Dong-Hwa Seo

Flexible energy storage devices based on graphene paper

Recently, great interest has been aroused in flexible/bendable electronic equipment such as rollup displays and wearable devices. As flexible energy conversion and energy storage units with high energy and power density represent indispensable components of flexible electronics, they should be carefully considered. However, it is a great challenge to fabricate flexible/bendable power sources. This is mainly due to the lack of reliable materials that combine both electronically superior conductivity and mechanical flexibility, which also possess high stability in electrochemical environments. In this work, we report a new approach to flexible energy devices. We suggest the use of a flexible electrode based on free-standing graphene paper, to be applied in lithium rechargeable batteries. This is the first report in which graphene paper is adopted as a key element applied in a flexible lithium rechargeable battery. Moreover graphene paper is a functional material, which does not only act as a conducting agent, but also as a current collector. The unique combination of its outstanding properties such as high mechanical strength, large surface area, and superior electrical conductivity make graphene paper, a promising base material for flexible energy storage devices. In essence, we discover that the graphene based flexible electrode exhibits significantly improved performances in electrochemical properties, such as in energy density and power density. Moreover graphene paper has better life cycle compared to non-flexible conventional electrode architecture. Accordingly, we believe that our findings will contribute to the full realization of flexible lithium rechargeable batteries used in bendable electronic equipments.

Galvanic Replacement Reactions in Metal Oxide Nanocrystals

Hollowing Out Metal Oxide Nanoparticles Corrosion is normally a problem, but it can be useful, for example, when you wish to create hollow metal nanoparticles, whereby the reduction of one metal species in solution drives the dissolution of the core of the particle. Oh et al. (p. 964; see the Perspective by Ibáñez and Cabot) adapted this approach to metal oxide nanoparticles by placing Mn3O4 nanocrystals in solution with Fe2+ ions, which replaces the nanocrystal exterior with γ-Fe2O3. At sufficiently high Fe2+ concentrations, hollow γ-Fe2O3 nanocages formed. These hollow structures could be used as anode materials for lithium ion batteries. Hollow mixed-metal oxide nanoparticles can be made by replacing the metal cations through redox reactions in solution. [Also see Perspective by Ibáñez and Cabot] Galvanic replacement reactions provide a simple and versatile route for producing hollow nanostructures with controllable pore structures and compositions. However, these reactions have previously been limited to the chemical transformation of metallic nanostructures. We demonstrated galvanic replacement reactions in metal oxide nanocrystals as well. When manganese oxide (Mn3O4) nanocrystals were reacted with iron(II) perchlorate, hollow box-shaped nanocrystals of Mn3O4/γ-Fe2O3 (“nanoboxes”) were produced. These nanoboxes ultimately transformed into hollow cagelike nanocrystals of γ-Fe2O3 (“nanocages”). Because of their nonequilibrium compositions and hollow structures, these nanoboxes and nanocages exhibited good performance as anode materials for lithium ion batteries. The generality of this approach was demonstrated with other metal pairs, including Co3O4/SnO2 and Mn3O4/SnO2.

A New High-Energy Cathode for a Na-Ion Battery with Ultrahigh Stability

Large-scale electric energy storage is a key enabler for the use of renewable energy. Recently, the room-temperature Na-ion battery has been rehighlighted as an alternative low-cost technology for this application. However, significant challenges such as energy density and long-term stability must be addressed. Herein, we introduce a novel cathode material, Na1.5VPO4.8F0.7, for Na-ion batteries. This new material provides an energy density of ~600 Wh kg(-1), the highest value among cathodes, originating from both the multielectron redox reaction (1.2 e(-) per formula unit) and the high potential (~3.8 V vs Na(+)/Na) of the tailored vanadium redox couple (V(3.8+)/V(5+)). Furthermore, an outstanding cycle life (~95% capacity retention for 100 cycles and ~84% for extended 500 cycles) could be achieved, which we attribute to the small volume change (2.9%) upon cycling, the smallest volume change among known Na intercalation cathodes. The open crystal framework with two-dimensional Na diffusional pathways leads to low activation barriers for Na diffusion, enabling excellent rate capability. We believe that this new material can bring the low-cost room-temperature Na-ion battery a step closer to a sustainable large-scale energy storage system.

