Daeun Kim's research while affiliated with Korea Polytechnic University and other places

The surface of micrometer‐sized CoO particles is finely tuned by a nitride produced through one‐step process with various heat treatments with solid‐state urea, which has been frequently used as a surfactant for the nanosynthesis of CoO in aqueous media. After conducting simple surface nitridation processes without a solvent using an autogenic reactor, it is found that the CoO surface is clearly modified by a cobalt‐rich nitride (Co5.47N) by X‐ray diffraction analyses. As the heat treatment temperature increases, the content of the cobalt nitride also grows. A temperature above 500°C for the surface modification causes an initial specific capacity loss because of the growth of the inert cobalt nitride; however, the surface‐modified samples exhibit a more prolonged cycle life than the pristine CoO due to decreased kinetic barrier and enhanced surface stability. Additionally, the surface nitridation leads to a high rate capability, which can solve poor power density for conversion‐type anode materials. It is found that the surface nitridation allows a high electrical conductivity and reduces the surface charge transfer resistance.

Graphene nanocomposite with ordered mesoporous carbon–silica–titania was prepared via one-pot evaporation-induced self-assembly. Due to the shear stress of surfactant self-assembly, a novel structure having a parallel alignment of the hexagonally ordered mesopores against graphene nanosheets plane was firstly observed. When applied into anode lithium ion batteries, the novel nanocomposite exhibited high reversible capacity of 574 mA h g⁻¹ with improved initial efficiency of 65%, significant reduction of impedance value and very stable cycle performance, which was attributed to the facile transport of electron through the graphene sheets and the easy penetration of electrolyte via the ordered mesopores. © 2018 The Korean Society of Industrial and Engineering Chemistry

Herein, charge storage behavior of graphene oxide electrode was investigated after its electrochemical activation. X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), galvanostatic chargedischarge method (GCD), and electrochemical impedance spectroscopy (EIS) were employed for this analysis. From XPS analysis, a decrease of atomic ratio of C=C bond was observed after electrochemical activation from 86.94 to 79.64%. Also, enhancement of specific capacitance appeared from 54.1 to 65.7 F g−1 at 20 mV s−1, and rectangular shape in CV became more collapsed in activated grapheme oxide electrode. In the GCD profiles, similarly, difference of resistance values at high and low cut-off potential became larger after activation, indicative of increased polarization. From EIS, detailed resistance components were compared, which reflected that the increased resistance and higher capacitance after activation was probably attributed to larger amount of surface functional groups after activation. Open image in new window

The concentration of sulfate ions was continuously monitored by Raman spectroscopy during the preparation of cathode material precursors. The obtained spectra were used for the quantitative analysis of sulfate ions by comparison with a reference solution, and a gradual increase of sulfate ion concentration was detected. Using a mathematical approach, a time-dependent equation describing sulfate ion concentration was theoretically derived, and the calculated concentrations were compared with the measured ones, showing good agreement between these values. Due to the increased sulfate ion concentration, the metal hydroxide cathode precursor became more contaminated with elemental sulfur. Two as-obtained hydroxide powders with different sulfur impurity contents were calcined with a lithium source, producing cathode materials. Electrochemical analysis of the prepared cathode materials showed that morphological aspects (such as particle size) more highly affected the cathode performance more than the sulfur impurity content.

Using a stabilizing agent-assisted co-assembly method, a novel nanocomposite of mesoporous carbon embedded with uniform tungsten oxide nanorods is obtained, which is converted into carbon-sheathed tungsten oxide nanoparticles by delicate calcination and further reduction. Through optimization of tungsten content, it is found that highly crystalline tungsten oxide nanoparticles are uniformly coated with an ultra-thin carbon layer. When applied into electrochemical charge-storage electrodes for supercapacitor and lithium-ion battery, an excellent average capacitance (129 F g⁻¹, above 400 F cm⁻³), higher rate performance and significantly advanced cycle stability are observed. These improved charge storage properties are attributed to improved electrical conductivity and enhanced structural stability, which is induced by uniform carbon coating on partially reduced tungsten oxide nanoparticles.

