리튬 이온 배터리 (LIBs)에서 유기 전극은 비용 효율성 및 자원 의 풍부하다는 장점을 가져 경제적 잠재성을 가지고 있다. 그러나 전해질에서의 유기 활물질의 용해는 LIBs의 활용 측면에 있어서 주된 한계이다. 이 문제를 해결하기 위해 고농도 전해질 (HCE)가 제안되었다. 하지만 기존 HCE 연구는 높은 점도, 낮은 젖음성, 낮 은 이온 전도도 등과 같은 문제를 가진다. 이에 이 연구는 HCE의 물리적 한계를 극복하고 유기 전극의 용출을 억제하기 위해 비용 해 용매로서 희석제를 활용한 국부 고농도 전해질을 도입하였다. 희석제의 도입에 따른 용매화 구조의 변화는 라만 분광법, RDF (Radial Distribution Functions) 및 결합 에너지 계산을 통해 확인 을 하였으며 용출 확인은 UV-Vis spectrometry를 통해 확인을 하였다. 본 연구에서는 희석제로서 1,1,2,2-tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether (TTE)를 활용하였으며 TTE를 첨가함으 로써 용매화 구조의 변화 및 영향을 파악하기 위해 라만 분석, RDF를 계산을 했다. 그 결과 TTE를 활용한 국부 고농도 전해질 (LHCE)에서 자유 음이온 TFSI-가 감소하고 CIP, AGGs가 증가 함을 확인하였다. 또한 Li+에 가장 근접하게 존재하는 분자들은 DME와 TFSI-이며 TTE는 Li+에 거의 존재하지 않음을 확인함으 로써 TTE가 용매화 구조의 가장 바깥 껍질에 존재하여 전극과 전 해질의 두 유기 소재 사이의 직접적인 접촉을 억제하는 것을 확인 할 수 있음을 계산적으로 확인을 하였다. 또한 계산을 통해 중성상태에서 전자를 주입할 경우의 에너지 차이를 밝혀냈다. LHCE에서는 전자를 주입하였을 때 비교적 좁은 구역에서 용출이 발견이 되었으며 용출이 발생이 관찰되는 영역 역시 희석제에 의해 하나의 층이 형성되어 넓은 범위에서의 용출 을 억제함을 확인하였다. 반면, 고농도 전해질 (HCE)에서는 넓은 범위에서의 PTCDA의 선형 그래프를 확인할 수 있었고 극심한 용 출이 발생함을 확인할 수 있었다. 이를 통해 희석제를 활용했을 때 희석제에 의해 하나의 층이 형성이 되어 용출 억제가 가능함을 증 명하였다. 전해액의 용출 정도 파악은 방전된 perylene-3,4,9,10- tetracarboxylic dianhydride (PTCDA) 전극을 활용하였다. 해당 전극을 각 전해질에 0, 12, 24시간 동안 담근 후 전해질의 색 변화 를 관찰하였다. 기본 전해질 및 HCE의 색이 PTCDA의 색처럼 붉 게 변화는 것과 달리 LHCE에서는 색 변화가 관찰되지 않았다. 또 한 UV-Vis spectrometry를 활용하여 관찰했을 때에도 LHCE에 서는 PTCDA가 가지는 고유의 피크 영역에서 흡광도가 관찰되지 않았다. 이를 통해 희석제를 활용한 전해질이 유기 전극의 용출 억 제에 도움을 준다는 것을 확인하였다. 마지막으로 용출의 억제가 실제로 셀 성능 향상에 도움이 되는 지 확인을 하였다. 그 결과 용출 억제에 의한 셔틀 효과에 의해 빠 르게 용량 감소를 보이는 기본 전해질 및 HCE와 다르게 LHCE는 장기 사이클 동안 높은 용량 유지율을 보였으며 고율속에서의 안 정성 또한 가지고 있음을 확인하였다. 희석제를 활용하여 LIBs의 높은 가역성을 확보할 수 있다는 점에서 유의미한 연구이다. 더불 어 해당 연구를 통해 얻은 결과를 바탕으로 한 전해질의 제어는 이후에도 고출력, 장기 수명을 갖는 유기 전극 배터리 연구에 도움 을 줄 것이다.|Organic electrodes in lithium-ion batteries (LIBs) have economic potential due to their cost-effectiveness and abundant resources. However, the dissolution of organic electrode materials in the electrolyte has a significant limitation to their use in LIBs. To address this issue, high concentrated electrolytes (HCEs) have been proposed, but conventional HCEs suffer from several problems such as high viscosity, poor wettability, and low ion conductivity. This study introduces localized high concentrated electrolytes (LHCEs) with non-solvating diluents to overcome the physical limitation of HCEs and suppress the dissolution of organic electrodes. Control of solvation structure due to the introduction of diluents were confirmed through Raman spectroscopy, Radial Distribution Functions (RDFs), and binding energy calculations, while dissolution was verified using UV-Vis spectrometry. In this study, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was used as a diluent. Raman analysis and RDF calculations were performed to understand the changes and effects in the solvation structure due to the TTE. The results showed that in LHCEs with TTE, free TFSI- decreased while CIPs and AGGs increased. Additionally, it was confirmed that the molecules closest to Li+ are DME and TFSI-, with TTE rarely present near Li+. This indicates that TTE exists in the outer shell of the solvation structure, preventing direct contact between the organic electrode and the electrolyte. Furthermore, the energy difference when injecting electrons in the neutral state was determined through calculations. In LHCEs, dissolution was observed in relatively confined areas upon electron injection, and a layer formed by the diluent was shown to suppress dissolution over a broad range. In contrast, HCEs exhibited a linear dissolution graph of PTCDA over a wide range, indicating severe dissolution. This demonstrates that the use of diluents forms a layer that can effectively inhibit dissolution. To assess the extent of dissolution in the electrolyte, discharged perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) electrodes were used. These electrodes were soaked in each electrolyte for 0, 12, and 24 hours, and the color change of the electrolyte was observed. Unlike the conditional electrolyte and