Hun-Gi Jung

Nanostructured high-energy cathode materials for advanced lithium batteries

Nickel-rich layered lithium transition-metal oxides, LiNi(1-x)M(x)O(2) (M = transition metal), have been under intense investigation as high-energy cathode materials for rechargeable lithium batteries because of their high specific capacity and relatively low cost. However, the commercial deployment of nickel-rich oxides has been severely hindered by their intrinsic poor thermal stability at the fully charged state and insufficient cycle life, especially at elevated temperatures. Here, we report a nickel-rich lithium transition-metal oxide with a very high capacity (215 mA h g(-1)), where the nickel concentration decreases linearly whereas the manganese concentration increases linearly from the centre to the outer layer of each particle. Using this nano-functional full-gradient approach, we are able to harness the high energy density of the nickel-rich core and the high thermal stability and long life of the manganese-rich outer layers. Moreover, the micrometre-size secondary particles of this cathode material are composed of aligned needle-like nanosize primary particles, resulting in a high rate capability. The experimental results suggest that this nano-functional full-gradient cathode material is promising for applications that require high energy, long calendar life and excellent abuse tolerance such as electric vehicles.

Ruthenium-Based Electrocatalysts Supported on Reduced Graphene Oxide for Lithium-Air Batteries

Ruthenium-based nanomaterials supported on reduced graphene oxide (rGO) have been investigated as air cathodes in non-aqueous electrolyte Li-air cells using a TEGDME-LiCF3SO3 electrolyte. Homogeneously distributed metallic ruthenium and hydrated ruthenium oxide (RuO2·0.64H2O), deposited exclusively on rGO, have been synthesized with average size below 2.5 nm. The synthesized hybrid materials of Ru-based nanoparticles supported on rGO efficiently functioned as electrocatalysts for Li2O2 oxidation reactions, maintaining cycling stability for 30 cycles without sign of TEGDME-LiCF3SO3 electrolyte decomposition. Specifically, RuO2·0.64H2O-rGO hybrids were superior to Ru-rGO hybrids in catalyzing the OER reaction, significantly reducing the average charge potential to ∼3.7 V at the high current density of 500 mA g(-1) and high specific capacity of 5000 mAh g(-1).

NaCrO2 cathode for high-rate sodium-ion batteries

Sodium-ion batteries offer a potential alternative or complementary system to lithium-ion batteries, which are widely used in many applications. For this purpose, layered O3-type NaCrO2 for use as a cathode material in sodium-ion batteries was synthesized via an emulsion-drying method. The produced NaCrO2 was modified using pitch as a carbon source and the products were tested in half and full cells using a NaPF6-based non-aqueous electrolyte solution. The carbon-coated NaCrO2 cathode material exhibits excellent capacity retention with superior rate capability up to a rate of 150 C (99 mA h (g oxide)−1), which corresponds to full discharge during 27 s. The surface conducting carbon layer plays a critically important role in the excellent performance of this cathode material. We confirmed the reaction process with sodium using X-ray diffraction and X-ray absorption spectroscopy. Thermal analysis using time-resolved X-ray diffraction also demonstrated the structural stability of carbon-coated Na0.5CrO2. Due to the excellent performance of the cathode material described herein, this study has the potential to promote the accelerated development of sodium-ion batteries for a large number of applications.

Study on the Catalytic Activity of Noble Metal Nanoparticles on Reduced Graphene Oxide for Oxygen Evolution Reactions in Lithium-Air Batteries.

