| 摘要: | 本論文前半部探討以溶膠凝膠法合成LixNi1-yCoyO2陰極材料之合成條件，首先以熱重量分析儀分析以酒石酸為螯合劑，所合成之先導物，接著利用XRD鑑定各製程所得材料之結構變化，進而測試各材料之電池性能，以求出最佳製程條件，其合成變因有煆燒溫度、時間、鋰計量及摻雜鍶離子等。後半部則為以高溫固態法合成一系列高電壓陰極材料LiMyNi0.5-yMn1.5O4（M = Fe, Mg, Al, Cu；y = 0.1~0.4）及LiMyCr0.5-yMn1.5O4（M = Fe, Al；y = 0.1~0.4）等，利用XRD探討各種不同金屬摻雜所得材料之結構變化，並以循環伏安法（Cyclic Voltammetry）測試材料氧化還原行為。 1. 以溶膠凝膠法合成LixNi0.8Co0.2O2陰極材料 首先以硝酸鋰、硝酸鎳及硝酸鈷等三種為起始物，以酒石酸為螯合劑，無水酒精為溶劑，將所得的先導物以600、700及800℃三種不同煆燒溫度，分別於不同煆燒時間6、12及24小時，在氧氣氣氛下進行煆燒。由XRD分析圖譜中可發現在煆燒溫度700℃以上之條件可合成出純相產物。本實驗最佳製程條件為以800 ℃為煆燒溫度，12小時為煆燒時間。其合成材料於充放電截止電壓分別為4.2及3.0 V，充放電速率為0.1 C測試時，第一次循環放電電容量為174 mAh/g，且經十次充放電測試後，其放電電容量為165 mAh/g，電荷維持率為95％。 為避免高溫熱處理下造成鋰的揮發及鋰鎳位置互換而造成電容量損失，因此擬藉助加入過量鋰金屬，以改善此一現象。吾人針對x ＝1.00、1.05、1.10及1.15進行研究。結果發現計量數仍以鋰正計量x＝1.00所合成之Li1.00Ni0.8Co0.2O2陰極材料電池性能最佳，第一次可逆電容量為174 mAh/g，經十次充放電測試後，放電電容量為165 mAh/g，電荷維持率為95％。 為了增進材料本身的導電度，擬藉由摻雜的方式，加入微量之金屬離子，改善鎳含量較多的材料，結構穩定性較差之缺點。當鍶對鋰之莫爾數比為10-6時有最佳之電池性能，其第一次及第十次放電電容量分別為182及174 mAh/g，電荷維持率為96 %。 2. 以高溫固態法合成高電壓LiMyNi0.5-yMn1.5O4陰極材料（M = Fe, Mg, Al, Cu；y = 0.1~0.4） 以固態法合成LiMyNi0.5-yMn1.5O4陰極材料，擬藉由摻雜各種不同金屬鐵、鎂、鋁及銅及改變不同金屬計量莫爾比例y = 0.1、0.2、0.3及0.4等，探討材料的電化學行為，瞭解鋰錳氧材料摻雜不同金屬元素電池性能的差異。由XRD可發現各項摻雜系統之結構均為立方體結構，其產物繞射峰面不隨鎳離子的改變而改變，是一良好的固溶相。圖中可發現在（400）位置的繞射峰位置，隨著鎳離子摻雜量的增加，而往高角度偏移，顯示晶格常數a值隨鎳摻雜的增加而降低。 由CV測試結果得知當摻雜金屬離子莫爾數比例由0.1逐漸增加至0.4時，可發現在4.0 V 區域之氧化還原峰之強度有增強的趨勢，而且高於4.5 V以上之氧化還原峰有逐漸向更高電壓區偏移的行為產生。這意指氧化還原電位會隨鎳與摻雜金屬的莫爾比例不同而有所改變，且摻雜金屬之莫爾數比愈高，電位就更朝向高電壓區域發展。 在電池性能方面，四種金屬摻雜莫爾數比仍以0.1為最佳摻雜計量，初始可逆電容量皆有100 mAh/g以上，且隨著摻雜計量數的增加，電容量呈現遞減的趨勢。在改變不同金屬元素為摻雜系統時，發現以鐵離子之摻雜且莫爾計量比例為0.1時，電池性能最佳，第1次可逆電容量為117 mAh/g，第10次循環可逆電容量為113 mAh/g，電荷維持率為97 %，且經60次循環後，放電電容量為90 mAh/g，電荷維持率為78 %。 3. 以高溫固態法合成高電壓LiMyCr0.5-yMn1.5O4陰極材料（M = Fe, Al,；y = 0.1~0.4） 針對上述第2部分所得最佳之摻雜金屬元素鐵，以及次佳之金屬元素鋁，吾人以鉻取代鎳離子，於相同製程條件下，改變不同金屬計量莫爾比例y=0.1、0.2、0.3及0.4等，合成一系列LiMyCr0.5-yMn1.5O4陰極材料（M = Fe, Al）。由XRD結構分析圖中可發現各個摻雜系統其在（400）位置的繞射峰位置，隨著鉻離子摻雜量的增加，而往高角度偏移，顯示晶格常數a值隨鉻摻雜的增加而降低。 由CV圖中可發現氧化還原反應發生的位置分成兩個區域，分別為4.0及5.0 V兩區域。在圖形中可發現其接近5.0 V附近的氧化峰電位極高，約5.0 V左右，相較於前述第2點以鎳的摻雜約4.8 V左右還要高，顯示鉻的摻雜系統其工作電壓較鎳的摻雜系統為高。 電池性能方面，鉻鐵與鉻鋁兩摻雜系統均在金屬摻雜計量莫爾比例y = 0.1時電池性能最佳，以鉻鐵系統為例，其初始放電電容量為117 mAh/g，第10次放電電容量為109 mAh/g。電荷維持率為93 %，且經40次循環後，放電電容量為88 mAh/g，電荷維持率為75 %。而鉻鋁系統其第1次放電電容量為112 mAh/g，第10次放電電容量為107 mAh/g。電荷維持率為96 %，且經40次循環後，放電電容量為70 mAh/g，電荷維持率為73 %，鉻鐵摻雜系統之電池性能優於鉻鋁之摻雜系統。 This dissertation work deals with the synthesis and characterization of two classes of lithium-intercalating cathode materials: layered LixNi1-yCoyO2 prepared by a sol-gel process and LiM’’yM’0.5-yMn1.5O4 (M’’= Fe, Mg, Al, Cu ; M’= Ni, Cr ; y = 0.0~0.4) spinels prepared via a solid-state route. The physico-chemical characterization of LixNi1-yCoyO2 was carried out by TGA/DTA, XRD and charge-discharge studies. The synthesis parameters – calcination temperature and duration, lithium stoichiometry, dopant (Sr2+) levels, etc. – were optimized in order to obtain products with the best electrochemical activity. The effect of simultaneously doping the spinels on the structural characteristics was examined by XRD and the effect on electrochemical features was analyzed by cyclic voltammetry. 