Jihyun Hong

Noncovalent Functionalization of Graphene with End-Functional Polymers

Stable dispersion of reduced graphene in various organic solvents was achieved via noncovalent functionalization with amine-terminated polymers. An aqueous dispersion of reduced graphene was prepared by chemical reduction of graphene oxide in aqueous media and was vacuum filtered to generate reduced graphene sheets. Good solvents and nonsolvents for the dried reduced graphene were evaluated using a solubility test. To achieve stable dispersion in the evaluated nonsolvents, amine-terminated polystyrene was noncovalently functionalized to the graphene, while graphene sheets were phase transferred via sonication from aqueous phase to the organic nonsolvent phase, including the amine-terminated polymers. Thorough FTIR and Raman spectroscopy investigation verified that the protonated amine terminal group of polystyrene underwent noncovalent functionalization to the carboxylate groups at the graphene surface, providing the high dispersibility in various organic media.

Aqueous Rechargeable Li and Na Ion Batteries

Haegyeom Kim,†,∥ Jihyun Hong,†,∥ Kyu-Young Park,†,∥ Hyungsub Kim,†,∥ Sung-Wook Kim, and Kisuk Kang*,†,‡ †Department of Materials Science and Engineering, Research Institute of Advanced Materials (RIAM), Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 151-742, Republic of Korea ‡Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 151-742, Republic of Korea Nuclear Fuel Cycle Development Group, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea

Superior Rechargeability and Efficiency of Lithium–Oxygen Batteries: Hierarchical Air Electrode Architecture Combined with a Soluble Catalyst†

The lithium-oxygen battery has the potential to deliver extremely high energy densities; however, the practical use of Li-O2 batteries has been restricted because of their poor cyclability and low energy efficiency. In this work, we report a novel Li-O2 battery with high reversibility and good energy efficiency using a soluble catalyst combined with a hierarchical nanoporous air electrode. Through the porous three-dimensional network of the air electrode, not only lithium ions and oxygen but also soluble catalysts can be rapidly transported, enabling ultra-efficient electrode reactions and significantly enhanced catalytic activity. The novel Li-O2 battery, combining an ideal air electrode and a soluble catalyst, can deliver a high reversible capacity (1000 mAh g(-1) ) up to 900 cycles with reduced polarization (about 0.25 V).

Recent progress on flexible lithium rechargeable batteries

Flexible lithium ion batteries (LIBs) have received considerable attention as a key component to enable future flexible electronic devices. A number of designs for flexible LIBs have been reported in recent years; in this article, we review recent progress. We focus on how flexibility can be introduced into each component of the LIB, including the active materials, electrolytes, separators, and current collectors. Approaches to integrating each component into a single device are described and the corresponding changes in the electrochemical and mechanical properties are discussed. Finally, the key challenges in the development of flexible LIBs are summarized.

Structural evolution of layered Li1.2Ni0.2Mn0.6O2 upon electrochemical cycling in a Li rechargeable battery

Recently Li1.2Ni0.2Mn0.6O2, one of the most promising cathode candidates for next generation Li rechargeable batteries, has been consistently investigated especially because of its high lithium storage capacity, which exceeds beyond the theoretical capacity based on conventional chemical concepts. Yet the mechanism and the origin of the overcapacity have not been clearly understood. Previous reports on simultaneous oxygen evolution during the first delithiation may only explain the high capacity of the first charge process, and not of the subsequent cycles. In this work, we report a clarified interpretation of the structural evolution of Li1.2Ni0.2Mn0.6O2 upon the electrochemical cycling, which is the key element in understanding its anomalously high capacity, through careful study of electrochemical profiles, exsitu X-ray diffraction, HR-TEM, Raman spectroscopy, and first principles calculation. Moreover, we successfully resolved the intermediate states of structural evolution upon electrochemical cycles by intentionally synthesizing sample with large particle size. All observations made through various tools lead to the result that spinel-like cation arrangement and lithium environment are gradually created and locally embedded in layered framework during repeated electrochemical cycling. Moreover, through analyzing the intermediate states of the structural transformation, this gradual structural evolution could explain the mechanism of the continuous development of the electrochemical activity below 3.5 V and over 4.25 V.

Sodium intercalation chemistry in graphite

The insertion of guest species in graphite is the key feature utilized in applications ranging from energy storage and liquid purification to the synthesis of graphene. Recently, it was discovered that solvated-Na-ion intercalation can occur in graphite even though the insertion of Na ions alone is thermodynamically impossible; this phenomenon enables graphite to function as a promising anode for Na-ion batteries. In an effort to understand this unusual behavior, we investigate the solvated-Na-ion intercalation mechanism using in operando X-ray diffraction analysis, electrochemical titration, real-time optical observation, and density functional theory (DFT) calculations. The ultrafast intercalation is demonstrated in real time using millimeter-sized highly ordered pyrolytic graphite, in which instantaneous insertion of solvated-Na-ions occurs (in less than 2 s). The formation of various stagings with solvated-Na-ions in graphite is observed and precisely quantified for the first time. The atomistic configuration of the solvated-Na-ions in graphite is proposed based on the experimental results and DFT calculations. The correlation between the properties of various solvents and the Na ion co-intercalation further suggests a strategy to tune the electrochemical performance of graphite electrodes in Na rechargeable batteries.

