| 摘要: | 本論文共分為五大部分，(A)部分為以草酸當作螯合劑，溶膠凝膠法合成LiNi0.8Co0.2O2材料，利用XRD探討各製程所得材料之結構變化，並以SEM觀察合成材料之表面型態，進而測試各材料之電池性能，以求出最佳合成條件，其合成變因為煆燒溫度、時間、鋰計量及摻雜鍶離子等；(B)部分和(A)部分雷同，只改變螯合劑種類，主要以順丁烯二酸為探討的對象，其合成變因為煆燒溫度、時間、鋰計量、螯合劑量、溶劑種類及摻雜鍶離子；(C)部分為在相同之最佳煆燒條件下，以丙二酸、丁二酸、己二酸、辛二酸、癸二酸、酒石酸、丙酸及檸檬酸等有機酸為螯合劑，探討不同碳數 (分子內之-CH2-群數目)對於合成LiNi0.8Co0.2O2陰極材料之電池性能的影響。 (D)部分和前三部分之合成方法不同，此合成方法為高溫固態法，探討摻雜鋅金屬離子於LiNi0.8Co0.2O2陰極材料，研究是否會改善其電池性能，並以DSC研究材料之熱穩定性，且利用交流阻抗法及循環伏安法分析材料的電化學性質；(E)部分和(D)部分相似，皆以高溫固態法為合成方法，並摻雜其它非過渡金屬離子 (鎂、鋅、鋁)於Li1.05Ni0.7Co0.2Ti0.05O2材料中，探討材料之電池性能，並利用DSC研究材料之熱穩定性，且使用循環伏安法分析材料的電化學性質。 (A) 以草酸為螯合劑合成LixNi0.8Co0.2O2陰極材料 本實驗所得的最佳煆燒溫度為800 ℃及煆燒時間為12小時，其合成材料之第一次循環放電電容量為163 mAh/g，且經十次循環之後，其放電電容量為158 mAh/g。加入微量的鍶離子以取代鋰離子位置的效應，並改善材料的電池性能，而摻雜鍶離子濃度以Sr2+/Li+值 (10-4~10-8)表示。當Sr2+/Li+值為10-6時，材料之第一次可逆電容量最高，且循環穩定性亦為所有摻雜材料中之最佳，其第一次循環放電電容量為173 mAh/g，且經一百次循環之後，放電電容量為138 mAh/g，電荷維持率為80.1 %。接著探討加入過量的鋰之效應，結果發現LxNi0.8Co0.2O2 (x=1.10)材料之電池性能最佳，其第一次循環可逆電容量為182 mAh/g，經五十次循環測試後，可逆電容量為153 mAh/g，電荷維持率為84.1 %。雖然此材料比市售FMC LiNi0.8Co0.2O2之電荷維持率 (100 %)低，但LixNi0.8Co0.2O2 (x=1.10)材料之可逆電容量卻高於市售FMC 之LiNi0.8Co0.2O2 (第一次循環電容量為166 mAh/g)。 (B) 以順丁烯二酸為螯合劑合成LixNi0.8Co0.2O2陰極材料 以順丁烯二酸為螯合劑，並採用溶膠凝膠法合成LiNi0.8Co0.2O2陰極材料。利用不同的合成條件 (溶劑、煆燒時間、煆燒溫度、酸對金屬離子比(R)與鋰計量比等)以改質材料，並求得理想的合成條件，製備出擁有最佳電化學性質材料。本研究所得到的最佳合成條件為以乙醇為溶劑，煆燒溫度800℃，煆燒時間12小時，並在氧氣氣氛之下合成LiNi0.8Co0.2O2陰極材料。上述合成條件下所得出的材料，在充放電速率為0.1 C-rate及充放電截止電壓分別為4.2與3.0 V時，第一次循環放電電容量為190 mAh/g，且經十次循環之後，其放電電容量為183 mAh/g。 (C) 以各種螯合劑合成LiNi0.8Co0.2O2陰極材料 草酸至癸二酸 (碳數從0至8)為螯合劑，所合成之LiNi0.8Co0.2O2材料的電池性能，以己二酸 (碳數為4)表現最佳，第一次及第十次循環可逆電容量分別為178 mAh/g與166 mAh/g，電荷維持率為93.3 %，且發現隨碳數的增加 (草酸至癸二酸)，其pH值亦隨之增加 (0.05至1.58)。 (D) 摻雜鋅金屬離子合成Li1.05ZnxNi0.8-xCo0.2O2 (x=0.00至0.01) 以傳統高溫固態法合成Li1.05Ni0.8Co0.2O2，並以鋅摻雜於此材料中。由X光繞射分析得知，少量的鋅佔據鈷及鎳的位置，因而改變晶格結構。以2032硬幣型電池測試所合成材料之電池性能，發現摻雜鋅之Li1.05Ni0.8Co0.2O2材料之可逆電容量與循環效率均顯著增加，尤其Li1.05Zn0.0025Ni0.8Co0.2O2材料，在充放電截止電壓範圍分別為4.2 與3.0 V時，第一次可逆電容量為170 mAh/g，經過一百次充放電之後，可逆電容量為138 mAh/g，其循環效率為81.0 %；未摻雜鋅之Li1.05Ni0.8Co0.2O2材料，第一次可逆電容量為158 mAh/g，經過一百次充放電之後，可逆電容量為97 mAh/g，其循環效率為61.4 %，且由較大範圍之充放電截止電壓分別為4.4與2.5 V及高溫 (55 ℃)電池測試之後，發現摻雜鋅可有效地強化材料之晶體結構，並改善其電化學性質。 (E) 摻雜兩種不同金屬於Li1.05Ni0.8Co0.2O2合成Li1.05Ni0.7Co0.2Ti0.05M0.05O2 (M= Mg, Zn或Al) 本實驗是以摻雜三種不同金屬於Li1.05Ni0.7Co0.2Ti0.05O2材料中，結果得知摻雜鋁於Li1.05Ni0.7Co0.2Ti0.05O2陰極材料，第一次可逆電容量為153 mAh/g，經十次循環後，電荷維持率為98 %，且經一百次循環後，電荷維持率高達84.3%；摻雜鎂之材料，第一次可逆電容量為145 mAh/g，經十次循環後，電荷維持率為100 %，且經一百次循環後，電荷維持率高達91.0 %；摻雜鋅之材料，第一次可逆電容量為140 mAh/g，經十次循環後，電荷維持率為98 %，且經一百次循環後，電荷維持率高達82.1 %，故摻雜鋁之材料，其電荷維持率卻不及摻雜鎂之材料，而電池性能最差者應屬於摻雜鋅之材料。由DSC測試得知，摻雜鎂可有效地改善材料之熱穩定性，但摻雜鋅卻無助益。 