Hyeokjo Gwon

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.

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).

Enhanced power and rechargeability of a Li-O2 battery based on a hierarchical-fibril CNT electrode

Recently Li-air batteries have been considered to be a promising candidate for EV and HEV applications due to their exceptionally high energy density. A key factor for the practical application of Li-air batteries is to solve the poor reversibility of nonconductive discharge products, which remains a significant limiting factor for Li-air batteries. Therefore, the air electrode needs to be designed such that it minimizes the undesirable clogging and promotes the electrochemical reactivity. As the control of the morphology and porosity of the electrode greatly affects on the capacity and rate capability, various nanostructured air electrodes have been reported using carbon nanoparticles, graphene, graphene oxide, or carbon nanotubes (CNTs). However, the poor cyclability and low rate capability remain as critical drawbacks of the Li−O2 batteries, and the ideally designed electrode architecture is still awaited.

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.

Fabrication and Electrochemical Characterization of TiO2 Three-Dimensional Nanonetwork Based on Peptide Assembly

The three-dimensional network of TiO(2) hollow nanoribbons designed from a peptide assembly using atomic layer deposition is demonstrated as a promising Li secondary battery electrode in this study. The nanoribbon network ensures effective transport of electrons and Li ions due to (i) a well-connected network of nanoribbons and (ii) the hollow structure of each nanoribbon itself, into which Li ions in the electrolyte can readily diffuse. The improved specific capacity, rate capability, and cyclability of the nanonetwork show that the utilization of a nanonetwork of individual hollow ribbons can serve as a promising strategy toward the development of high-performance electrode for Li secondary batteries.

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.

A new catalyst-embedded hierarchical air electrode for high-performance Li–O2 batteries

The Li–O2 battery holds great promise as an ultra-high-energy-density device. However, its limited rechargeability and low energy efficiency remain key barriers to its practical application. Herein, we demonstrate that the ideal electrode morphology design combined with effective catalyst decoration can enhance the rechargeability of the Li–O2 battery over 100 cycles with full discharge and charge. An aligned carbon structure with a hierarchical micro-nano-mesh ensures facile accessibility of reaction products and provides the optimal catalytic conditions for the Pt catalyst. The new electrode is highly reversible even at the extremely high current rate of 2 A g−1. Moreover, we observed clearly distinct morphologies of discharge products when the catalyst is used. The effect of catalysts on the cycle stability is discussed.

Phase Stability Study of Li1-xMnPO4 (0 ≤ x ≤ 1) Cathode for Li Rechargeable Battery

The phase stability of Li 1-x MnPO 4 (0≤x≤ 1) is investigated in this study for different Li compositions and temperatures by high temperature X-ray diffraction and electron microscopy. The map of stable phases is determined at temperature ranges between room temperature and 410°C. While pure LiMnPO 4 phase is stable at high temperature, partial phase transformation of MnPO 4 into Mn 2 P 2 O 7 is observed in delithiated phases above 210°C. Electron microscopy study also indicates the instability of the delithiated phase. The morphology of LiMnPO 4 is severely damaged upon delithiation. The instability of the delithiated phase and the phase transformation into Mn 2 P 2 O 7 may imply that safety concerns can be raised regarding the LiMnPO 4 cathode, unlike its Fe counterpart.

Sodium-Ion Storage in Pyroprotein-Based Carbon Nanoplates

Pyroprotein-based carbon nanoplates are fabricated from self-assembled silk proteins as a versatile platform to examine sodium-ion storage characteristics in various carbon environments. It is found that, depending on the local carbon structure, sodium ions are stored via chemi-/physisorption, insertion, or nanoclustering of metallic sodium.