New iron-based mixed-polyanion cathodes for lithium and sodium rechargeable batteries: combined first principles calculations and experimental study.

New iron-based mixed-polyanion compounds Li(x)Na(4-x)Fe(3)(PO(4))(2)(P(2)O(7)) (x = 0-3) were synthesized, and their crystal structures were determined. The new compounds contained three-dimensional (3D)sodium/lithium paths supported by P(2)O(7) pillars in the crystal. First principles calculations identified the complex 3D paths with their activation barriers and revealed them as fast ionic conductors. The reversible electrode operation was found in both Li and Na cells with capacities of one-electron reaction per Fe atom, 140 and 129 mAh g(-1), respectively. The redox potential of each phase was ∼3.4 V (vs Li) for the Li-ion cell and ∼3.2 V (vs Na) for the Na-ion cell. The properties of high power, small volume change, and high thermal stability were also recognized, presenting this new compound as a potential competitor to other iron-based electrodes such as Li(2)FeP(2)O(7), Li(2)FePO(4)F, and LiFePO(4).

Fabrication of FeF3 Nanoflowers on CNT branches and their application to high power lithium rechargeable batteries.

Growing interest in electric vehicles, storage of energy from renewable sources, and load-leveling has positioned Li rechargeable batteries at the center of great attention, as they provide outstanding performance in terms of energy storage. [ 1 , 2–6 ] In the past decade, Li rechargeable battery technology has monopolized portable electric device markets such as mobile phones and laptop computers. However, further research on battery performance is still necessary for new applications such as electric vehicles and large scale power storage systems. [ 2–5 ] In essence, new technologies require electrodes with higher energy and power density to store and deliver more energy faster . Therefore, the development of new electrode materials that meet the requirements mentioned above is of utmost importance. Since the pioneering work of Tarascon and coworkers revealed a new strategy for high capacity electrodes by demonstrating that metal oxides can store more than one Li ion per transition metal atom through conversion reaction, [ 7 ] many researchers have investigated various conversion reaction compounds, such as metal nitrides, [ 8,9 ] sulfi des, [ 10,11 ] fl uorides, [ 12–16 ]

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.

The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials

Lithium-ion batteries are now reaching the energy density limits set by their electrode materials, requiring new paradigms for Li(+) and electron hosting in solid-state electrodes. Reversible oxygen redox in the solid state in particular has the potential to enable high energy density as it can deliver excess capacity beyond the theoretical transition-metal redox-capacity at a high voltage. Nevertheless, the structural and chemical origin of the process is not understood, preventing the rational design of better cathode materials. Here, we demonstrate how very specific local Li-excess environments around oxygen atoms necessarily lead to labile oxygen electrons that can be more easily extracted and participate in the practical capacity of cathodes. The identification of the local structural components that create oxygen redox sets a new direction for the design of high-energy-density cathode materials.

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.

Unexpected discovery of low-cost maricite NaFePO4 as a high-performance electrode for Na-ion batteries

Battery chemistry based on earth-abundant elements has great potential for the development of cost-effective, large-scale energy storage systems. Herein, we report, for the first time, that maricite NaFePO4 can function as an excellent cathode material for Na ion batteries, an unexpected result since it has been regarded as an electrochemically inactive electrode for rechargeable batteries. Our investigation of the Na re-(de)intercalation mechanism reveals that all Na ions can be deintercalated from the nano-sized maricite NaFePO4 with simultaneous transformation into amorphous FePO4. Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO4 is the significantly enhanced Na mobility in the transformed phase, which is ∼one fourth of the hopping activation barrier. Maricite NaFePO4, fully sodiated amorphous FePO4, delivered a capacity of 142 mA h g−1 (92% of the theoretical value) at the first cycle, and showed outstanding cyclability with a negligible capacity fade after 200 cycles (95% retention of the initial cycle).

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.