The work embodied in this dissertation may be divided into five parts. (A): oxalic acid as a chelating agent for the sol-gel synthesis of LiNi0.8Co0.2O2. The various synthesis parameters such as calcination temperature, duration of heat treatment, lithium stoichiometry and dopant ion (Sr2+) concentration were optimized in order to obtain the best-performing cathode material. The structural and morphological characterizations of the products were done by XRD and SEM, respectively. The lithium intercalation properties were studied by galvanostatic charge-discharge cycling. (B): This part is similar to Part (A) except that the chelating agent was maleic acid. The effects of solvent and the acid-to-total cation ratio (R) were investigated. (C): Having identified the optimal calcination conditions (800°C and 12 h), the effect of the carbon number of the dicarboxylic acids (defined as the number of –CH2– groups in the molecule) on the sol-gel synthesis of LiNi0.8Co0.2O2 was investigated. (D): A solid-state procedure was adopted for the synthesis of Zn-doped LiNi0.8Co0.2O2 with the aim of unraveling the role of the size-invariant Zn ions towards the stabilization and, consequently, the cyclability of the cathode material. Cyclic voltammetry and electrochemical impedance measurements were made to understand the electrochemical features of the samples. DSC experiments were carried out to study the thermal stability of the doped materials. (E): The enhancement in the electrochemical and thermal characteristics of solid-state prepared LiM0.05Ti0.05Ni0.70Co0.20O2 (M = Mg, Al, Zn) were investigated by cyclic voltammetry, DSC and galvanostatic charge-discharge studies. (A) Oxalic acid as a chelating agent for the sol-gel preparation of LiNi0.8Co0.2O2 The best synthesis condition was a calcination treatment at 800°C for 12 hours. A product synthesized under this condition gave a first discharge capacity of 163 mAh/g, which faded to 158 mAh/g in the tenth cycle, registering charge retention of 96.4%. In order to improve the cathodic performance, doping with Sr was attempted. The amount of the dopant was such that the Sr2+/Li+ ratio was between 10-8 and 10-4. At a Sr2+/Li+ ratio of 10-6, the first and the hundredth cycle capacities of the material were 173 and 138 mAh/g, respectively, with charge retention of 80.1%. Among the lithium-rich phases studied (Li stoichiometries: 1.00 to 1.15), the most desirable results were obtained at a lithium stoichiometry of 1.10, with a first discharge capacity of 182mAh/g. The fiftieth cycle capacity of this material was 153 mAh/g, corresponding to charge retention of 84.1%. The synthesized sample was compared to a commercial sample obtained from the Foote Mineral Corporation (FMC). Although the charge retention value after ten cycles for the FMC sample was an impressive 100%, its first-cycle discharge capacity (166 mAh/g) was inferior to that of our samples. (B) Maleic acid as a chelating agent for the synthesis of LiNi0.8Co0.2O2 The various synthesis parameters such as calcination temperature, duration of heat treatment, solvent, acid-to-total cation ratio (R) and lithium stoichiometry were optimized in order to obtain a cathode material with