特異吸著於鋰離子二次電池之電化學特性研究

Abstract

人工石墨應用於鋰離子二次電池陽極材料的研究，已有數年之久。DPG人工石墨內含奈米碳管及富勒烯等奈米結構，奈米碳管之高表面積及奈米尺寸的管徑可使陽極人工石墨在充放電過程中儲存更多鋰離子於其中，使DPG人工石墨電容量高過理論值372mAh/g，達到400mAh/g以上，但相對的電池不可逆也高達30%以上，離商業上應用尚有進步的空間。 本實驗主要針對解決DPG人工石墨應用於鋰離子二次電池時所產生的不可逆現象。利用表面改質加熱處理，以及氫氯酸與氫氟酸分別在不同酸濃度下進行粉末酸洗處理。經由充放電測試(charge-discharge)、循環伏安分析儀(cyclic voltammetry)了解其電化學行為；交流阻抗分析儀(AC Impedance)量測電化學界面間之電荷傳送現象，找出不可逆發生之機制與解決方法。經由實驗結果發現，氫氟酸系統的人工石墨，不可逆有隨著酸濃度增加而遞減的趨勢，氫氟酸濃度 9N時可降至24.59%，為研究至目前所發現不可逆最低的條件。 經由電化學循環伏安法所得結果求取腐蝕電流密度與酸濃度之間的關係，比較氫氟酸與氫氯酸兩種系統下的不同，發現兩者之間有一氯離子與氟離子特異吸著(specific adsorption)現象的差異，此現象極可能為影響鋰離子二次電池不可逆現象之重要因素。

The application of artificial graphite as anode material for lithium ion secondary battery has been researched for several years. The reversible capacity of DPG over 400mAh/g, even higher than the theoretical value 372mAh/g, is contributed by the inner structures such as CNT and C60. Unfortunately, the difficulty of cutting down irreversibility is still the main problem looking forward to conquer to reach the market standard. The conception of experiments is resolving the irreversible phenomena in the DPG lithium secondary battery. Surface oxidative treatment, hydrochloric acid and hydrofluoric acid rinsing are three different methods carried out in this research. The electrochemical behavior of lithium secondary battery was confirmed by cyclic voltammetry. AC impedance was also used in this research to observe the charge transfer between the electrochemical interfaces and the possible irreversibility mechanisms. Results from the charge-discharge experiments indicate that the irreversibility is tended to ease with increasing hydrofluoric concentration. The irreversibility is lowered down to 24.59% for 9N hydrofluoric rinsed treatment, which is the best result since ever. According to the corrosion current density analyzed from the results of cyclic voltammetry, there is a distinction between two different systems and suggested by the specific adsorption between fluorine ion and chloride ion. This might be one of the reasons why hydrofluoric system shows a better irreversibility in lithium ion secondary battery.