최적 설계된 비수계 전해질을 이용한 리튬-공기전지의 싸이클 특성

Abstract

Li-air batteries have higher theoretical energy density than conventional rechargeable lithium-ion batteries. However, recent studies of Li-air batteries with organic electrolytes demonstrate that most of these solvents may not have long-term chemical stability in the presence of the superoxide radical. Thus, improving the stability of electrolyte toward reduced oxygen species is the main key for developing the non-aqueous Li-air batteries with good capacity retention. In Part1, we employed diethylene glycol diethyl ether (DEGDEE) as an electrolyte solvent and evaluated its performance in a Li-air cell. DEGDEE was selected as the electrolyte solvent for the lithium-air cell, because it has a high ionic conductivity (4.7 mS cm-1 with 1 M LiTFSI), low viscosity and high oxidative stability exceeding 5 V vs. Li/Li+, which are superior to those of TEGDME. Good reversibility for oxygen reduction and evolution reaction was investigated by XRD, SEM and XPS. In Part2, we tried to investigate ionic liquid-based electrolytes for the Li-air batteries. They showed an oxidative stability higher than 5 V vs. Li/Li+, high ionic conductivity and good stability towards the superoxide radical. However, the ionic conductivity of ionic liquid electrolytes is relatively low due to their high viscosity, and the solubility and diffusion coefficient of oxygen in ionic liquid electrolytes. So we blended with DEGDEE to solve this problem. As a result, the Li-air batteries employing these electrolytes exhibited good cycling stability exceeding 150 cycles at a current density of 0.1 mA cm-2.|리튬-공기 전지는 기존의 리튬-이차 전지보다 높은 이론 에너지 밀도를 갖는다는 장점을 가지고 있다. 하지만, 최근의 리튬-공기 전지 연구결과에 의하면, 리튬-이차 전지에 널리 사용되는 카보네이트 계열 전해질을 사용하는 경우, 전해질이 분해되어 Li2CO3, 리튬알킬카보네이트 등이 지속적으로 형성되는 것이 밝혀졌다. 이후 DME(1,2-dimethoxyethane) 및 TEGDME(tetraethylene glycol dimethyl ether) 등과 같은 ether 계 전해질이 carbonate계 보다 안정성이 뛰어나다고 밝혀졌지만, 여전히 Li2CO3가 지속적으로 형성되는 것으로 알려져 있다. 전해질이 분해되는 특성 이외에, 양극이 외부로 노출되어 있는 구조이기 때문에 비휘발성 및 소수성을 갖는 전해질의 도입이 필요하다. Part1에서는 DEGDEE(diethylene glycol diethyl ether)라는 새로운 ether계 전해질을 도입하였다. 이 전해질은 높은 이온전도도(4.7 mS cm-1)와, 우수한 전기화학안정성(5 V vs. Li/Li+,)을 갖는다. 또한 말단이 에틸기로 되어 있어, superoxide radical에 우수한 안정성을 갖어 기존에 널리 사용되던 TEGDME(tetraethylene glycol dimethyl ether)전해질보다 더 우수한 수명특성을 갖는 것을 확인하였다. 산소의 산화,환원반응에 대한 가역특성은 SEM, XRD 그리고 XPS 분석을 통해 확인하였다. Part2에서는 양극이 노출되어 있다는 단점을 극복하기 위해 비휘발성 및 소수성을 갖는 이온성액체를 도입하였다. 하지만, 이온성액체를 단독으로 사용할 경우 높은 점도 및 낮은 산소확산도 등의 문제가 발생하므로 DEGDEE와 블랜딩하는 실험을 진행하였다. 실험결과, 0.1 mA cm-2 전류밀도, 500 mAh g-1 용량 조건에서 150 cycle 이상의 우수한 수명특성을 갖는 시너지 효과를 내는 것을 확인하였다.; Li-air batteries have higher theoretical energy density than conventional rechargeable lithium-ion batteries. However, recent studies of Li-air batteries with organic electrolytes demonstrate that most of these solvents may not have long-term chemical stability in the presence of the superoxide radical. Thus, improving the stability of electrolyte toward reduced oxygen species is the main key for developing the non-aqueous Li-air batteries with good capacity retention. In Part1, we employed diethylene glycol diethyl ether (DEGDEE) as an electrolyte solvent and evaluated its performance in a Li-air cell. DEGDEE was selected as the electrolyte solvent for the lithium-air cell, because it has a high ionic conductivity (4.7 mS cm-1 with 1 M LiTFSI), low viscosity and high oxidative stability exceeding 5 V vs. Li/Li+, which are superior to those of TEGDME. Good reversibility for oxygen reduction and evolution reaction was investigated by XRD, SEM and XPS. In Part2, we tried to investigate ionic liquid-based electrolytes for the Li-air batteries. They showed an oxidative stability higher than 5 V vs. Li/Li+, high ionic conductivity and good stability towards the superoxide radical. However, the ionic conductivity of ionic liquid electrolytes is relatively low due to their high viscosity, and the solubility and diffusion coefficient of oxygen in ionic liquid electrolytes. So we blended with DEGDEE to solve this problem. As a result, the Li-air batteries employing these electrolytes exhibited good cycling stability exceeding 150 cycles at a current density of 0.1 mA cm-2.