Among many challenges present in Li-air batteries, one of the main reasons of low efficiency is the high charge overpotential due to the slow oxygen evolution reaction (OER). Here, we present systematic evaluation of Pt, Pd, and Ru nanoparticles supported on rGO as OER electrocatalysts in Li-air cell cathodes with LiCF3SO3-tetra(ethylene glycol) dimethyl ether (TEGDME) salt-electrolyte system. All of the noble metals explored could lower the charge overpotentials, and among them, Ru-rGO hybrids exhibited the most stable cycling performance and the lowest charge overpotentials. Role of Ru nanoparticles in boosting oxidation kinetics of the discharge products were investigated. Apparent behavior of Ru nanoparticles was different from the conventional electrocatalysts that lower activation barrier through electron transfer, because the major contribution of Ru nanoparticles in lowering charge overpotential is to control the nature of the discharge products. Ru nanoparticles facilitated thin film-like or nanoparticulate Li2O2 formation during oxygen reduction reaction (ORR), which decomposes at lower potentials during charge, although the conventional role as electrocatalysts during OER cannot be ruled out. Pt-and Pd-rGO hybrids showed fluctuating potential profiles during the cycling. Although Pt- and Pd-rGO decomposed the electrolyte after electrochemical cycling, no electrolyte instability was observed with Ru-rGO hybrids. This study provides the possibility of screening selective electrocatalysts for Li-air cells while maintaining electrolyte stability.

Li–O2 cells with LiBr as an electrolyte and a redox mediator

After many years of successful and disappointing results, the field of Li–O2 research seems to have reached an equilibrium state. The extensive knowledge that has accrued through advanced analytical studies enables us to delineate the weaknesses of the Li–O2 battery. It is now clear that the instability of the cell components toward extreme conditions existing during cell operation leads to early cell failure as well. One serious challenge is the high oxidation potential applied during the charge process. Redox-mediators may reduce the over-potential and, therefore, improve the efficiency and cyclability of Li–O2 cells. Their use in Li–O2 cells is mandatory. We have previously shown that LiI can indeed behave in such a manner; however, it also promotes the formation of side products during cell operation. We have, therefore, embarked on a comprehensive study of lithium halide salts as electrolytes for use in Li–O2 cells. We examine herein the effect of other components in the cell, such as solvents and contaminants, on the lithium halide salt activity. Based on the electrochemical behavior and the identity of the final cell products under various conditions, we can glean substantial information regarding the detailed operation mechanisms for each specific case. We have concluded that low concentration of LiBr in diglyme solution can improve the cell performance with fewer side effects than LiI. With LiBr, only the desired Li2O2 is formed during discharge. During charge, the bromine redox couple (Br−/Br3−) can reduce the oxidation potential to only 3.5 V. Higher efficiency and better cyclability of cells containing LiBr demonstrate that the electrolyte solution is the key to a successful Li–O2 battery.

Coating Lithium Titanate with Nitrogen-Doped Carbon by Simple Refluxing for High-Power Lithium-Ion Batteries

Nitrogen-doped carbon is coated on lithium titanate (Li4Ti5O12, LTO) via a simple chemical refluxing process, using ethylenediamine (EDA) as the carbon and nitrogen source. The process incorporates a carbon coating doped with a relatively high amount of nitrogen to form a conducting network on the LTO matrix. The introduction of N dopants in the carbon matrix leads to a higher density of C vacancies, resulting in improved lithium-ion diffusion. The uniform coating of nitrogen-doped carbon on Li4Ti5O12 (CN-LTO) enhances the electronic conductivity of a CN-LTO electrode and the corresponding electrochemical properties of the cell employing the electrode. The results of our study demonstrate that the CN-LTO anode exhibits higher rate capability and cycling performance over 100 cycles. From the electrochemical tests performed, the specific capacity of CN-LTO electrode at higher rates of 20 and 50 C are found to be 140.7 and 82.3 mAh g(-1), respectively. In addition, the CN-Li4Ti5O12 anode attained higher capacity retention of 100% at 1 C rate after 100 cycles and 92.9% at 10 C rate after 300 cycles.

