混合型超級電容器之鋰儲存現象及鋰再生利用行為研究

Study of Lithium Storage Phenomena and Li Recycling Behaviors in Hybrid Supercapacitor Devices

黃冠閔(Guan-Min Huang)

指導教授 ： 吳文偉

國立交通大學/工學院/材料科學與工程系所/博士(2019年)

摘要

混合型超級電容器，因其具備成為下個世代電能儲存(EES)系統之潛力而備受關注，其結合了超級電容器與鋰離子電池的操作原理，有助於同時獲得高功率密度及高能量密度的特性。在追求提升電容、延長循環壽命、優異充放電速度及無汙染的需求上，此種非傳統式EES系統的開發被視為一個全新開端。即便如此，在這種系統中，電極材料及電解質研發仍需取得一些更重大的進步與提升，尤其隨著近年來元件尺度微縮的趨勢，從奈米材料的觀點加以釐清其運作機制，顯得越來越重要。因此，本研究將重點放在過渡金屬氧化物/奈米碳管的複合材料(Co3O4/CNTs與NiFe2O4/CNTs)以及六氟磷酸鋰(LiPF6)液態電解質的穩定性上，利用臨場觀察技術來深入探討鋰的儲存現象與鋰的再生利用之行為。除此之外，我們也重新利用再生的鋰，觸發超快速鋰化反應。
在第一部分的研究中，我們透過臨場穿透式電子顯微鏡(in situ TEM)技術來探究Co3O4/CNTs材料中可逆的鋰儲存機制，並以高解析穿透式電子顯微鏡(high-resolution TEM)、電子繞射(DP)能量散佈光譜儀(EDS)和電子能量損失能譜(EELS)分析陽極材料的結構與成分組成。實驗採用我們獨特的臨場實驗設置，可歸納出兩種不同的操作機制，其中包含電極材料的氧化還原反應(電池型)與離子吸脫覆反應(超級電容型)。經過第一次充電後，立方體形貌的Co3O4會轉變成大量的Co奈米晶粒，並散布在Li2O的基質之中；而在後續的充放電循環中，則呈現Co/Li2O與CoO/Li2O之間的可逆反應。此外，CNT的多孔結構和Li2O基質的守恆現象使之能緩衝充放電伴隨的巨大體積變化，這將提升此元件的壽命。這部分研究不僅直接觀察到其中的電化學反應，更提供材料改質的依據，以促進混合型超高電容器更為優異的特性。
第二部分的研究，我們利用電子束從LiF (LiPF6液態電解質的降解產物)中回收鋰金屬。於降解過程中鋰損失的現象將會縮短電解質的使用壽命，進而導致電容衰退甚至元件失效。為了使鋰的再生機制更加明朗，我們運用in situ TEM的影片及即時電子繞射強度分布圖來證明LiF的形貌演變與相變化。此外，我們也導入EDS與EELS的分析，更進一步證實，在LiF被電子束照射後，鋰將以薄片的形式再生。本研究在鋰的重複使用及延伸電解質的永續性上，將會是一個新的思維。
第三部分的研究，我們延續了第二部分的內容，以再生之鋰來觸發後續NiFe2O4/CNTs的鋰化反應，達成延伸電解質的永續性及超快速鋰化的目標。因此，我們借助in situ TEM來探討其中的完整機制。首先，藉由電子束效應來進行鋰再生，再以這些鋰薄片作為鋰源(Li-source)供給後續NiFe2O4/CNTs的化學鋰化，過程亦伴隨著NiFe2O4奈米顆粒相轉變為Ni與Fe納米晶粒的現象。與一般的「電化學」鋰化相比，「化學」鋰化具有超快的鋰化速度且不需外加電壓即可完成充電，將可拓展未來更多樣化的應用。通過此研究，將有助於在能量採集領域中，為設計新穎儲能元件提供一種新策略。