desirable electrochemical properties. The ideal conditions were a heat treatment protocol of 800°C for 12 hours in flowing oxygen, with ethanol as the solvent, at an R value of 1 and a lithium stoichiometry of 1.00. A product synthesized under these conditions yielded a first-cycle capacity of 190 mAh/g at a discharge rate of 0.1 C between 3.0 and 4.2 V. The capacity of the material in the tenth cycle was 183 mAh/g. (C) Dicarboxylic acids as chelating agents for the sol-gel synthesis of LiNi0.8Co0.2O2 Six dicarboxylic acids (oxalic acid to sebacic acid, representing carbon numbers 0 to 8) were used as chelating agents for the synthesis of LiNi0.8Co0.2O2. The best results were obtained with adipic acid, which has a carbon number of 4. The first and tenth cycle capacities for the products obtained with this acid were 178 and 166 mAh/g, respectively. The charge retention after ten cycles was 93%. The pH of the as-prepared precursor (0.05 to1.58) was found to increase linearly with the carbon number (0 to 8). (D) Zn-doped lithium-nickel-cobalt oxides, Li1.05ZnyNi0.8-yCo0.2O2 (y = 0.0000 to 0.0100) Zn-doped Li1.05ZnyNi0.8-yCo0.2O2 compositions were synthesized by a conventional solid-state method. The products were characterized by XRD, galvanostatic cycling, cyclic voltammetry, electrochemical impedance spectroscopy and thermal analysis. For the Li1.05Zn0.0025Ni0.7975Co0.2O2 sample cycled between 3.0 and 4.2 V, the discharge capacities in the first and hundredth cycles were 170 and 138 mAh/g, respectively, registering charge retention of 81.0%. The corresponding values for the undoped material were 158 and 97 mAh/g, with charge retention of 61.4%. The improved electrochemical properties of the doped system were attributed to the structural stability derived from incorporating the size-invariant Zn2+ ions. The Zn-doped system also showed improved capacity and cyclability when the cycling was performed in a wider voltage window (2.5 to 4.4 V) as well as at an elevated temperature (55°C). (E) Electroanalytical and thermal stability studies of multi-doped lithium-nickel-cobalt oxides A solid-state fusion method was employed for the synthesis of LiM0.05Ti0.05Ni0.70Co0.20O2 (M = Mg, Al, Zn). Al as a co-dopant yielded a first-cycle capacity of 153mAh/g. The charge retention rates after ten and one hundred cycles were 98.0 and 84.3%, respectively. Although the first-cycle capacity for the Mg-doped material was 145 mAh/g, the charge retentions in the tenth and hundredth cycles were 100 and 91.0%, respectively. Zn as a co-dopant gave a first-cycle capacity of 140 mAh/g. In this case, the capacity retention after ten cycles was 98.0% and after 100 cycles it was 82.1%. DSC data revealed improved thermal stability for the Mg co-doped system. No improvement in the thermal stability of the Zn-doped system was noticed. |