1. Sol-gel synthesis of LiNi0.8Co0.2O2 cathode material Tartaric acid was used as the chelating agent for the sol-gel synthesis of LixNi1-yCoyO2. The optimized heat treatment protocol for the synthesis was a calcination temperature of 800℃ for 12 h. LiNi0.8Co0.2O2 prepared under this heat treatment protocol gave a first-cycle discharge capacity of 174 mAh/g, which faded to 165 mAh/g in the tenth cycle, registering a charge retention of 95 % (0.1 C; 3.0~4.2 V). To compensate for lithium that may evaporate during heat treatment and to pre-empt the occupation of Ni in the Li sites, lithium-rich phases, LixNi0.8Co0.2O2, (where x= 1.05~1.15) were synthesized. However, promising results were obtained only with the perfectly stoichiometric composition (x = 1.00). Sr2+ as a dopant was introduced in the LixNi0.8Co0.2O2 structures in order to enhance the electrical conductivity of the cathode material. Sr2+/Li+ ratios of 10-4 to 10-8 were studied. The most desirable electrochemical features were obtained at a Sr2+/Li+ ratio of 10-6, when the product gave a first-cycle discharge capacity of 182 mAh/g. The capacity and charge retention in the tenth cycle were 174 mAh/g and 96%, respectively. 2. Solid-state synthesis of high-voltage cathode materials, LiMyNi0.5-yMn1.5O4 (M=Fe, Mg, Al, Cu ; y=0.0~0.4) Solid-state synthesized LiMyNi0.5-yMn1.5O4 (M = Fe, Mg, Al, Cu ; y = 0.0~0.4) were studied as high-voltage cathode materials. Powder x-ray diffraction studies showed that all the substituents displayed a propensity for the 8a tetrahedral site at high concentrations. Cyclic voltammetric studies showed electrochemical activity around 4.0 V as well as above 4.4 V. While the 4-volt activity was related solely to the Mn4+/Mn3+ couple, the 5-volt activity was due to the redox reactions of Ni and the other transition metal ions. The co-substituents reduced the 5-volt capacity and shifted the redox potentials in the 5-volt region to higher values. At high concentrations, the co-substituents tend to occupy the 8a sites, leading to a blockage of lithium transport during the charge-discharge processes. LiFe0.1Ni0.4Mn1.5O4 registered the best performance with a first-cycle capacity of 117 mAh/g and capacity retention of 78% over 60 cycles (0.1C; 3.3~4.95V). Electrochemical impedance studies showed a decrease in the charge-transfer resistance at high deintercalation levels. 3. Solid-state synthesis of high-voltage cathode materials, LiMyCr0.5-yMn1.5O4 (M = Fe, Al ; y = 0.0~0.4) As with the Ni-substituted systems discussed above, an increase in the amount of Fe or Al increased the propensity of these co-dopants to occupy the 8a lithium sites. Electrochemical activity was noted in the 4-volt and 5-volt regions. Fe as a co-dopant increased the currents associated with the high-voltage peaks, while Al enhanced the high-voltage capability of the spinel. Irrespective of whether the co-dopant was a transition metal or a non-transition metal, it altered the electrochemical characteristics of both the Mn4+/Mn3+ and the Cr4+/Cr3+ couples, the effect being more pronounced in the 5-volt region. Although increased amounts of Fe or Al rendered the spinels high-voltage active, both the deliverable capacity and the capacity retention obtained with the Al-doped materials were less than those with the Fe-doped materials. At a 0.1 C rate between 3.3 and 5.1 V, the Fe-doped spinel (y = 0.1) gave a first-cycle capacity of 117 mAh/g, while that co-doped with Al gave 112 mAh/g. The corresponding values in the 40th cycle were 88 and 82 mAh/g, respectively. |