關鍵字

混合型超級電容器 ； 可逆鋰離子儲存 ； 鋰回收再生 ； 電子束輻射分解 ； 超快速鋰化 ； 臨場穿透式電子顯微鏡

並列摘要

Hybrid supercapacitors have received much interest for their potential to become next-generation electrical energy storage (EES) systems. The combination of the principles of supercapacitors and lithium-ions batteries contributes to the acquirement of high power density and high energy density simultaneously. This unconventional EES system marks a novel beginning toward the demand for elevated capacitance, long cycle life, quick charge-discharge speed and pollution free. Even so, either electrode materials or electrolytes still need to have more significant advances in this kind of EES system. In particular, as the devices scale down in recent years, unveiling the operation mechanism from the nanoscopic perspective of materials is becoming more and more important. In this work, we would put the emphasis on transition metal oxides, Co3O4/CNTs and NiFe2O4/CNTs nanocomposites, and the stability of LiPF6 liquid electrolyte. The lithium storage phenomena and Li recycling behaviors were fully revealed by in situ technique. Moreover, we also reused the recycled Li to trigger ultrafast lithiation subsequently.
In the first part, we investigated the reversible lithium storage mechanism of Co3O4/CNTs material via in situ transmission electron microscopy (TEM). Additionally, we analyzed the structure and composition of the anode material by high-resolution TEM, electron diffraction, energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Using our unique in situ experimental setup that employs liquid electrolyte, we elucidate two different mechanisms during operation, including the redox reaction (battery-type) and ions adsorption (supercapacitor-type) of the electrode material. The cube-like Co3O4 nanoparticles were converted to Co nanograins dispersed in the Li2O matrix after the first charging cycle. Subsequent cycles presented a reversible reaction between Co/Li2O and CoO/Li2O. Furthermore, the porous structure of the CNTs and conservation of the Li2O matrix allow for the excellent ability to accommodate tremendous volume expansion, which enhances the life of hybrid supercapacitors. Our observations not only provide direct evidence of the electrochemical behavior but also improve the structure to promote enhanced performance for the application of hybrid supercapacitors.
In the second part, we utilized an electron beam (e-beam) to retrieve the Li metals from LiF, the degradation product of LiPF6 liquid electrolyte. The loss of lithium in the degradation process will shorten the operating life of electrolyte, leading to capacitance fading or even device failed subsquently. To make the mechanism of Li recycling clear, in situ TEM imaging video and real-time identification of diffraction pattern (DP) intensity profile were used to demonstrate the morphology evolution and phase transformation during the process. Additionally, with the incorporation of EDS and EELS charaterizations, it was confirmed that Li would be recycled in the form of flakes upon e-beam irradiation on LiF. This study provides a new thinking of reusing Li in some application and extends the sustainability of the electrolyte.
In the third part, we employed the Li obtained in the second part to trigger the further lithiation of NiFe2O4/CNTs to extend the sustainability of the electrolyte and gain ultrafast lithiation. Accordingly, in situ transmission electron microscopy (in situ TEM) was used to investigate the comprehensive mechanism of the whole process. The e-beam acting on the degradation product LiF clusters led to the generation of Li flakes, which served as the Li-source for the subsequent lithiation. Then, with these Li flakes, the chemical lithiation of NiFe2O4/CNTs was triggered, resulting in the phase transformation to Ni and Fe nanograins. Compared to “electrochemical” lithiation, the ultrafast reaction speed and the ability to charge without directly applied potential in the “chemical” lithiation has the ability to extend to more diverse applications. Through this investigation, we provide a new strategy for designing novel energy storage devices for the energy-harvesting field.

並列關鍵字

Hybrid supercapacitor ； Reversible lithium storage ； Recycling of Li ； e-beam radiolysis ； Ultrafast lithiation ； in-situ TEM

參考文獻

Chapter 1

1-1. Muzaffar, A.; Ahamed, M. B.; Deshmukh, K.; Thirumalai, J., A review on recent advances in hybrid supercapacitors: Design, fabrication and applications. Renewable and Sustainable Energy Reviews 2019, 101, 123-145.

1-2. Brian Kihun Kim; S. S.; Aiping Yu; Jinjun Zhang, Electrochemical Supercapacitors for Energy Storage and Conversion. In Handbook of Clean Energy Systems, 2015, 1-25.

1-3. Whittingham, M. S., Electrical Energy Storage and Intercalation Chemistry. Science 1976, 192, 1126-1127.

1-4. M. Stanley Whittingham; F., N.J. CHALCOGENIDE BATTERY U.S. Patent 4009052, 1976.