Novel electrolytes in the mixed solvents of pyrrolinium-based ionic liquid and carbonate for a Li/LiFePO4 cell

Abstract

리튬이온 배터리는 현재 가장 많이 사용되고 있고, 늘어나는 휴대용 전기 기기의 수요로 가장 인기 있는 배터리가 되었다. 리튬이온 배터리는 카보네이트 (EC, EMC, DEC, DMC) 와 리튬염 을 기본으로 한 전해질을 사용 하고 있는데, 이들은 전해질로서 요구되는 유전상수와 점도에 부합되지만 여전히 가연성, 휘발성으로부터 폭발을 야기 시킬 수 있는 안전 문제가 있다. 이와 같은 문제점을 해결 하기 위하여 이온성 액체가 하나의 대안으로 이야기 되고 있다. 이온성 액체란 이온결합을 통하여 상온에서도 액체상태로 존재하는 물질이다. 이온성 액체는 낮은 증기압, 넓은 전기화학적 안정성, 넓은 액체범위, 높은 전도도 그리고 높은 열정 안정성과 같은 특성들에 의해 주목을 받고 있다. 특히 이온성 액체는 이가 가지는 불연성, 비휘발성과 같은 특별한 속성들 때문에 폭발과 같은 안전문제를 일으키는 카보네이트 계열의 전해질을 대체 할 수 있는 대체물질로서 높은 잠재력을 가지고 있다. 이온성 액체 전해질을 기반으로 한 리튬 배터리는 카보네이트 전해질을 사용한 것에 비해 안정 적이기는 하지만 높은 점도와 낮은 전도도 그리고 높은 단가와 같은 단점이 있고, 뿐만 아니라 좋은 SEI (solid electrolyte interphase)를 형성 함에 있어서도 어려움이 있다. 우리는 카보네이트와 이온성 액체 양면에서 모두 오는 단점들을 극복하고, 현재 상용화 되어있지만 안전문제를 가지고 있는 카보네이트 전해질의 대체물질 확보를 위하여 카보네이트와 피롤리늄계 이온성 액체를 기반으로 한 혼합용액을 준비하였다. 우리는 이미다졸리움계, 피롤리디늄계, 피페리디늄계, 피롤리늄계 이온성 액체 와 같은 양이온을 연구 개발 하고 있다. 잘 알려진 이온성 액체중의 하나인 이미다졸리움계 이온성 액체는 좋은 전기화학적 성질을 보이지만, 이미다졸리움 양이온의 불안정한 C-2 위치의 수소로 인해 환원 불안정성이라는 단점을 가지고 있다. 이를 해결하기 위해 우리는 알콕시 치환기와 환형내 이중결합을 가지며 불안정한 위치의 수소를 가지지 않는 피롤리늄계의 이온성 액체를 합성하였다. 여러 가지 합성된 피로리늄계의 이온성 액체 중에서 하프셀 충-방전테스에서 가장 좋은 결과를 보인 [E(OMe)]Pyrl-FSI (6), [P(OMe)]Pyrl-FSI (7), 그리고 [A(OMe)]Pyrl-FSI (8)를 혼합 전해질의 후보로 선정 한 후, 카보네이트 (EC:EMC=3:7(v/v)+1M LiPF6+vc(3%)) 용액과 각각 의 피롤리늄계 이온성 액체를 다른 비율로(20, 40, 60, 80wt%) 섞어 혼합용액을 준비했다. 그 중에서 [E(OMe)]Pyrl-FSI (6) 를 60% 함유 하고 있는 혼합용액이 가장 높은 전도도 (13.76 mS cm-1) 를 보였고, LiFePO4 양극에서 100 싸이클 동안 하프셀 충-방전테스트를 한 결과 가장 높은 방전 용량과 초기용량대비 99%의 높은 수준의 가역 용량이 유지되었다(155 mAh g-1). 또한, 우리는 난연성 확보를 위해 가스 토치를 이용한 버닝 테스트를 진행 하였다. 이 테스트를 통하여 이온성 액체 함유에 의한 감소된 가연성을 확인 하였고, 특히 60%이상의 이온성 액체를 함유한 모든 혼합용액은 불연성을 보였다. 이 연구에서는 피롤리늄기반의 이온성 액체와 카보네이트의 혼합용액을 사용 함으로써 카보네이트 전해질로부터 발생되는 가연성문제를 개선 하고, 고가의 이온성 액체를 단독 사용하지 않음으로써 가격절감을 할 수 있으며, 높은 충-방전 속도와 100 사이클 이상의 횟수에서도 좀더 안정적이고 높은 효율을 가지는 새로운 혼합 전해질의 가능성을 보여줬다.

