以四甲基哌啶氧化物之氧化還原介質應用於半固態鎂氧氣電池

Abstract

近年隨電動車、人工智慧及資料中心之蓬勃發展，儲能需求日益提升。然而鋰離子電池已趨近於理論極限，金屬空氣電池具高能量密度、成本優勢及潛在應用價值，逐漸受產業與學術界關注。相較於鋰氧電池易因鋰枝晶生成導致電池短路、鋅氧電池受析氫腐蝕且放電可逆性差及鈉空氣電池因於空氣中反應性高易導致安全性風險，鎂氧電池(magnesium–oxygen battery)兼具高能量密度、永續性及安全性，故被視為極具潛力之金屬空氣電池。然其實際應用仍受限於放電產物堆積、界面極化、過電位過高等問題，關鍵瓶頸主要來自於放電產物氧化鎂之絕緣特性。 本研究藉四甲基哌啶氧化物(2,2,6,6-tetramethylpiperidine-1-oxyl; TEMPO)作為氧化還原介質(redox mediator; RM)應用於半固態電解質聚偏氟乙烯-六氟丙烯共聚物(polyvinylidene fluoride hexafluoropropylene; PVDF-HFP)，以促進放電產物分解並提升鎂氧電池之電化學效能。實驗結果證實，添加0.75 M TEMPO之鎂氧電池相較於未添加TEMPO之鎂氧電池，其循環壽命由20圈提升至45圈，增幅達125%，同時過電位由1.51 V降低至0.77 V，降幅達49%，證實TEMPO有助於提升鎂氧電池之循環壽命與降低過電位。 本研究新穎性乃藉同步輻射中心之X光吸收光譜、Tauc plot及理論計算，揭示放電產物內部存在鎂氧缺陷並進行能隙分析討論。經實驗計算，放電生成之氧化鎂因內部鎂氧缺陷之形成，使其能隙(band gap)由7.8 eV顯著降低至4.3 eV。進一步藉密度泛函理論計算(density functional theory; DFT)證實，當氧化鎂引入氧空缺後，禁帶(forbidden band)內將形成額外缺陷態(defect states)與中間能階(mid-gap state)，進而調控其電子結構與電荷傳輸行為，使原本絕緣之放電產物轉化為具電子傳導能力之缺陷型結構，有利於後續充電反應中之可逆分解，提升鎂氧電池之效能。

With the rapid development of electric vehicles, artificial intelligence, and data centers, the demand for energy storage has increased significantly. Due to the theoretical limitations for lithium-ion batteries, metal–air batteries have attracted further attention owing to their high energy density and low cost. Compared with other metal–oxygen batteries, magnesium–oxygen (Mg–O2) batteries have emerged as highly promising candidates for metal–air batteries owing to their high energy density, sustainability, and intrinsic safety. However, their practical application is still hindered by discharge product accumulation and high overpotential. Among these issues, the key bottleneck mainly arises from the insulating nature of the discharge product. To enhance the performance of magnesium–oxygen batteries, the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was introduced as a redox mediator into a PVDF-HFP semi-solid-state electrolyte to promote the decomposition of discharge products and improve the electrochemical performance. The addition of 0.75 M TEMPO extended the cycle life by 125% and reduced the overpotential by 49%. This study validates that TEMPO markedly enhances the cycling stability and reaction kinetics. This work elucidates the internal magnesium–oxygen defect states in the discharge product and provides a detailed band gap analysis through experimental techniques and density functional theory (DFT) calculations. Experimental results reveal that the formation of these defects significantly reduces the band gap of the discharged MgO from 7.8 eV to 4.3 eV, effectively lowering the barrier to charge transport. This transition promotes reversible decomposition during charging, providing a critical pathway for optimizing the electrochemical efficiency of magnesium–oxygen batteries.