鋰電池系統(Lithium battery system)目前正面臨資源短缺、電容量限制以及安全性等的問題。鎂金屬因同時具有高體積電容量、高自然豐沛度與充電時不易形成枝晶等特色，以其作為負極材料的鎂電池系統(Magnesium battery system)成為具有開發潛力的選項之一。大部分的鎂電池系統研究主要專注於電解質開發以及其與鎂電池系統的匹配度，卻忽略了鎂金屬電極本身的充放電行為和經過不同循環次數後的電極顯微結構。因此，本論文將以純鎂、AZ31與LZ91鎂合金作為可充電鎂金屬電池負極，於All Phenyl Complex (APC)電解質中進行特定循環圈數的定電流放電/充電循環、電化學阻抗圖譜與動態極化掃描曲線分析，並針對特定循環圈數之電極顯微結構進行觀察，再與電化學結果作連結以探討不同鎂金屬負極的放電/充電機制。此外，也會比較純鎂、AZ31與LZ91作為可充電鎂金屬電池負極的放電/充電行為差異。
當鎂金屬負極在進行放電時，可觀察到純鎂、AZ31與LZ91均為不均勻的剝除形貌，伴隨著多個局部放電孔洞，但AZ31與LZ91和純鎂相比呈現較高的放電孔洞密度。當放電完成後，電極表面的放電孔洞內與周圍區域會充滿錯合物離子團。若隨即進行後續的充電，所有鎂金屬負極皆會因放電孔洞內充滿著穩定態的錯合物離子團(Mg2Cl3+∙6THF)而無法在放電孔洞內沉積鎂鍍層。主要的還原反應則會由放電孔洞周圍的介穩態錯合物離子團(MgCl++∙5THF)進行，使得鎂鍍層在純鎂與AZ31電極表面偏好圍繞著放電孔洞進行沉積；而LZ91電極上的鎂鍍層則沒有明顯的繞孔形貌，此現象應和LZ91在剛放電完的電極表面有較純鎂與AZ31分布均勻且數量更多的介穩態MgCl+∙5THF有關。
隨後對三種鎂金屬負極進行後續的放電/充電循環，發現純鎂在放電時會同時進行鎂鍍層與底材的剝除，使得部分鎂鍍層殘留於電極表面上，造成後續充電時會形成粒狀結構與多面體結構兩種明顯不同形貌的鎂鍍層。之後放電時則會因多面體結構鍍層相較於粒狀鍍層較為穩定且難以剝除，使後續剝除主要發生在粒狀鍍層區域。此行為也將導致純鎂放電時所需的過電位隨著循環圈數的增加而有所增加。AZ31與LZ91相較於純鎂具有更好的抵抗剝除能力，因此在放電時會優先進行鎂鍍層的剝除。當鍍層被完全剝除而底材裸露後，放電所需的過電位也隨之增加。因為AZ31和LZ91主要都是在合金表面進行鎂鍍層的沉積與剝除，所以能夠表現出較純鎂均勻的鍍層形貌與分布。
最後，無論是在哪種鎂金屬負極上進行放電/充電循環，同一圈內的充電過電位皆會隨著時間的進行而增加，且每一圈的充電過電位也會隨著循環圈數的增加而增加，此趨勢與隨著循環進行而下降的電極表面錯合物離子團濃度有關。

Lithium battery systems are currently facing problems including resources shortage, capacity limitation, and safety. Magnesium (Mg) metal has the advantages of high volumetric capacity, high natural abundance, and less dendritic growth during charging, so Mg battery systems using Mg as negative electrode become one of the promising alternatives to lithium batteries. Most studies about Mg battery systems focus on developing novel electrolytes and their feasibilities, while the charge/discharge behavior and the resultant microstructure of the Mg metal electrode after cycles are overlooked. Therefore, this study used pure Mg, AZ31, and LZ91 Mg alloys as the negative electrodes for Mg battery systems. Constant current discharge/charge cycles, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization curves were conducted in an all phenyl complex (APC) electrolyte. After specific cycles, the resultant microstructure of the electrode was investigated and combined with the electrochemical results to discuss the discharge/charge mechanism. In addition, the difference in discharge/charge behavior of pure Mg, AZ31, and LZ91 will also be compared.
When discharging the pristine Mg metal negative electrodes, the stripping morphology of pure Mg, AZ31, and LZ91 was inhomogeneous with numerous localized discharge holes. However, AZ31 and LZ91 show a higher density of discharge holes than pure Mg. Right after discharge, complex ions are distributed inside and near the discharge holes on the electrode surface. If the subsequent charge was conducted right after discharge, no Mg plating can be observed in the discharge holes because they are filled with stable complex ions (Mg2Cl3+∙6THF). The reduction reaction proceeds through the metastable complex ions (MgCl+∙5THF) near the discharge holes, resulting in preferential deposition of Mg along the circumferences of the discharge holes on pure Mg and AZ31 electrodes. In contrast, the plated Mg on LZ91 does not have this preference, which could be related to the more uniform and higher amounts of metastable MgCl+∙5THF on the LZ91 electrode surface after discharge.
Subsequent discharge/charge on the three Mg metal negative electrodes reveals that, on pure Mg, stripping of plated Mg and Mg substrate proceeds in parallel during discharge, resulting in incomplete stripping of the plated Mg. This leads to two distinct morphologies on pure Mg during charging: granular and faceted Mg structures. Because the faceted Mg structure is more stable and harder to be stripped than the granular structure, later stripping mainly happens on the granular Mg structure region. This also results in the increase in discharge overpotential with cycle number on pure Mg electrode. In contrast, AZ31 and LZ91 show a higher stripping resistance than pure Mg, so the plated Mg was preferentially stripped during discharge. When the plated Mg was completely stripped, a larger discharge overpotential was needed to strip the alloy substrate. Consequently, the plating and stripping morphologies are more uniform on AZ31 and LZ91 than pure Mg because they proceed on the alloy surface.
Lastly, no matter on which Mg negative electrode the discharge/charge cycle performed, the charge overpotential increases with charging time within a cycle, and the charge overpotential of every cycle increases with cycle number. This trend is related to the decreasing complex ion concentrations near the electrode surface as the cycle goes on.

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