Lithium-ion batteries are widely used and have been the one of the most popular batteries as the needs for electrical portable devices increase. The electrolytes used currently are based on the mixture of PC, EC, EMC, DEC, DMC carbonates with a soluble lithium salts. While these solvents are satisfied with required dielectric constants and viscosities, they still remain safety problems such as flammability and volatility. The major concerns of lithium-ion batteries with conventional carbonate electrolytes are that short circuits often lead to overheating and ignition of the organic carbonate solvents, which are part of conventional electrolytes. Carbonate solvents that contribute to the dissociation of lithium salts are volatile and potentially combustible and can lead to the thermal runaway of batteries at any abuse conditions. Recently, an interest in non-flammable materials is greatly becoming high as a mean for improving battery safety. Thus, to solve the problems caused by conventional carbonate solvents, ILs have been picked as a candidate of electrolytes to replace conventional carbonate solvents of lithium-ion batteries. Ionic liquids(ILs) have come into the spotlight in the field of electrochemistry because of their interesting properties. Due to the ionic nature in bonding, ionic liquids are salts existing in liquid at room temperature and show unique properties such as non-volatility, non-flammability, low vapor pressure, large electrochemical stability, wide liquid range, high ionic conductivity and high thermal stability. ILs have been issued since it possess high potential to be applicable as components of electrolytes for lithium-ion batteries on account of its unique properties. Lithium-ion batteries on the basis of ILs are safer than those with conventional electrolytes which are based on organic carbonate solvents. However, there are still some drawbacks as electrolytes of lithium-ion batteries. ILs normally have higher viscosity, high cost and lower conductivity than conventional electrolytes. Another problem is that most ionic liquids have low tendencies to form SEI (solid electrolyte interphase) layers on either anode or cathode. No complete solution to overcome the disadvantages of using ionic liquids has yet been found. To deal with the problems from both ILs and conventional electrolytes, we prepared novel electrolytes in the mixed solvents of pyrrolinium-based ionic liquid and carbonate as advanced electrolytes that could be replacement of conventional electrolytes. We have been developing ILs that would be competitive and comparable to that of conventional ILs such as imidazolium-based, pyrrolidinium-based, pipridine-based and pyrrolinium-based ILs. Nevertheless, imidazolium-based ILs are well known as one of electrolytes that show good electrochemical properties compared to conventional ILs for lithium-ion batteries, it still has several defects as electrolytes of lithium-ion batteries. Having an alternative plan for the improvement of all the drawbacks of which is resulting from imidazolium-based ILs such as high viscosity, low conductivity and unstability at low potential, we synthesized several types of pyrrolinium-FSI ionic liquids containing two substituents attached at N-position and C-2 position of pyrrolinium ring and it can remedy defects in imidazolium-based ILs. Among them, three types of selected ILs that are 1-ethyl-2-methoxy-pyrrolinium bis(fluorosulfonyl)imide ([E(OMe)]Pyrl-FSI) (6), 1-propyl-2-methoxy-pyrrolinium bis(fluorosulfonyl)imide ([P(OMe)]Pyrl-FSI) (7) and 1-allyl-2-methoxy-pyrrolinium bis(fluorosulfonyl)imide ([A(OMe)]Pyrl-FSI) (8) showed better cyclability than other pyrrolinium-based ILs with different types of substituents. Therefore, these ILs have been chosen to be treated as a suitable component for preparing novel electrolytes. In addition, we have selected the organic carbonate solvent (EC:EMC=3:7(v/v)+1M LiPF6+vc(3%)), which is one of commonly used electrolyte for lithium-ion batteries, as the other component of novel electrolytes. We expected that the novel electrolytes we prepared will have positive effect on not only flammability problems set by organic solvent but also cycling efficiency set by forming good SEI layer in Lithium-ion batteries. LiFePO4-based half cells were assembled using the novel electrolytes in the mixed solvents of pyrrolinium-based IL and organic carbonate in different IL contents, and their cycling performances were evaluated. The operating cycling conditions of coin cells containing any electrolytes at 1C-rate have been rarely reported. Thus, we have focused on investigation of cell operation condition with faster C-rate. In our study, a cell containing the novel electrolyte showed good cycling performance comparable to that of a cell assembled with organic carbonate electrolyte. Especially, the electrolyte consisted of 60% of 1-ethyl-2-methoxy-pyrrolinium bis(fluorosulfonyl)imide ([E(OMe)]Pyrl-FSI) (6) and carbonate solvent (EC:EMC=3:7(v/v)+1M LiPF6+vc(3%)), which have the highest conductivity (13.76 mS cm-1), showed the highest discharge capacity value and capacity retention ratio at 1C-rate ( after 100 cycles : 99% and 155 mAh g-1 ) on the LiFePO4-based half cells among three types of ILs of which are a part of the novel electrolyte after 100 cycles. Furthermore, as the amount of ILs increases, we were able to confirm reduced flammability of the novel electrolytes after taking a burning test. The novel electrolytes have not caught on fire, especially more than 60 ILs weight percentages. This study indicates both that safety and cycling performance of the lithium-ion batteries can be improved by using the novel electrolytes consisted of ionic liquid and organic carbonate solvent and it can reduce the cost resulting from the independent use of expensive ILs.