Synthesis of 2-Alkoxy-Pyrrolinium-based Ionic Liquids with Bis(trifluoromethanesulfonyl)imide or Bis(fluorosulfonyl)imide Anion and its Characterization in LiFePO4 cells of Lithium Ion Batteries

Abstract

리튬 이차전지의 전해액으로 사용하기 위한 이온성 액체(IL)를 합성하고 물리화학적, 전기화학적 성능을 평가하였다. 기존 이미다졸륨계 양이온의 불안정한 C-2위치의 수소로 인해 환원 불안정성이라는 단점을 가지고 있다. 과거, 이를 해결하기 위하여 포화된 고리형 양이온인 피롤리디늄과 피페리디늄을 사용하여 연구한 결과, 환원 불안정성을 대폭 향상시킬수가 있었다. 하지만, 높은 점도와 낮은 전도도가 또다른 문제로 남았다. 그리하여 알콕시 치환체를 가진 피롤리늄 양이온을 도입하게 되었다. 즉, 피롤리늄의 이중결합은 양이온의 구조를 점도면에서 유리한 평평한 구조로, 알콕시 치환체는 낮은 점도와 전기화학적 안정성을 도모하기 위해 도입되었다. 지금까지 bis(trifluoromethanesulfonyl)imide (TFSI) 가 주로 연구되어왔는데, 이는 다소 높은 점도와 녹는점이라는 단점을 가지고 있다. 그래서 우리는 bis(fluorosulfonyl)imide (FSI) 를 도입하여 보다 더 낮은 점도와 전도도 뿐만 아니라 낮은 녹는점을 가진 이온성액체를 얻을 수 있었다. 우리는 여러가지 피롤리늄계 이온성액체를 합성하였는데, 그중에서 [A(OEt)]Pyrl-FSI (16)이 가장 낮은 점도 (22 cP)와 높은 전도도 (14.3 mS cm-1)를 보여주었다. 또한, 1M LiFSI가 녹은 [A(OMe)]Pyrl-FSI (12)이 4.4 V로 가장 넓은 전위창을 보여주었고, 1M LiTFSI가 녹은 [E(OMe)]Pyrl-FSI (10)과 [P(OMe)]Pyrl-FSI (11) 는 LiFePO4 양극에서 17 싸이클 동안 하프셀 충-방전테스트를 한 결과 초기용량대비 99%의 높은 수준의 가역 용량이 유지되었다. 이와 같은 연구를 통하여 기존 이온성 액체들이 높은 점도, 낮은 전도도, 전기화학적 불안정성 및 낮은 효율성 등으로 인해 리튬 이차전지에 적용되기 어려웠던 여러 문제점에 대한 새로운 해결책을 제시 할수 있었다.

Ionic liquids (ILs) are salts with low melting point which is below 100 oC and composed of ionic species such as cations and anions. ILs have been issued since it has high potential to be applicable for lithium secondary batteries (LIBs) due to its unique properties such as wide liquid range, high ionic conductivity, wide voltage window, non-volatility, and non-flammability. One of the well known ionic liquids is imidazolium-based ILs which have low viscosity and high ionic conductivity compared to other conventional ILs. However, there still are some defects as electrolyte of lithium secondary batteries. That is, imidazolium-based ILs were found electrochemically unstable at low potential called cathodic polarization, which is mainly because of the decomposition of acidic proton at C-2 position of imidazolium ring. Few years ago, we reported saturated cyclic ammonium-based ILs called pyrrolidinium and piperidinium-based ILs to see the stability at cathode. As we expected, it showed more stable behavior compared to imidazolium-based ILs, but viscosity and ionic conductivity problems happened. To deal with it, using heteroatom substituents such as ether and sulfide is one of the solutions due to the fact that hetroatom containing substituents such as methoxymethyl group have positive effects on not only cathodic stability, but also viscosity and ionic conductivity. Therefore, in this work, we tried to overcome the defects that conventional ionic liquids have by using pyrrolinium cation with an alkoxy substituent which accompanies with low viscosity. Bis(trifluoromethanesulfonyl)imide (TFSI) have been widely used as an anion of ionic liquids. However, it contains several defects such as somewhat high viscosity and high melting point. Therefore, we modified an anion of pyrrolinium-based ILs for bis(fluorosulfonyl)imide (FSI) that gives low viscosity and melting point. We synthesized pyrrolinium FSI ionic liquids with several different types of substituents including an alkoxy group and characterized its electrochemical and physicochemical properties. Among them, 1-allyl-2-ethoxy-pyrrolinium bis(fluorosulfonyl)imide (16) ([A(OEt)]Pyrl-FSI) showed the lowest viscosity of 22 cP and high conductivity of 14.6 mS cm-1 and we were able to see the widest electrochemical window of 4.4 V from [A(OMe)]Pyrl-FSI (12). In addition, [E(OMe)]Pyrl-FSI (10) and [P(OMe)]Pyrl-FSI (11) have shown good cycleability on the LiFePO4-based half cell (capacity retention ratio after 17 cycles: 99%) From these results, we believe that ILs with enhanced physicochemical and electrochemical properties can provide a lot of applications as electrolytes in LIBs.