| 摘要: | 先進鋰離子電池(LIBs)，對電池循環壽命和安全性的要求極為嚴苛。我們發現改進這些物性的主要關鍵在於改善固態電解質界面(Solid electrolyte interface, SEI)。理想的SEI層必須能保護電極，減少其與電解液直接接觸產生物理或化學變化，進而增加電池的循環壽命和安全性。但SEI層也必須仍保有恰當通道有利於鋰離子嵌入/脫出，才不至於影響電池的充放電性表現。
鑑於此，本實驗設計將由開發新型的電解液添加劑來改質SEI結構，藉以完成下世代鋰離子電池的目標。本研究設計以1,3-二甲基巴比妥酸(1,3-DBTA)與N,N′-(4,4′-亞甲基二苯基)雙馬來醯亞胺(BMI)和N-苯基馬來醯亞胺(PMI)進行共聚反應，形成高分岐網狀結構的添加劑p-BPMI。p-BPMI粉末可順利溶解在商用電解液(1M LiPF6 in EC+DMC+EMC 1:1:1 V/V)中，並作為電解液添加劑運用在鋰離子電池(LIBs)上。經組成NMC(Li[Ni1/3Mn1/3Co1/3]O2) /Li鈕扣型半電池測試，於化成過程中生成的固態電解質界面（SEI）藉由電子掃描顯微鏡（SEM）、X-射線光電子光譜（XPS）、電化學阻抗頻譜（EIS）進行分析探討，可以證實p-BPMI順利披覆於電極活性物質並參與SEI的生成。此新的SEI層包覆性較好可減少電解液和鋰鹽的消耗，非但未增加離子傳輸的電阻影響充放電，反而能改善在高溫操作之循環壽命及提升電池安全性能表現。
加入1wt％的p-BPMI於電解液中組成NMC/Li鈕扣型半電池進行測試，在室溫(25oC)以0.5C-rate充放電80圈進行循環測試，與不含添加劑之電解液比較，容量保持率從原本的85.21％提高到96.53％；高溫(在60oC)測試的部分，以0.5C-rate充放電30圈進行循環測試，將同樣1wt％的p-BPMI和Vinylene carbonate (VC)相比，容量保持率從93.71％提高到96.43％。最後將1wt% p-BPMI運用在NMC /graphite軟包裝全電池(500毫安)上，在充滿電狀態(4.3V)進行穿刺測試，並沒有任何燃燒或爆炸的跡象發生，反觀只有添加VC的電解液發生燃燒反應。這種新型電解液添加劑p-BPMI不僅改善了鋰電池室溫和高溫循環性能，更提升了電池使用的安全性。
;Cycle life and safety are the two most sought-after properties in advanced lithium battery (LIBs). We have previously discovered the improvement hinges on solid electrolyte interface (SEI) layer. An idea SEI layer must be able to isolate the direct contact of electrolyte with the active electrode materials, which prevents physical or chemical reaction and eventually leads to longer cycle life and safety. However, this SEI structure should also bear fluent ion transport path that avails lithium intercalation and de-intercalation during charge and discharge cycle, thus not become a hindrance to cell performance.
In this study, we have developed a new type of electrolyte additive to facilitate formation of idea SEI structure, which leads to improved cycle life and effectively raised battery safety. The additive is based on a family of hyperbranched polyimine derivative composed of 1,3-dimethyl-barbituric acid (1,3-DBTA), copolymerized with N,N’-(4,4’-methylenediphenyl)dimaleimide (BMI) and N-phenylmaleimide(PMI), (termed as p-BPMI). The p-BPMI powder is fully dissolvable in common commercial electrolytes such as (1M LiPF6 in EC+DMC+EMC 1:1:1 V/V), and administered the same fashion as other electrolyte additives. The interface composition and structure after pre-formation and cycling of coin cells using the Li(Ni1/3Mn1/3Co1/3)O2/Li half-cell were characterized by SEM、EIS and XPS techniques and confirmed that the hyper-branched p-BPMI is homogeneously dispersed on the active component surface and participated with the growth of SEI. The newly formed SEI is robust, thinner and porous which not only allows fluent lithium transport, it also reduces the electrolytes and salt decomposition leading to longer cycle life and improved cell safety.
With the addition of 1wt% p-BPMI in the electrolyte, the capacity retention is raised from 85.21% to 96.53% compared with the base electrolyte after 0.5C-rate for 80 cycles at 25°C. When operating at higher temperature (60oC), with the addition of 1wt% p-BPMI in the electrolyte, the capacity retention is raised from93.71% to 96.43% compared to the use of electrolyte with 1wt% Vinylene carbonate (VC) as the additive. Fully charged NMC/graphite pouch full cell (500mA) with 1wt% p-BPMI-added electrolyte at 4.3V shows no sign of combustion or explosion after internal shortage inflicted by nail-penetration, while the regular cell with only VC additive shows thermal run away and finally exploded. This new type of electrolyte additive, p-BPMI is confirmed to be effective in improving both the room temperature and high temperature cycling performance after high temperature storage; and also assured LIB’s safety. |
