金屬空氣電池之反應機制與其回收應用

Abstract

本研究乃探討Li–CO2與Na–CO2電池中陰極反應之放電機制，並進行Li–O2電池中空氣陰極之再生研究。Li–CO2電池因其卓越之能量密度與碳利用而備受推崇，然而對其陰極反應之深入理解仍然難以確定。傳統觀點認為，CO2還原與碳作為還原產物相關，反應式為：3CO2 + 4Li+ + 4e− → 2Li2CO3 + C，E° = 2.8 V vs. Li+/Li。然而，現有研究中對碳之形成證據有限。反應限制電位步驟涉及CO2 + e− → CO2− (E° = 1.1 V vs. Li+/Li)，與2.6 V之工作電位形成鮮明對比，故須重新評估。此外，2.8 V之氧化還原電位之推估存在缺陷，因其涵蓋不涉及電子轉移之化學反應。此些觀察結果均令人懷疑所提出電池反應之正確性。本研究提出新觀點，認為O2與H2O參與電化學反應，導致過往所報導之高工作電位。經由軟X光吸收光譜之研究，證明僅形成Li2CO3作為放電產物。此外，經由研究發現，僅於較低之1.1 V電位下，方發生純CO2還原，產物非不定型碳而為CO。故於研究Li–CO2電池時，必須全面考慮O2與H2O之可能污染，對放電產物進行仔細鑑定以制定正確之反應方程。Na–CO2電池同樣以其高能量密度存儲與CO2利用而著名，然於闡明其反應機制與提高電池性能方面面臨類似之挑戰。經由採用原位環境下之X光光電子能譜(APXPS)，本研究揭示CO2還原反應機制，其與以往研究不同，本研究發現純CO2環境表現出較差之電化學活性。引入O2與H2O等添加劑可增強CO2之反應性。然二者產生之放電產物均使離子液體分解生成烯類，此乃過往Na–CO2電池中產生元素碳訊號之可能來源。最後本研究將焦點轉向Li–O2電池回收。其中已廣泛研究空氣陰極之發展，然對其再生之關注有限。本研究引入一種水洗再生方法，作為再生空氣陰極並為其進行鑑定之有效技術。該方法成功地將兩種類型之空氣陰極再生至少五循環，而不導致容量下降。於去除放電產物後，更容易觀察到結構與化學/電化學變化。再生後，容積膨脹與表面官能基化之碳為主要觀察現象。此外，經處理後具LiOH之流出物可用於CO2捕獲而形成Li2CO3，有助於減少CO2排放。此項研究強調Li–O2電池不僅可作為高能量密度設備，更可作為實現環保之解決方案。

This study is to explore the discharge mechanism of the cathode reaction in Li–CO2 and Na–CO2 batteries, as well as the regeneration of air cathodes in Li–O2 batteries. Li–CO2 batteries are highly regarded for their remarkable energy density and carbon utilization, yet a comprehensive understanding of their cathodic reaction remains elusive. The conventional perspective proposed CO2 reduction with carbon as the reduced product, with the reaction formula: 3CO2 + 4Li+ + 4e− → 2Li2CO3 + C (Eeq = 2.8 V vs. Li+/Li). However, the substantiation of carbon formation is limited in existing studies. The potential-determining step involving CO2 + e− → CO2− (Eeq = 1.1 V vs. Li+/Li) stands in stark contrast to the working potential of 2.6 V, thus prompting a reevaluation. Moreover, the redox potential at 2.8 V is flawed, considering it encompasses a chemical reaction without electron transfer. These observations cast doubt on the correctness of the proposed cell reaction. This study introduces a novel perspective, suggesting the involvement of O2 and H2O in the electrochemical reaction, leading to the high working potential. The exclusive formation of Li2CO3 as the discharge product is confirmed through soft X-ray absorption spectroscopy. Additionally, it is found that sole CO2 reduction occurs at a lower potential of 1.1 V, yielding CO instead of C as the discharge product. Thus, a comprehensive examination of O2 and H2O contamination is essential when investigating Li–CO2 batteries, and careful characterization of the discharge product is crucial for accurate reaction formulation. The Na–CO2 battery, renowned for its high-energy-density storage and CO2 utilization, faces analogous challenges in elucidating its reaction mechanism and enhancing battery performance. Employing in situ ambient pressure X-ray photoelectron spectroscopy (APXPS), this study unravels the CO2 reduction reaction mechanism. Unlike previous studies, this study reveals that pure CO2 environments exhibit poor electrochemical activity. Introducing additives such as O2 and H2O enhances CO2 reactivity leads to the degradation of ionic liquid and the formation of alkenes. The side products (alkenes) then generate the misleading Csp2 signals which can be generated by elemental carbon. Therefore, carbon can not be formed even with the presence of additives such as O2 and H2O. The focus shifts to Li–O2 batteries, where the development of air cathodes has been extensively studied, yet attention is limited to recycling these batteries. The thesis introduces an H2O-wash method as a potent technique for regenerating air cathodes and preparing them for characterization. This method successfully regenerates two types of air cathodes for at least five cycles without capacity degradation. Structural and chemical/electrochemical changes are readily observable following discharge product removal. Volume expansion and functionalized carbon are evident post-regeneration. Furthermore, the LiOH-containing effluent resulting from the treatment can be harnessed for CO2 capture, contributing to the reduction of CO2 emissions. This study underscores that Li–O2 batteries can serve not only as high-energy-density devices but also as a solution for a more environmentally sustainable future.