| 摘要: | 鋰離子電池安全性一直是受矚目的重要議題之一，隨著高耗能3C隨身器材及電動汽機車（Hybrid Electric Vehicle, HEV、Electric Vehicle, EV）的快速發展，鋰離子電池能量密度和電容量也不斷提升，來滿足使用需求，因而解決鋰離子電池安全性的方案更為迫切。先前研究指出，高溫下引發鋰離子電池熱失控的主要導火線為:(a)鈍化膜在高溫下分解，活性物質裸露於電解液中，造成電解液分解，釋放大量的熱﹔(b)正極材料在高溫下充電時，易釋出氧，而活性氧進一步與電解液作用，加速電池內部的熱量累積，並引發更多副反應及熱量的生成，即可能造成電池的危害，甚至發生燃燒及爆炸事件。
本研究利用自由基捕捉劑巴比妥酸的衍生物1,3-Dimethylbarbituric acid(1,3DBTA)與雙馬來醯亞胺1,1’-(Methylenedi-4,1-phenylene)bismaleimide（BMI）及其衍生物（C4BMI, Si(MI)3, PMI）進行聚合，聚合出以1,3DBTA為核心的高分岐寡聚物，來做為鋰離子電池三元系正極材料Li[Ni1/3Mn1/3Co1/3]O2（NMC333）中的添加劑。主要目的是藉由高分岐寡聚物在三元材料表面形成不阻礙鋰離子穿透且具高溫穩定性的鈍化膜，達到保護電極之功用；當電池溫度過高時，1,3DBTA截取正極材料在高溫釋出的氧並阻斷燃燒鏈鎖反應，避免了熱爆衝現象，達到保障安全之目標。由高溫循環壽命測試可觀察到添加劑有效提升鈍化膜高溫的穩定性；由電性表現得知，添加核心式的高分岐寡聚物（o-BMI）其6C-rate的電容量保持率達76.0% （v.s 0.2C-rate）；雖然添加劑造成內電阻增加約10 Ohm但鋰離子擴散係數沒有顯著下降，得知其鋰離子通道仍然通暢不受到阻塞；進一步由XPS光譜分析添加劑對鈍化膜的影響，證實添加劑有效降低電解液（鋰鹽及溶劑）的分解，並抑制LiF及Li2CO3的生成。由以上結果推測添加劑作用機制是經由充放電驅動添加劑尾端剩餘的C=C進行二次聚合，或與電解質液中的Vinylene carbonate（VC）及電解液分解生成的乙烯發生聚合反應，使得個別的高分岐添加劑在鈍化膜形成的化成過程中，再次聚合於電極表面，形成具多孔隙包覆的保護層（鈍化膜），而此保護層不阻礙到鋰離子嵌入嵌出的孔道。並實際應用於不同比例之NMC正極活性材料，證實此添加劑對正極材料具特定選擇性及適用性，可藉由不同MI結構數量設計來達到添加劑對特定材料的應用。
本研究證實以具碳碳雙鍵的高分岐添加劑能在化成期間參與鈍化膜形成。在正極材料表面包覆不阻礙鋰離子嵌入嵌出之鈍化膜，提升鈍化膜的熱穩定性，形成高溫仍穩定的鈍化膜，增進高溫下電池循環壽命能力。此一添加物兼具了改善鋰離子電池安全性，以及達成保護電極之功效。
;Safety issue has always been the primary concern for lithium ion battery in all field of applications. The lithium battery safety issue is aggravated with the increasing demand for higher power and energy density in advanced electronic devices and in electric car applications. Resolving the safety issue becomes the most urgent goal for lithium battery technology for wider adaptation, especially by transportation sector.
The safety issue is primarily originated from the use of volatile organic solvent such as,EC, PC, and DMC.etc that contains thermally decomposable lithium salts （LiPF6, LiClO4 etc.）. Few approaches are explored to improve the lithium battery safety: (a). The use of electrode materials without oxygen release （LiFePO4）. (b). use of non- volatile solid-state electrolytes （ionic liquids）. (c). use of non-flammable or fire retardant separator material （ceramic） (d). External electronic circuit protection, and (e) shutting down the thermal run-away by the use of additives.
In this study, we disclosed a net-work like solid electrolyte interface formed from a hyper branched additive which circumference electrode active materials. The additive forms a porous protective layer （in the order of less than few nm） which hinders thick SEI formation and prevents direct electrolyte contact with the active material thus improves cycle-life. The net-work SEI layer formed by hyper branch oligomer with flexible side chain created fast lithium ion transport path across/from the active materials, and improves high rate performance at elevated temperature. The protective layer can be thermally triggered to neutralize radical species released by the electrodes upon heating over 135 0C; which cut-down the combustion chain reaction, thus quenched thermal run-away. Cycling at elevated temperature （50-60 oC） indicates this protective layer stabilizes the electrode surface and giving it better cyclability. Charging at 6C, the protective cathode yielded 76% of capacity compared to that arrived at 0,2C rate , which is higher than the one without applying the additive when. Although the interfacial resistance reached about 10 Ohm, lithium diffusion constant is un- affected due to the porous nature of the net-work like solid electrolyte interface. Furthermore, we have examined and compared the thermal-quenching effects using other BTA derivatives such as 1,3DBTA、5,5DBTA and determined the plausible mechanisms to quenching thermal run away. Finally, we have also examined the effectiveness of this additive when applying on other cathode materials such as LiMnO2 and LiCoO2.
A surface characterization show the additive has participate with the SEI formation during the first few charge/discharge cycle and forms a porous and stable protective layer over-coated the cathode materials. These results indicated this material has served as an active protective shut down agent in the event of internal shortage and/or over charging thus elevated safety of lithium ion battery. |
