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標題: | 草酸沉降法合成鋰離子電池富鎳鋰鎳鈷錳氧正極材料 Synthesis and Characterization of Layered Ni-rich LiNi0.8Co0.1Mn0.1O2 Cathode for Lithium-ion Batteries via Oxalate Co-Precipitation Approach |
作者: | Chia-Hsin Lin 林佳歆 |
指導教授: | 吳乃立(Nae-Lih Wu) |
關鍵字: | 鋰離子電池,層狀富鎳三元系過渡金屬氧化物,草酸共沉降法,雙重功能改質,混合正極材料系統,體積能量密度, Li-ion batteries,layered Ni-rich NCM811,oxalate co-precipitation,dual functions modification,bi-modal cathode system,volumetric energy density, |
出版年 : | 2020 |
學位: | 碩士 |
摘要: | 近年來隨著環保意識的抬頭,電動車的發展成為市場上注目的焦點,而對於追求高品質、高效能鋰離子電池研究也成為不可或缺的一環。層狀富鎳鋰離子三元系過度金屬氧化物LiNixCoyMnzO2 (x≥0.5) 系列之材料,由於具有甚高之能量密度、低成本與相對毒性小的優點,使其成為新世代鋰離子電池中最具發展潛力的正極材料。然而,它們仍有著許多棘手的問題如鋰/鎳離子錯位、電容量快速衰退、相轉變及在空氣環境中的不穩定性等,大幅限制其實際運用之範圍。因此,吾人將從此方面切入,用簡單卻有效的手法建構出具有高能量密度並兼顧高循環壽命及穩定性的富鎳正極材料。 首先,本研究採用新穎的草酸共沉降法合成富鎳正極之前驅物。有別於傳統氫氧化物共沉降法複雜、高成本與能量消耗的製程,利用草酸能大大減低程序繁瑣性,且具備量產的能力。透過調控溶液中的pH值及反應時間,吾人獲得具有完整形貌、粒徑約3-5微米的小顆粒前驅物。而由於草酸在高鎳材料上對燒結溫度的敏感性,兩階段固相反應法被採用以解決顆粒破碎及陽離子排列問題。此外,退火溫度與粒子形貌、結晶性及電化學表現間的關聯性也將完整的討論以得到高容量的層狀富鎳材料。從結果顯示,在最適化燒結溫度下不論是電容量或常溫/高溫循環壽命皆能得到與主流氫氧化物共沉降法不相上下的表現。 於第二部分中,在合成出高性能的小顆粒層狀富鎳正極材料後,吾人透過具有雙重功能的鋯元素進行粉體改質。利用在鍛燒程序前加入適量的氧化鋯,經高溫退火後,部分的鋯進入內部晶格中,而剩餘的氧化鋯則留在表面形成保護層,並能與表面殘鋰反應,在增強結構穩定性的同時阻隔電解液與活物的直接接觸,達成穩定整體正極材料的卓越表現。在分析儀器的輔助下,吾人也證實了此改質對於材料內部及表面所帶來細微的變化。另一方面,吾人也將改質前後的材料於高電壓(4.5V 4.7V)與高溫(55oC)下進行測試,經由阻抗、活化能及形貌分析,進一步探討此種雙功能改質方法對於富鎳正極材料的影響性。 在此研究的最後部分,吾人運用上述改質後的富鎳正極小顆粒粉體,與來自工研院(ITRI)的大顆粒商用粉體以固相法在不同比例下混合,以提升極板密度和單位體積的電容量。由於在快充電池中,必須犧牲掉些許的電極密度以滿足在高速率充放電的性能,而透過摻雜小顆粒於大顆粒空隙中,不僅能提升原有的密度,也受惠於小顆粒本身具有的高速充放電能力。經由電性測試後,吾人也觀察到在特定比例下的混合材料,兩種顆粒間產生的協同作用,不論是對循環壽命與倍率放電表現皆有所提升。而最後在計算出的體積能量密度中,混合正極材料亦有著顯著地增加。 總結來說,吾人所使用的草酸共沉降法,展現出低成本及操作簡單的特色;而建構出的雙重功能改法,在穩定正極活物與電解液介面上有著顯著的成效;運用於混合正極系統中,也達成我們預期的結果-提升能量密度。本研究有系統地從合成方法一步步地改善,最後成功產出具有商業化應用的正極材料,同時也兼顧內部原理與機制的探討,開創了一條有別於以往合成鋰離子電池層狀三元系富鎳正極材料的道路。 In recent years, with the raising awareness of environmental conservation,electric vehicles have gradually caught the focused spotlight in the market. Research in pursuing high quality with high performance Lithium-ion batteries (LIBs) become an indispensable issue. Layered nickel-rich lithium ternary transition metal oxides, LiNixCoyMnzO2 (x≥0.5, abbreviated as Ni-rich NCM), have attracted significant attention and regarded as the potential cathode materials for next generation LIBs owing to their high energy density, low cost, and less toxicity. Nevertheless, several intractable obstacles including Li/Ni cation mixing, rapid capacity fading, phase transition, as well as air-sensitivity still restrict the broadly practical applications. Therefore, we will start from this point of view, and manage to construct the Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode that enables high energy, long cycle life and stability structure through simple but effective approach. First of all, our research employs novel oxalate co-precipitation method to synthesize NCM811 precursor. In comparison with traditional hydroxide co-precipitation that is complicated, costly, and energy consuming, using oxalate can dramatically reduce the complexity of synthesis procedure. By virtue of controlling pH value and reaction time in the solution, we obtain the intact small particle with diameter around 3-5μm. Furthermore, because of the intrinsic fracture properties for oxalate during high temperature sintering, we apply the steps calcination to take both local nucleation and cation ordering into consideration. Effect of calcination temperature on the particle morphology, crystallinity and electrochemical performance are also been systematically discussed. The results show that optimal sintering temperature facilitate better cycle life regardless of room or elevated temperature, which is highly competitive compare to mainstream hydroxide co-precipitation approach. During second part, after successfully acquiring the high-performance layered Ni-rich NCM811 cathodes, dual functions Zr-modified technique are adopted on these powders. The combination of Zr4+ doping with ZrO2 coating strategy are introduced to act as the bulk and surface stabilizer of NCM811 cathode through addition of moderate nano-ZrO2 before heat treatment. By using analytical instruments, we confirm the subtle change in host structure as well as surface characteristic. On top of that, severe conditions involving in high cut-off potential (4.5V 4.7V) and elevated temperature (55oC) operation are further studied to make a fully comparison. From the impedance, activation energy and morphology observation, the influence of this dual modification on NCM811 cathode are realized in detail. In the final section, we combine our small particle with large particle (from ITRI) to establish the bi-modal cathode system via solid mixing in various blended ratio. The insertion of small particle into the void space of large one are proved to increase the electrode density as well as volumetric energy. Surprisingly, from the electrochemical test, synergistic effect is found in specific blended ratio that dramatically promote the cycle life and rate capability, which is promising for fast-charging batteries. Some novel perspective is also unveiled in this paragraph. All in all, the oxalate co-precipitation method exhibit the features of simple and low cost;the dual functions modification approach have significant improvement on stabilizing overall structure as well as electrode/electrolyte interface;the utilization of bi-modal system also meets our expectation-enhance the energy density. This research systematically explores the principle and mechanism in the processing of NCM811, which opens up a new avenue for synthesizing layered Ni-rich materials in LIBs. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/15475 |
DOI: | 10.6342/NTU202001627 |
全文授權: | 未授權 |
顯示於系所單位: | 化學工程學系 |
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