Fast and Sustainable Energy Storage in Flavins Inspired by Biological Energy Metabolism for Li Rechargeable Batteries
Authors: Hong Jihyun , Lee Minah , Kim Haegyeom , Seo Dong-Hwa , Lim Hee-Dae , Park Chan-beum , Kang Kisuk 
Published: February 2020

Cellular metabolism comprises energy transduction machineries that operate by a series of redox-active components for storing energies from nutrients, which are transduced into high-energy intermediates for cellular works such as chemical synthesis, transport, and movement.
Biological energy transduction mechanism hints at the construction of a man-made energy storage system. Since the pioneering work by Tarascon et al.[1] towards a sustainable lithium rechargeable battery received significant resonance, organic materials such as carbonyl, carboxyl, or quinone-based compounds have been demonstrated to be bio-inspired organic electrodes [2, 3]. The imitation of redox-active plastoquinone and ubiquinone cofactors [4] through the utilization of redox active C=O functionalities in organic electrodes is a significant step-forward to biomimetic energy storage. However, the biological energy transduction is based on numerous redox centers of versatile functionalities available in nature, not limited to the simple redox active C=O functionalities. Consideration of how natural energy transduction systems function at organelle or cellular levels by elucidating the basic components and their operating principles selected by evolution will enrich the biomimetic strategy for efficient and green energy storage.
Herein, we propose new-type of redox active organic molecules containing C=N functionality, where most interest in organic electrodes has focused on the molecules with C=O functionalities, such as carbonyl, carboxylate, or quinone-based molecules [5]. Flavins, a key redox element in respiration and photosynthesis, facilitate either one- or two-electron-transfer redox processes accompanying proton transfer at nitrogen atoms of diazabutadiene motif during cellular metabolism. We have discovered that the protonation sites of a riboflavin molecule can capture two lithium ions reversibly exhibiting a high capacity of 174 mAh/g, which is comparable to that of LiFePO4. The combined ex situ characterizations and density-functional theory(DFT)-based calculation revealed that the redox reaction occurs via the two successive single-electron transfer steps at nitrogen atoms of diazabutadiene motif, which is analogous to the proton-coupled electron transfer of flavoenzymes in nature. We also demonstrated that the capacity and voltage can be tuned by the substitution of flavin cofactors, which opens up new principles in electrode design. This kind of bio-electrode is particularly unprecedented for lithium rechargeable batteries.
In terms of the practical use of organic-based electrodes, however, they suffer from sluggish kinetics and poor capacity retention, which originate from low electronic conductivity and dissolution of electroactive chemicals into electrolytes.
To address this issue, we further demonstrate a novel and facile design strategy for organic electrodes possessing high energy and power densities combined with excellent cyclic stability [6]. Non-covalent nanohybridization of electroactive aromatic molecules with single-walled carbon nanotubes (SWNTs) leads to reassembly of electroactive molecules from bulk crystalline particles into molecular layers on conductive scaffolds. The simple fabrication of this nanohybrid electrode in the form of a flexible, free-standing paper (free of binder/additive and current collector) results in ultrafast kinetics delivering 510 Wh/kg within 30 minutes (204 mAh/g ≈ 98% of theoretical capacity) and 272 Wh/kg of energy even within 46 seconds. Moreover, the stable anchorage of electroactive organic molecules on the sidewall of SWNTs enables above 99% capacity retention upon 100 cycles, which was hardly achieved for organic electrodes. Our approach can be extended to other aromatic organic electrode systems, bringing bio-inspired organic materials a step closer to practical cathodes in rechargeable batteries.
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