放電截止電壓分別為4.2與3.0 V，當材料充電至高電壓4.4 V時，此時材料之結構易受破壞，故可迅速分辨出層狀結構陰極材料改質前後循環穩定性之差異，故此部分之充放電截止電壓設定為4.4與2.75 V。若改質後之材料可在高電壓條件下擁有較佳的循環穩定性，相信在一般的充放電條件下，更可提高循環壽命。 (A) 以溶膠凝膠法合成LixNi0.8Co0.2O2陰極材料 本研究以丙二酸為螯合劑，並採用溶膠凝膠法合成LixNi0.8Co0.2O2陰極材料。所得到的最佳合成條件為煆燒溫度800℃、煆燒時間12小時、以乙醇為溶劑、溶液pH值為0.6，鋰計量數為1.00時，在充放電截止電壓分別為4.2與3.0 V時，以0.1 C定速率充放電，可得初始可逆電容量為173 mAh/g，經61次循環之後，電容量為138 mAh/g，電荷維持率為80 %。其初次可逆電容量略高於市售FMC LiNi0.8Co0.2O2及EIC LiNi0.8Co0.2O2 約4~7 mAh/g，在循環壽命方面則與市售陰極材料相當接近，顯示以丙二酸為螯合劑之溶膠凝膠法，製備混合金屬氧化物LixNi0.8Co0.2O2時，所得材料之電池性能僅略優於市售陰極材料LiNi0.8Co0.2O2。 (B) 以MgAl2O4塗佈於層狀結構陰極材料 以不同重量百分比濃度之MgAl2O4，對Com-LiCoO2進行表面改質時，當以1.0 wt % MgAl2O4進行改質時，可得到初始電容量為164 mAh/g，經131次循環測試後，電荷維持率方衰退至80 %；而未改質材料之初始電容量為167 mAh/g，經26次循環後，電荷維持率已衰退至80 %，由此可知，此表面塗佈改質技術可有效地改善材料於高電壓(4.4V)時，循環穩定性不佳之問題。然而，以相同MgAl2O4處理Com-LiCoO2之合成方法，控制濃度為1.0 wt%，將金屬氧化物MgO與Al2O3分別塗佈於Com-LiCoO2表面時，材料之循環壽命分別為34次與28次，與未改質材料相差不大；但是，將此兩金屬氧化物混合製成MgAl2O4時，其改質效果明顯提升，可提高循環壽命至131次，為未改質材料之五倍。 若將MgAl2O4濃度控制為1.0 wt%時，以不同莫爾數乙基乙醯醋酸形成不同表面型態之MgAl2O4，並將它同時地塗佈改質於Com-LiCoO2材料表面，以莫爾數x = 1時形成之MgAl2O4塗佈改質時，具有最佳之循環穩定性，其初始電容量為163 mAh/g，經154次循環測試後，電荷維持率才衰退至80 %，其循環壽命為未改質材料之六倍。 當被改質系統為FMC LiNi0.8Co0.2O2材料時，控制MgAl2O4重量百分比濃度為1.0 wt%，以莫爾數x = 1之乙基乙醯醋酸，所形成的MgAl2O4表面型態，將它同時地表面塗佈於99.0 wt% FMC LiNi0.8Co0.2O2材料表面。改質後之材料，其初次可逆電容量為184 mAh/g，直至120次循環後，電荷維持率始低於80 %；然而未改質之材料，其初次可逆電容量為183 mAh/g，在第46次循環後，其電荷維持率已低於80%。 The first part of this dissertation deals with the sol-gel synthesis of LixNi0.8Co0.2O2 with malonic acid as a chelating agent. The effects of the temperature and duration of calcination, pH of the precursor solution, nature of the solvent used, and excess lithium stoichiometry on the characteristics of the products were investigated. The second part covers the enhanced cyclability of oxide-coated commercial LiCoO2 and LiNi0.8Co0.2O2 materials. The coatings were electro-inactive oxides such as MgO, Al2O3, MgAl2O4 applied by a sol-gel process. (A) The malonic acid-assisted synthesis of LixNi0.8Co0.2O2 Polycrystalline LiNi0.8Co0.2O2 was synthesized by a solution combustion method, with malonic acid as the chelating agent. A 12-h calcination at 800°C was found to be optimal for the production of phase-pure LiNi0.8Co0.2O2 powders with high specific capacities. The electrochemical performance of the products was correlated with the crystallographic parameters of the products obtained under different heat-treatment protocols. The first- and tenth-cycle capacities of the product obtained by a 12-h calcination at 800°C were 173 and 169 mAh/g, respectively. The initial discharge capacity was about 4–7 mAh/g higher than the capacities of commercial FMC LiNi0.8Co0.2O2 and EIC LiNi0.8Co0.2O2 samples. However, the cycle lives of the three materials were similar. The improved performance of the products obtained from ethanolic solutions, compared to those from aqueous solutions, is attributed to an easier combustion process aided by coordinated ethanol molecules in the precursors. Moreover, at a pH of 7 in aqueous solutions, the coordination of the cations by the dissociated malonic acid is complete and free from competition from other nucleophiles such as hydroxyl ions. Therefore, it was ideal for the synthesis of LiNi0.8Co0.2O2 with good characteristics. An excess lithium stoichiometry of 1.05 was detrimental to the capacity of the cathode material. (B) MgAl2O4-coated cathode materials A semi-alkoxy sol-gel method was employed for coating MgAl2O4 spinel oxide on commercial LiCoO2. XRD studies suggested the formation of a substitutional compound of the type LixMyCo1–yO2 (M = Al/Mg) on the surface of the cathode particles. Electron microscopic images of the coated particles indicated that the spinel existed as a loosely-held kernel. ESCA depth profiles showed that Al3+ and Mg2+ diffused into the bulk of the cathode material during the calcination process. Cycling studies showed that the cathode with a 1.0 wt.% coating had the maximum improvement in cyclability, a fact supported by its lowest R-factor. This study showed that at a 1.0 wt.% coating level, the cyclability of LiCoO2 improved five-fold: from 26 for the bare LiCoO2 to 131 for the spinel-coated LiCoO2. However, spinel coatings on commercial LiNi0.8Co0.2O2 did not register such dramatic improvements. The sol-gel approach was adopted for coating with MgO and Al2O3 too, which at a 1.0 wt.% coating level gave materials that could sustain only 34 and 28 cycles, respectively. Thus, coating with a mixed oxide was found to bestow desirable improvements in the cycling behavior of LiCoO2. |