SOFCs with Sc-Doped Zirconia Electrolyte and Co-Containing Perovskite Cathodes

Recently, much attention has been directed toward the study of Sc-doped zirconia electrolyte for intermediate-temperature SOFCs (IT-SOFCs) due to its fairly good ionic conductivity compared with conventional Y-doped zirconia, which can reduce the internal ohmic loss of the cell. For improving unit cell performance, another important point to be considered is the selection of appropriate electrode materials to reduce the polarization loss at the electrode. In this study, we fabricated anode-supported SOFCs with a 1 mol % CeO 2 codoped 10 mol % Sc 2 O 3 -ZrO 2 (CeSSZ) electrolyte. Various kinds of cathode materials such as La-Sr-Mn-O, La-Sr-Co-O (LSCo), La-Sr-Co-Fe-O, and Sm-Sr-Co-O (SSCo) have been applied in order to identify the most suitable cathode material for Sc-doped zirconia. We evaluated the power-generating characteristics of 5 X 5 cm scaled unit cells as well as the cathode polarization effect via a dc current-voltage measurement, a dc current interruption method, and ac impedance spectroscopy. We also thoroughly investigated the interfacial reaction between the electrolyte and the cathode in order to identify appropriate heat-treatment conditions for each candidate cathode material on the CeSSZ electrolyte. According to the investigation, unit cells with a SSCo and LSCo cathode showed superior power density of 1.13 and 1.33 W/cm 2 , respectively, at 700°C and fairly stable cell performance without any serious interfacial reaction.

Silicon/copper dome-patterned electrodes for high-performance hybrid supercapacitors

This study proposes a method for manufacturing high-performance electrode materials in which controlling the shape of the current collector and electrode material for a Li-ion capacitor (LIC). In particular, the proposed LIC manufacturing method maintains the high voltage of a cell by using a microdome-patterned electrode material, allowing for reversible reactions between the Li-ion and the active material for an extended period of time. As a result, the LICs exhibit initial capacities of approximately 42u2005F g−1, even at 60u2005A g−1. The LICs also exhibit good cycle performance up to approximately 15,000 cycles. In addition, these advancements allow for a considerably higher energy density than other existing capacitor systems. The energy density of the proposed LICs is approximately nine, two, and 1.5 times higher than those of the electrochemical double layer capacitor (EDLC), AC/LiMn2O4 hybrid capacitor, and intrinsic Si/AC LIC, respectively.

A high-capacity Li[Ni0.8Co0.06Mn0.14]O2 positive electrode with a dual concentration gradient for next-generation lithium-ion batteries

To increase the reversible capacity of layered lithium nickel-cobalt-manganese oxide, a Li[Ni0.8Co0.06Mn0.14]O2 positive electrode with a two-sloped full concentration gradient (TSFCG) was successfully synthesized via co-precipitation. The TSFCG maximizes the Ni concentration in the particle core and the Mn concentration on the particle surface. The TSFCG Li[Ni0.8Co0.06Mn0.14]O2 positive electrode showed improved overall electrochemical properties (i.e., reversible capacity, cycle life, and rate capability) and thermal stability compared to a conventional positive electrode (CC) Li[Ni0.8Co0.06Mn0.14]O2 without a concentration gradient. Electrochemical impedance spectroscopy showed that the high stability of the outer surface composition of Li[Ni0.64Co0.06Mn0.30]O2 is responsible for reduction in surface resistance and charge transfer resistance by decreasing the parasitic reaction with the electrolyte. These reduced resistances explain the superior rate capability of TSFCG positive electrodes.

Self-Rearrangement of Silicon Nanoparticles Embedded in Micro-Carbon Sphere Framework for High-Energy and Long-Life Lithium-Ion Batteries

Despite its highest theoretical capacity, the practical applications of the silicon anode are still limited by severe capacity fading, which is due to pulverization of the Si particles through volume change during charge and discharge. In this study, silicon nanoparticles are embedded in micron-sized porous carbon spheres (Si-MCS) via a facile hydrothermal process in order to provide a stiff carbon framework that functions as a cage to hold the pulverized silicon pieces. The carbon framework subsequently allows these silicon pieces to rearrange themselves in restricted domains within the sphere. Unlike current carbon coating methods, the Si-MCS electrode is immune to delamination. Hence, it demonstrates unprecedented excellent cyclability (capacity retention: 93.5% after 500 cycles at 0.8 A g-1), high rate capability (with a specific capacity of 880 mAh g-1 at the high discharge current density of 40 A g-1), and high volumetric capacity (814.8 mAh cm-3) on account of increased tap density. The lithium-ion battery using the new Si-MCS anode and commercial LiNi0.6Co0.2Mn0.2O2 cathode shows a high specific energy density above 300 Wh kg-1, which is considerably higher than that of commercial graphite anodes.