Regulating Steric Hindrance in Redox Coupling of Single Organic Molecule for High Performance Organic Batteries
Authors: Ri Vitalii , Son Giyeong , Kim Chunjoong 
Published: December 2024

Since the use of tricarbonyl compounds (e.g. dichloroisocyanuric acid) as electrode materials for primary lithium batteries dates back to the 1960s, organic molecules have received significant attention as a means to further advance the state-of-the-art electrode materials for rechargeable batteries due to their advantages, including abundance, high energy density, design flexibility, environmentally benign nature, mild synthesis processes, and cost competitiveness. Numerous molecular design strategies have been employed to achieve high performance in organic electrode materials for rechargeable batteries. For examples, the organic molecular structure can be optimized to provide high specific capacity by minimizing the redox-inactive parts, exploiting multiactive centers, or introducing electron activating and deactivating groups. In addition, cycle stability can be controlled by polymerization of the active-molecules and electrical conductivity can be increased by enhancing electron delocalization, inter- (p-d) and intra-molecular (p-p) conjugation in aromatic rings of the organic molecules.

Herein, we provide a novel design of organic electrode for high discharge voltage with superior cycling stability in rechargeable battery by using steric effect of substituted the single molecule. 16,17-dihydroxyviolanthrone (DHV) molecule was used as a single molecule model for control group because the violanthrone compound has a planar conformation overlapping sp2 hybridized carbon and its hydroxyl groups that can be substituted with a variety of substituents. The electrochemical performance of DHV cathode and its reversible redox process in rechargeable battery system were demonstrated through experimental and theoretical identification at the first time. To further advance the battery performance of the DHV through the steric hindrance effect, we substituted main functional group (-OH) of DHV nearby p-type redox center with long-chain alkoxy group (-OC8H17 or -OC10H21) leading to ether, which are dioctiloxyviolanthrone (DOV) and didecyloxyviolanthrone (DDV), respectively.

In comparison to DHV, DOV and DDV showed significant increase of redox potential of the oxidation reaction allowing battery with around 0.6 V higher operating voltage. We further substantiated the steric hindrance effect from long-chain alkoxy groups on coupling electrolyte anion (PF6
-) with charged DOV and DDV via computational simulation and theoretical studies based on density functional theory (DFT). Moreover, we showed that the bipolar nature of the violanthrones allows its application in symmetrical cells, though the cycling performance was limited by the reduction reaction stability. Additionally, we demonstrated that finding of the more suitable electrolyte could lead to greatly enhanced stability of the violanthrone electrodes allowing up to 2000 cycles without capacity reduction and reduced voltage decay. Finally, based on various analytical methods, we demonstrated that the oxidation occurs through withdrawing of the delocalized electrons from the large conjugated structure of the violanthrones, and that the main the p-type redox site could be determined to be the moiety of (-(ROC)C-C-C-C(COR)-). Thus, this molecular design approach allows development of organic cathode with a high redox potential, comparable to the conventional cathode materials, and potentially may lead to future all-organic symmetric batteries.