| 摘要: | 鋰電池之陰極經過處理產生特殊結構之固態電解質介面(solid electrolyte interface，SEI )，有助於充放電過程中鋰離子之傳遞，並提升電極之穩定性，同時亦有助於提高鋰電池的電性及安全性。 針對此本研究提出兩種改進固態電解質介面的方法達成製作高性能高安全性鋰電池之目標。本研究第一部份藉由添加導電高分子修飾 SEI，增進鋰離子電池的快充效能及電池壽命。由多種單體中篩選出thiophene單體衍生物3,4-Ethylenedioxythiophene (EDOT)並將之進行電化學聚合附著於陰極材料表面。導電高分子具有電子傳遞主鏈及離子傳遞側鏈，在陰極上構建了有效的磷酸鋰鐵電荷轉移。添加了EDOT可看出在C-Rate及壽命上皆有改進，高濃度可提高電容量，但並不是越高越好。隨著濃度的增加，所需活化穩定的圈數也越長，也越容易阻塞電極上離子的傳遞，本研究以0.03M pEDOT為最佳化條件。 第二部分為經由在電解質液中添加活性分子改善鋰離子電池的安全性。目前市售鋰離子電池廣泛採用的陰極材料鈷酸鋰，其整體安全性仍待突破。本研究將巴比妥酸(BTA)及其衍生物分別添加到以鈷酸鋰為陰極材料的電解質液中，藉由13DBTA與BTA的化學特性觀察其改變，由熱差式掃描分析儀(DSC)顯示放熱溫度延後，但在電池的測試中也發現有電容量的損失。因此藉由添加比例的調控，可控制電池性能與安全效果的提升。相較於一般電池使用耐燃劑，雖可穩定電池的安全性，卻大量犧牲了電池的電容量，本研究則兼顧循環壽命與熱穩定性的特性達到雙贏的效果。 The cathode of Li-ion battery after charge-discharged can produce a special structure named solid electrolyte interface (solid electrolyte interface, SEI), which helps lithium ion transmission during charge and discharge process, and improves the electrode stability. Charge capacity and safety can also be improved by improving SEI structure. The research presents two approaches to the improvement of SEI structure with the aim to produce high performance and safe lithium battery. This study is divided into two parts. The first part describes the modification of SEI through the addition of conducting polymer, which enhances both the C-rate capacity and cycle life. After screening several monomers, the thiophene monomer derivatives: 3,4-Ethylenedioxythiophene (EDOT) is found to be most effective for this purpose after electrochemical polymerization on the cathode surface. The conductive polymer is composed of electronic conducting main chain and ion conductive side-chain, enabling an efficient charge transfer on the interface of the LiFePO4 cathode particles. The addition, EDOT has effectively improved the C-Rate capability and life cycle. The improvement is found to increase with EDOT concentration but optimized at certain threshold. In presence of excess EDOT, longer activation cycle (during which polymerization is achieved) will be required to completely polymerize EDOT, and the interface may be too thick whcih blocks the litium insertion. In this system, 0.03M is the optimized concentration. The second part of the study is to improve lithium battery safety feature with the addition of functional molecules in the electrolytes. LiCoO2 is the most widely used cathode material in commercial lithium ion batteries, but the safety remains an issue which urgently needing improvement. In this research, barbituric acid (BTA) and its derivatives as well as conducting polymer were added to the electrolyte to improve lithium battery safety. The delay of exothermic temperature can be observed by differential scanning thermal calorimetry (DSC). Carefully balancing the component composition, it is found the battery performance and safety features can both be enhanced. Conventional safety technology uses flame retardants to reduce electrolyte flammability, temperature control is not satisfactory, and the charge capacity usually suffers greatly. In contrast, present approach achieved the thermal stability while still maintaining the charge capacity and long cycle life. |