High Ni content layered oxide materials for the positive electrode in lithium-ion batteries have high specific capacity. However, their poor electrochemical and thermal stability at elevated temperature restrict the practical use. A small amount of Al_2O_3 was added to the mixture of transition metal hydroxide and lithium hydroxide. The LiNi_{0.6}Co_{0.2}Mn_{0.2}O_2 was simultaneously doped and coated with Al_2O_3 during heat-treatment. Electrochemical characteristics of modified LiNi_{0.6}Co_{0.2}Mn_{0.2}O_2 were evaluated by the galvanostatic cycling and the LSTA(linear sweep thermmametry) at the constant voltage conditions. The nano-sized Al_2O_3 added materials show better cycle performance at elevated temperature than that of micro-sized Al_2O_3. As the added amount of nano-Al_2O_3 increased, the thermal stability of electrode also enhanced, but the use of 2.5 mol% Al showed the best high temperature performance.

A mesostructured TiO2–graphitic carbon (TiO2–gC) composite was synthesized through a simple and scalable hydrothermal method to be employed as an anode material in Li-ion batteries. In a wide voltage range (0.0–2.5 V), the TiO2–gC composite anode possesses a high initial lithiation capacity (598 mA h g−1) at 0.1 C (1 C: 150 mA g−1), and it still retains 369 mA h g−1 after 50 cycles at 0.5 C. Furthermore, under a high current density of 2 C, the TiO2–gC anode exhibits stable capacity (252 mA h g−1) retention for up to 200 cycles. This excellent electrochemical performance could be ascribed to a synergistic effect of well-developed mesoporosity with a high surface area (345.4 m2 g−1), the conductive graphitic carbon wall, and uniformly dispersed TiO2 nanoparticles, resulting in improved Li+ penetration, fast electron transport and high structural stability during cycling.

Hexagonally ordered mesoporous carbon materials were prepared and used as electrode materials in an electric double-layer capacitor. Using this electrode, the change of electrolyte resistance within the mesopores was investigated according to the bulk electrolyte concentration. Using three different electrochemical transient experiments—imaginary capacitance analysis, chronoamperometry, and chronopotentiometry—the time constant associated with electrolyte transport was determined, which was then used to obtain the electrolyte resistance within the mesopores. With decreasing electrolyte concentration, the increase in electrolyte resistance was smaller than the increase in the resistivity of the bulk electrolyte, which is indicative of a different environment for ionic transport within the mesopores. On using the confinement effect within the mesopores, the predicted higher concentration within mesopore probably results in lower electrolyte resistance, especially under low bulk concentrations.

We investigate the effect of surface area of the lithium manganese spinel (LMO) cathode on the high-temperature storage performance of lithium-ion pouch cells. Using ~200 mAh design capacity, pouch cells were fabricated and their storage performance at high temperature was investigated according to changes in the LMO surface area. Cell performance and electrode analysis revealed that LMO with a large surface area exhibited a poor storage with a large decrease in capacitance and a more severe increase in resistance. This was mostly attributed to the change in surface morphology of anodes in each cell, indicative of the fast growth of a solid electrolyte interface film caused by high dissolution of Mn2+ ions in the high-surface-area LMO.

SnO2-reduced graphene oxide nanocomposite in the form of a free-standing film was prepared by simple chemical synthesis. The homogeneous and compact formation of the nanocomposite of SnO2 and reduced graphene oxide was confirmed by various analysis methods. When incorporated as anode in lithium-ion batteries, a high capacity (503 mAh g-1) and very stable cycle life were observed. These favorable properties probably arise from the efficient relaxation of high mechanical stress by the reduced graphene-oxide layers during the lithiation-delithiation process within SnO2. [Figure not available: see fulltext.] © 2015, The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht.