학술논문
열 안전성이 향상된 고분자 분리막 제조 및 이차전지소재 개발 / Preparation of polymeric membrane with highly enhanced thermal property and materials for secondary battery applications
Document Type
Dissertation/ Thesis
Source
Subject
Language
Korean
Abstract
리튬이온전지는 다른 이차전지에 비해 높은 에너지 밀도를 보이는 것을 특징으로 하며, 최근에는 전자기기용 소형전지뿐만 아니라 중대형 전지가 큰 각광을 받고 있다. 현재 일부 상용화를 시작한 HEV (hybrid electric vehicles) 외에 PHEV (plug in hybrid electric vehicles) 및 EV (electric vehicles)로 인해 중대형 전지시장은 매우 급격하게 증가하고 있는 상태이다. 한편, 태양열, 풍력, 조력 등의 대체 에너지 개발로 생산된 전기를 저장할 수 있는 전력저장장치(ESS)용 고성능 리튬이온전지는 매우 중요한 에너지 저장시스템으로서 차세대 스마트 그리드 기술의 핵심요소로 인식 되고 있다. 이에 따라 세계 각국에서는 리튬이온전지를 미래형 고부가가치 산업으로 정하고 적극적인 산업 육성을 진행해오고 있다. 그러나 최근에 리튬이온전지의 과도한 용량과 고집적을 위한 소재들의 경량화로 인하여 전지의 고온 안전성 특성에 많은 제약이 따르고 있다. 따라서 전지로 인해 화재 사고가 많이 발생 하게 되었고, 이에 본 연구에서는 이차전지의 열 안전성을 향상할 목적으로 이차전지의 핵심 소재를 개발하게 되었다. 현재 상용화된 리튬이온전지용 분리막 소재는 대부분 폴리올레핀 계열 분리막이 사용되고 있으나 전해액에 대한 낮은 젖음성 문제를 안고 있다. 또한, 고온 특성이 약하여 전지 고출력시 안전성의 위험이 있다. 해결방안으로 화학적, 열적 안정성이 우수한 고분자소재인 polyimide(PI)를 선택하였다. PI분리막은 2단계 공정으로 제조되어야 하며, 1단계로 polyamic acid(PAA)과 같은 가용성 전구체를 이용하여 전기방사법으로 나노섬유를 제조하고, 2단계로 hot press 공정으로 이미드화(imidization)를 거쳐 PI 분리막을 얻었다. 제조된 분리막의 형태구조, 기공특성, electrolyte uptake, 기계적 물성, 열적 안정성을 평가하여 분리막에 대한 특성을 관찰하였다. PI 분리막은 이차전지용 분리막에 적합한 특성을 가졌으며, PE 상용 분리막 보다 열적 안정성이 우수하였다. 전지의 전기특성 평가는 PE상용 분리막과 초기용량이나 충/방전 효율이 비슷하였으며, 전지의 hot box 평가에서는 PE 상용 분리막보다 열적 안정성이 훨씬 우수하였다. 이로써 본 연구에서 개발된 PI 분리막은 현재 사용되고 있는 PE 상용 분리막을 대체할 수 있으며, 특히 고출력용 이차전지에서 안전하게 활용될 수 있음을 기대할 수 있다. 전해액으로 갖추어야 할 특성은 높은 이온전도도를 가져야하고 전극(양극/음극) 에 대한 전기화학적 안정성이 높아야 하며, 열적안정성으로 인해 사용 가능한 온도 범위가 넓고, 가격 경쟁력이 있어야 한다. 현재 이온전도도나 전기화학적 안정성이 좋은 액체 전해액을 사용하고 있으나, 고온 열적 안전성에 문제가 노출되고 있다. 이러한 문제를 해결하기 위해 전해액용 열 안정성을 향상 시킬 수 있는 첨가제를 개발하게 되었다. 본 연구에서 PVA-CN이 첨가제의 합성을 위해 PVA 원료를 용해하는 공정 (Step 1)과 PVA-CN을 합성하는 공정 (Step 2) 공정으로 나누어서 진행을 하였다. 합성된 PVA-CN이 첨가제의 우수한 열적 안정성을 TGA를 이용하여 확인하였다. PVA-CN이 첨가된 전해액의 이온전도도가 기존 상용의 전해액보다 약간 떨어졌지만 PVA-CN 첨가된 전해액을 활용한 이차전지는 고온 swelling 특성이 매우 우수한 결과를 보여주었으며, 이차전지의 가장 큰 사고 원인인 고온 swelling 문제를 해결 할 수 있다는 결과를 얻었다.
Li-ion batteries have shown a higher energy density than typical types of secondary batteries to be actively utilized in portable electronic devices and relatively large-scale energy storage systems (ESS). Thus, Li-ion battery markets have been rapidly growing recently because of the continuous development of commercially available large storage systems including HEV (hybrid electric vehicles), PHEV (plug in hybrid electric vehicles), and EV (electric vehicles). In the meantime, alternative energy generating systems such as solar, wind, and tidal power energies also require a electrical energy storage (EES) system based on Li-ion batteries, which can serve as a key element of next generation Smart Grid technology. As such, further improving Li-ion batteries has been designated as future high value markets around world. Although the electrical properties of Li-ion batteries have been greatly improved, the battery-induced fire accidents are occasionally reported due to the limited thermal stability. Herein, the focus of this study involves the improvement of core components of the Li-ion secondary battery systems to minimize their thermal safety issues. Currently available membranes for the Li-ion batteries are derived from polyolefin possessing a low wettability against electrolytes which often exhibit a risk of safety during the generation of high output from batteries. We speculated that chemically and thermally stable PI (polyimide) nanofiber membranes could resolve this problematic issue, where the PI membranes were prepared in two steps; the first step involved the electrospinning of PAA (polyamic acid) as soluble precursor to form nanoscale fibers, and the second step involved the formation of PI membranes via hot press-induced imidization. The morphology, pore property, electrolyte uptake, mechanical property, and thermal stability of the prepared nanofiber membranes were then thoroughly investigated. The overall properties of PI membranes were found to be suitable to serve as a membrane in the secondary ion battery systems whose thermal stability was much superior to that of commercially available polyethylene (PE) membranes. Thus, our developed PI membranes can be applicable to the battery systems requiring high outputs that can replace with the PE membranes. At the same time, another key component in the battery systems is an electrolyte whose ideal characteristics are high ion conductivity, broader range of operational temperature, thermal stability, and relatively low cost. In addition, electrolytes should not alter the stability of electrodes chemically and electrically. Liquid electrolytes are currently available forms, which exhibit high ion conductivity and chemical/electrical stability, but limited thermal stability. To resolve this problem, we attempted to develop electrolyte additives in order to improve their thermal stability. In this study, the development of PVA-CN (polyvinyl alcohol-acrylonitrile) additives was carried out with two steps; the first step was the dissolution of PVA raw materials, and the second step was the synthesis of PVA-CN. The thermal property of the prepared PVA-CN was quantitatively analyzed using TGA. The significantly improved thermal stability of the electrolytes containing PVA-CN additives was also confirmed by monitoring swelling behavior of the membranes at high temperature, although they exhibited slightly lower ion conductivity than commercially available electrolytes. This finding clearly suggests the possibility of preventing the swelling issue at high temperature which is the main cause of dangerous accidents from secondary battery systems.
Li-ion batteries have shown a higher energy density than typical types of secondary batteries to be actively utilized in portable electronic devices and relatively large-scale energy storage systems (ESS). Thus, Li-ion battery markets have been rapidly growing recently because of the continuous development of commercially available large storage systems including HEV (hybrid electric vehicles), PHEV (plug in hybrid electric vehicles), and EV (electric vehicles). In the meantime, alternative energy generating systems such as solar, wind, and tidal power energies also require a electrical energy storage (EES) system based on Li-ion batteries, which can serve as a key element of next generation Smart Grid technology. As such, further improving Li-ion batteries has been designated as future high value markets around world. Although the electrical properties of Li-ion batteries have been greatly improved, the battery-induced fire accidents are occasionally reported due to the limited thermal stability. Herein, the focus of this study involves the improvement of core components of the Li-ion secondary battery systems to minimize their thermal safety issues. Currently available membranes for the Li-ion batteries are derived from polyolefin possessing a low wettability against electrolytes which often exhibit a risk of safety during the generation of high output from batteries. We speculated that chemically and thermally stable PI (polyimide) nanofiber membranes could resolve this problematic issue, where the PI membranes were prepared in two steps; the first step involved the electrospinning of PAA (polyamic acid) as soluble precursor to form nanoscale fibers, and the second step involved the formation of PI membranes via hot press-induced imidization. The morphology, pore property, electrolyte uptake, mechanical property, and thermal stability of the prepared nanofiber membranes were then thoroughly investigated. The overall properties of PI membranes were found to be suitable to serve as a membrane in the secondary ion battery systems whose thermal stability was much superior to that of commercially available polyethylene (PE) membranes. Thus, our developed PI membranes can be applicable to the battery systems requiring high outputs that can replace with the PE membranes. At the same time, another key component in the battery systems is an electrolyte whose ideal characteristics are high ion conductivity, broader range of operational temperature, thermal stability, and relatively low cost. In addition, electrolytes should not alter the stability of electrodes chemically and electrically. Liquid electrolytes are currently available forms, which exhibit high ion conductivity and chemical/electrical stability, but limited thermal stability. To resolve this problem, we attempted to develop electrolyte additives in order to improve their thermal stability. In this study, the development of PVA-CN (polyvinyl alcohol-acrylonitrile) additives was carried out with two steps; the first step was the dissolution of PVA raw materials, and the second step was the synthesis of PVA-CN. The thermal property of the prepared PVA-CN was quantitatively analyzed using TGA. The significantly improved thermal stability of the electrolytes containing PVA-CN additives was also confirmed by monitoring swelling behavior of the membranes at high temperature, although they exhibited slightly lower ion conductivity than commercially available electrolytes. This finding clearly suggests the possibility of preventing the swelling issue at high temperature which is the main cause of dangerous accidents from secondary battery systems.