以尖晶石二氧化錳流電極於液流式電容去離子技術 鋰離子選擇性富集之研析

Abstract

隨著再生能源和電動車輛的快速成長，鋰作為鋰離子電池中的關鍵角色，在市場上將面臨供不應求的情況。然而，傳統方法如蒸發法通常具有耗時或效率不足等問題。因此，勢必要開發嶄新的鋰回收方法以克服這一問題。近年來，液流式電容去離子(flow-electrode capacitive deionization, FCDI)因其在處理高鹽度水體所具有的高脫鹽容量以及可連續操作的特性而備受矚目。傳統的碳基材料具有高比表面積以及良好的導電特性，已被廣泛運用於FCDI系統中，然而，其在廢水中離子之選擇性分離方面仍備受挑戰。尖晶石二氧化錳(Spinel manganese oxide, λ-MnO2)作爲一種鋰離子篩材料，由於其具有良好的Li+選擇性，近年來被廣泛運用在電化學鋰回收系統中。本研究旨在通過使用鋰離子篩λ-MnO2作為FCDI流電極中的活性材料，探討選擇性鋰富集之可行性。循環伏安法結果顯示，λ-MnO2在LiCl溶液中具有明顯且對稱的氧化還原峰，證明了Li+嵌入λ-MnO2涉及了可逆的氧化還原反應。同時，λ-MnO2在LiCl中的比電容值(237.01 F/g)也高出 NiCl2 (53.88 F/g)許多，表明了λ-MnO2對Li+的親和力以及對Li+的選擇性捕捉的潛力。在FCDI實驗中，電壓之增加不僅提升λ-MnO2之鋰離子富集速率(Average lithium enrichment rate, ALER)，亦增加Li+儲存在λ-MnO2中的比例。在2.4 V的電壓條件下，λ-MnO2具有最高的ALER(0.185 mmol Li/g-h)以及Li+儲存在λ-MnO2的比例(99.9%)。此外，陰極槽的pH下降表明了λ-MnO2中所發生的H+與Li+之間置換反應，證明了FCDI中λ-MnO2的儲存Li+的機制涉及了離子交換反應。另外，隨著初始LiCl的濃度提升，離子交換膜之間的濃度梯度也會隨之增加，導致擴散現象加劇從而得到更高的鋰離子去除率(Lithium removal rate, LRR)。例如，LRR從20 mM的0.45 mmol/h提升到100 mM的2.08 mmol/h，最後將初始濃度提升至200 mM後，LRR提升至2.55 mmol/h。同時，擴散現象的加劇也反應到了高估的鋰離子富集效率(Lithium enrichment efficiency, ηLi)。然而，在100以及200 mM條件下，ALER差距並不明顯，這可能是由於λ-MnO2的活性點位已趨於飽和所致。在Li+選擇性富集實驗中，相較於傳統對稱之AC//AC流電極，非對稱之AC//λ-MnO2展現出較高的選擇性(SLi/Ni=2.54)以及ALER (2.23 mmol Li/g-h)，顯示出了λ-MnO2作為流電極所具有的選擇性鋰富集的潛力。綜上所述，本研究成果證明了透過添加Li+選擇性材料作為流電極能夠使FCDI達到選擇性富集Li+的效果，展現了FCDI技術於未來作為鋰選擇性回收技術的應用潛力。

With the rapid expansion of renewable energy and electric vehicle markets, the demand for lithium, essential in energy storage systems, particularly lithium-ion batteries, is anticipated to grow swiftly, potentially surpassing supply. Current commercial lithium recovery methods, such as solar evaporation, are often time-consuming and economically inefficient. Consequently, the development of alternative lithium recovery techniques is imperative. Recently, flow-electrode capacitive deionization (FCDI) has garnered attention due to its high deionization capacity and capability for continuous operation. While traditional carbon-based materials with high surface area and excellent electrical conductivity have been utilized as flow electrode in FCDI, it is still a big challenge to selectively recover valuable ions from wastewater. Spinel manganese oxide (λ-MnO2), a lithium-ion sieve material, is renowned for its high selectivity towards Li+ and has been extensively employed in electrochemical lithium recovery in recent years. This study aims to investigate the feasibility of using spinel λ-MnO2 as the flow-electrode in FCDI for selective lithium enrichment. Cyclic voltammetry displayed significant redox peaks for λ-MnO2 in the presence of LiCl, exhibiting reversible Faradaic reaction. Additionally, the specific capacitance of λ-MnO2 with LiCl (237.01 F/g) was significantly higher than that with NiCl2 (53.88 F/g), indicating a preferential intercalation of Li+. In single ion experiments, an increase in applied voltage corresponded to an increased fraction of Li+ stored in λ-MnO2. At 2.4 V, the highest fraction of Li+ (99.9%) and the average lithium extraction rate (ALER, 0.185 mmol/g-h) were achieved, surpassing other applied voltages (0.8 and 1.6 V). The decrease in pH in the cathode chamber indicated an ion exchange reaction involving the substitution of H+ by Li+ during the intercalation process. Increasing concentrations enhance the concentration gradient across the ion exchange membranes (IEMs), causing more significant diffusion which results to higher lithium removal rate (LRR). This relationship is evident as the LRR rises from 0.45 mmol/h at 20 mM to 2.08 mmol/h at 100 mM, and further to 2.55 mmol/h at 200 mM. Higher initial concentration with more significant diffusion also causes overestimated lithium enrichment efficiency (ηLi). However, the quantity of Li+ in λ-MnO2 was 3.76 and 3.54 mol for 100 and 200 mM, respectively, possibly due to the saturation of active sites in λ-MnO2. In selective lithium enrichment experiments, asymmetric FCDI utilizing AC//λ-MnO2 flow-electrodes outperformed traditional symmetric AC//AC flow-electrodes, achieving a higher selectivity coefficient (SLi/Ni=10.21) and ALER (2.23 mmol Li/g-h). In conclusion, λ-MnO2 flow electrode in FCDI shows the potential of selective lithium enrichment. This study provides significant insights into the development of efficient lithium recovery technologies using FCDI.