All-graphene-battery: bridging the gap between supercapacitors and lithium ion batteries

Herein, we propose an advanced energy-storage system: all-graphene-battery. It operates based on fast surface-reactions in both electrodes, thus delivering a remarkably high power density of 6,450 W kg−1total electrode while also retaining a high energy density of 225 Wh kg−1total electrode, which is comparable to that of conventional lithium ion battery. The performance and operating mechanism of all-graphene-battery resemble those of both supercapacitors and batteries, thereby blurring the conventional distinction between supercapacitors and batteries. This work demonstrates that the energy storage system made with carbonaceous materials in both the anode and cathode are promising alternative energy-storage devices.

Organic nanohybrids for fast and sustainable energy storage.

A nanohybridization strategy is presented for the fabrication of high performance lithium ion batteries based on redox-active organic molecules. The rearrangement of electroactive aromatic molecules from bulk crystalline particles into molecular layers is achieved by non-covalent nanohybridization of active molecules with conductive scaffolds. As a result, nano-hybrid organic electrodes in the form of a flexible self-standing paper-free of binder/additive and current collector-are synthesized, which exhibit high energy and power densities combined with excellent cyclic stability.

Redox Cofactor from Biological Energy Transduction as Molecularly Tunable Energy-Storage Compound†

Energy transduction and storage in biological systems involve multiply coupled, stepwise reduction/oxidation of energycarrying molecules such as adenosine triphosphate (ATP), nicotinamide, and flavin cofactors. These are synthesized as a result of oxidation during citric acid cycles in mitochondria or during photosynthesis in chloroplasts, and high energies stored in their chemical bonds are consequently harnessed for many biological reactions. Phosphorylation and protonation are key underlying mechanisms that allow for reversible cycling and regulate the molecule-specific redox potential. A sequential progression of electron transfer through the redox cascades as well as continuous recycling of the redox centers enables efficient energy use in biological systems. The biological energy transductionmechanism hints at the construction of a man-made energy storage system. Since the pioneering work by Tarascon and co-workers towards a sustainable lithium rechargeable battery received significant resonance, organic materials such as carbonyl, carboxy, or quinone-based compounds have been demonstrated to be bio-inspired organic electrodes. The imitation of redoxactive plastoquinone and ubiquinone cofactors through the use 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. Flavins are one of most structurally and functionally versatile redox centers in nature, catalyzing an enormous range of biotransformations and electron-transfer reactions, which occur over a wide potential range (> 500 mV). The extraordinary versatility of flavins stems from their ability to engage in either oneor twoelectron-transfer redox processes, accompanying proton transfer at the nitrogen atoms of diazabutadiene motif. In the respiratory electron transport chain, for example, electrons from reduced flavin adenine dinucleotide (FADH2) are transported along a group of proteins located in the inner membrane of mitochondria to induce proton pumping across the membrane, as illustrated in Figure 1a (left). This process generates an electrochemical proton gradient, which results in the formation of high-energy ATP. FAD is reduced again in the citric acid cycle of mitochondria, which enables continuous recycling of flavin redox centers. A close analogy exists between the key components, facilitating respiration and battery operation (Figure 1a); charged ions (H or Li) and electrons, which are derived from flavin redox centers, are unidirectionally transported in a stoichiometric manner using separated paths. This creates chemical gradients across membranes, and finally results in the formation of highenergy species such as ATP and metallic lithium. Herein, we report on the possibility of using the energystorage mechanism of flavin redox cycling in mitochondria to lithium rechargeable batteries. According to our results, flavin electrodes were capable of reversibly storing and releasing two lithium ions and two electrons per formula unit. Redox reactions in flavin electrodes were thoroughly investigated using the combined analyses of ex situ characterizations and density functional theory (DFT)-based calculations. We found that the flavin redox reaction occurs during battery operation at the nitrogen atoms of the diazabutadiene motif in flavin molecules using two successive single-electron transfer steps, in a similar way to the proton-coupled electron transfer in flavoenzymes. Molecular tuning by chemical substitution on the isoalloxazine ring significantly improved electrochemical performances in terms of an average redox potential, a gravimetric capacity, and stability, resulting in a high-energy density comparable to that of LiFePO4, the [*] M. Lee, D. H. Nam, Prof. C. B. Park Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology Daejeon 305-701 (Korea) E-mail: parkcb@kaist.ac.kr J. Hong, Dr. D.-H. Seo, Prof. K. T. Nam, Prof. K. Kang Center for Nanoparticle Research Institute for Basic Science (IBS) Department of Materials Science and Engineering Research Institute of Advanced Materials Seoul National University, Seoul 151-742 (Korea) E-mail: matlgen1@snu.ac.kr [] These authors contributed equally to this work.

Biologically inspired pteridine redox centres for rechargeable batteries

The use of biologically occurring redox centres holds a great potential in designing sustainable energy storage systems. Yet, to become practically feasible, it is critical to explore optimization strategies of biological redox compounds, along with in-depth studies regarding their underlying energy storage mechanisms. Here we report a molecular simplification strategy to tailor the redox unit of pteridine derivatives, which are essential components of ubiquitous electron transfer proteins in nature. We first apply pteridine systems of alloxazinic structure in lithium/sodium rechargeable batteries and unveil their reversible tautomerism during energy storage. Through the molecular tailoring, the pteridine electrodes can show outstanding performance, delivering 533 Wh kg(-1) within 1 h and 348 Wh kg(-1) within 1 min, as well as high cyclability retaining 96% of the initial capacity after 500 cycles at 10 A g(-1). Our strategy combined with experimental and theoretical studies suggests guidance for the rational design of organic redox centres.