Leveraging Secondary Interactions Through Indigo Redesign for Improved Organic Cathode Materials in Li-Organic Batteries

Lithium-ion batteries (LIBs) are crucial in today's digital society as the predominant energy storage devices. However, the environmental impact of inorganic materials like lithium, manganese, and cobalt limits their further application. Additionally, their rigid crystal structures and intercalation processes induce extreme volume expansion, which ultimately worsens battery life. Redox-active organic materials (ROMs) are promising alternative cathode materials for next-generation rechargeable batteries. Composed of abundant, lightweight elements such as carbon, oxygen, and sulfur, ROMs offer flexible molecular designs that can be optimized for voltage and capacity, and facilitate fast ion diffusion and rapid charge/discharge processes.<br/><br/>Small organic molecules have gained attention as cathode materials due to their simple synthesis, well-defined structure, and high specific capacity. Unfortunately, the biggest problem with small-molecule-based cathodes is that they dissolve easily in organic electrolytes, which negatively impacts overall battery cycle stability. To overcome these challenges, we focused on leveraging secondary interactions such as hydrogen bonding (H-bond), dipole-dipole interactions, and π–π interactions, which induce strong intermolecular interactions and thus reduce solubility in the electrolytes. Among many promising molecules, indigo, well-known as a natural dye, has been utilized in organic electronic devices due to its fast charge mobility and possesses both inter- and intramolecular H-bonds. In previous studies, although indigo shows poor solubility in the neutral state, it has been shown that the H-bond is weakened in the reduced state. Therefore, it is important that secondary interactions are maintained throughout the entire redox reaction, necessitating another secondary interaction to replace the H-bond.<br/><br/>Herein, we explored the relationship between the solubility of charged species and secondary interactions by inducing different types of secondary interactions that persist throughout the redox reaction. Inspired by indigo, we developed two indigo derivatives that can intentionally eliminate H-bonds or induce dipolar ionic interactions through S atoms. From single crystal analysis and DFT results, it was confirmed that H-bonds clearly exist in the neutral state and can effectively reduce solubility. However, during the redox process, the inserted Li cation disrupted the intermolecular H-bonds, thereby significantly increasing the solubility of the charged state. Notably, the secondary interaction induced by sulfur not only marginally reduced the solubility of the neutral state but also significantly lowered the solubility of the charged state. This is because the Li cation inserted during the redox reaction attracts the partially negative S atom, thereby forming a linkage that connects the dimer. Suppression of solubility by dipole-ion interaction effectively increased battery performance, demonstrating high stability in long-term cycles and retaining over 88% capacity at a very fast charging speed (6 minutes).