HCEs, which turned red like PTCDA, LHCEs showed no color change. UV-Vis spectrometry also showed no absorbance in the characteristic peak region of PTCDA in LHCEs. This confirms that electrolytes using diluents help inhibit the dissolution of organic electrodes. Finally, the study examined whether inhibiting dissolution actually improves cell performance. Unlike the conditional electrolyte and HCEs, which rapidly decreased in capacity due to the shuttle effect from dissolution, LHCEs maintained the high capacity retention over long cycles and exhibited stability at high rates. This research is significant in that it demonstrates the potential to achieve high reversibility in LIBs using diluents. Moreover, the results obtained from this study on electrolyte control will be helpful for organic electrode batteries with high reversibility and long lifespans.
Alternative Abstract
Organic electrodes in lithium-ion batteries (LIBs) have economic potential due to their cost-effectiveness and abundant resources. However, the dissolution of organic electrode materials in the electrolyte has a significant limitation to their use in LIBs. To address this issue, high concentrated electrolytes (HCEs) have been proposed, but conventional HCEs suffer from several problems such as high viscosity, poor wettability, and low ion conductivity. This study introduces localized high concentrated electrolytes (LHCEs) with non-solvating diluents to overcome the physical limitation of HCEs and suppress the dissolution of organic electrodes. Control of solvation structure due to the introduction of diluents were confirmed through Raman spectroscopy, Radial Distribution Functions (RDFs), and binding energy calculations, while dissolution was verified using UV-Vis spectrometry. In this study, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was used as a diluent. Raman analysis and RDF calculations were performed to understand the changes and effects in the solvation structure due to the TTE. The results showed that in LHCEs with TTE, free TFSI- decreased while CIPs and AGGs increased. Additionally, it was confirmed that the molecules closest to Li+ are DME and TFSI-, with TTE rarely present near Li+. This indicates that TTE exists in the outer shell of the solvation structure, preventing direct contact between the organic electrode and the electrolyte. Furthermore, the energy difference when injecting electrons in the neutral state was determined through calculations. In LHCEs, dissolution was observed in relatively confined areas upon electron injection, and a layer formed by the diluent was shown to suppress dissolution over a broad range. In contrast, HCEs exhibited a linear dissolution graph of PTCDA over a wide range, indicating severe dissolution. This demonstrates that the use of diluents forms a layer that can effectively inhibit dissolution. To assess the extent of dissolution in the electrolyte, discharged perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) electrodes were used. These electrodes were soaked in each electrolyte for 0, 12, and 24 hours, and the color change of the electrolyte was observed. Unlike the conditional electrolyte and HCEs, which turned red like PTCDA, LHCEs showed no color change. UV-Vis spectrometry also showed no absorbance in the characteristic peak region of PTCDA in LHCEs. This confirms that electrolytes using diluents help inhibit the dissolution of organic electrodes. Finally, the study examined whether inhibiting dissolution actually improves cell performance. Unlike the conditional electrolyte and HCEs, which rapidly decreased in capacity due to the shuttle effect from dissolution, LHCEs maintained the high capacity retention over long cycles and exhibited stability at high rates. This research is significant in that it demonstrates the potential to achieve high reversibility in LIBs using diluents. Moreover, the results obtained from this study on electrolyte control will be helpful for organic electrode batteries with high reversibility and long lifespans.
