Thin Single-Ion Solid-State-Electrolyte Film using Covalent Organic Frameworks for Lithium-Ion Cells

Organic solid-state electrolytes have emerged as a solution for developing less flammable and flexible batteries. While polymeric electrolytes have been developed for decades, their practical applications have been hindered by low ionic conductivity and high activation energy at room temperature. Alternatively, a new class of porous and crystalline organic materials called covalent organic frameworks (COFs) shows promise. COFs are highly crystalline materials where desired molecules are periodically bonded to form a two-dimensional layer. The well-ordered stacking of these organic layers extends the three-dimensional structure while creating porous channels. COFs are also mechanically robust and chemically stable, unlike soft and non-crystalline polymer films. By introducing anionic pendants and Li⁺ ions, COFs have been demonstrated as single-ion conductors in previous studies [1,2]. However, the ionic conductivity was still moderated (&gt;10^-5 S cm^-1) at room temperature. This limitation can be attributed to the powder-like form of COFs and their typical fabrication into pellet-type electrolytes, which results in high thickness (&gt;100 μm) and significant interfacial resistances (&gt;1 kΩ), even when mechanical pressure is applied. These factors contribute to challenges such as nonuniform interfacial contact with electrodes and long Li⁺ transport paths.<br/>Here, we developed thin-film COF electrolytes and demonstrated improved Li-ion cell performances. We fabricated COF films with a thickness of approximately 35 μm using a one-pot synthetic method at room temperature. The interfacial resistance of the COF film with metallic Li was approximately 224 Ω, which was 8 times lower than that of the pellet-type ones. For molecular designs, Li⁺ ions and sulfonate functional groups were included in a hexagonal unit of COF. The Li⁺ conductivity of the COF film was highly dependent on the concentration of sulfonic groups and the crystallinity of the film. Higher Li⁺ contents, corresponding to a higher number of sulfonic groups, and better crystallinity resulted in higher ionic conductivity and lower activation energy. Consequently, we achieved an optimum Li⁺ conductivity and transference number of 1.01 × 10^-4 S cm^-1 and 0.91, respectively, at room temperature. The COF film also exhibited excellent performance in Li|Li symmetric cells at room temperature, with over 1000 hours at 0.1 mAcm^-2 and 0.1 mAhcm^-2. Additionally, Li|LiFePO₄ cells using the COF electrolyte delivered a capacity of 150 mAhg^-1 at 0.1 C for 150 cycles with negligible capacity fading. In this presentation, I will discuss details of COF film synthesis, several COF designs and their characteristics, and COF-based Li-ion cell performances.<br/><br/><br/><b>References:</b><br/>[1] Kihun Jeong, Sodam Park, Gwan Yeong Jung, Su Hwan Kim, Yong-Hyeok Lee, Sang Kyu Kwak, and Sang-Young Lee., <i>J. Am. Chem. Soc</i>,<i> </i><b>2019</b>, 141, 5880-5885.<br/>[2] Xing Li, Qian Hou, Wei Huang, Hai-Sen Xu, Xiaowei Wang, Wei Yu, Runlai Li, Kun Zhang, Lu Wang, Zhongxin Chen, Keyu Xie, and Kian Ping Loh., <i>ACS Energy Lett</i>,<b> 2020</b>, <i>5</i>, 3498-3506.