Kyungbae Kim

Effects of alloying elements on the electrochemical characteristics of iron aluminides

Effects of alloying elements on the electrochemical characteristics of iron aluminides in the H2SO4, H2SO4+KSCN and HCl solutions were investigated using electrochemical tests. The corrosion morphologies in iron aluminides were analysed by utilising optical microscopy. It was found that the addition of Cr and Mo to iron aluminides increased the corrosion potential, pitting potential and repassivation potential. The active current density, passive current density and reactivation current density decreased as Cr and Mo were added. In the case of Mo addition, the passive current density was slightly higher in the H2SO4 solution than in solutions containing SCN- and Cl-. When B was added to samples, the corrosion potential and repassivation potential decreased, whereas the active current density, passive current density, reactivation current density and pitting potential increased. Iron aluminides containing Mo and Cr showed remarkably improved intergranular and pitting corrosion resistance to SCN- and Cl- solution. On the other hand, B addition accelerated granular and intergranular corrosion by precipitation of borides.

Mechanochemically Reduced SiO2 by Ti Incorporation as Lithium Storage Materials

This study presents a simple and effective method of reducing amorphous silica (a-SiO2 ) with Ti metal through high-energy mechanical milling for improving its reactivity when used as an anode material in lithium-ion batteries. Through thermodynamic calculations, it is determined that Ti metal can easily take oxygen atoms from a-SiO2 by forming a thermodynamically stable SiO2-x /TiOx composite, meaning that electrochemically inactive a-SiO2 is partially reduced by the addition of Ti metal powder during milling. This mechanically reduced SiO2-x /TiOx composite anode exhibits a greatly improved electrochemical reactivity, with a reversible capacity of more than 700 mAh g(-1) and excellent cycle performance over 100 cycles. Furthermore, an enhancement in the mechanical and thermal stability of the composite during cycling can be mainly attributed to the in situ formation of the SiO2-x /TiOx phase. These findings provide new insight into the rational design of robust, high-capacity, Si-based anode materials, as well as their reaction mechanism.

Anode Design Based on Microscale Porous Scaffolds for Advanced Lithium Ion Batteries

Considering the increasing demands for advanced power sources, present-day lithium-ion batteries (LIBs) must provide a higher energy and power density and better cycling stability than conventional LIBs. This study suggests a promising electrode design solution to this problem using Cu, Co, and Ti scaffolds with a microscale porous structure synthesized via freeze-casting. Co3O4 and TiO2 layers are uniformly formed on the Co and Ti scaffolds, respectively, through a simple thermal heat-treatment process, and a SnO2 layer is formed on the Cu scaffold through electroless plating and thermal oxidation. This paper characterizes and evaluates the physical and electrochemical properties of the proposed electrodes using scanning electron microscopy, four-point probe and coin-cell tests to confirm the feasibility of their potential use in LIBs.

Si-SiOx-Al2O3 nanocomposites as high-capacity anode materials for Li-ion batteries

Nanocrystalline Si-embedded SiOx-Al2O3 composite materials were synthesized by a high-energy mechanical milling method, and their potential as an anode material for Li-ion batteries was examined. The starting materials were amorphous SiO2 and Al metal powders. To increase the initial coulombic efficiency of the SiO2-based electrode materials, the amorphous SiO2 was reduced by Al. The reducing medium was decided by calculating the thermodynamic formation energy. During the highenergy milling process, SiO2 was partially reduced and Al was simultaneously oxidized to aluminum oxide, yielding nano Si-embedded composite. The composite was characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission microscopy. In electrochemical tests, the reversible capacity of the composite electrode was approximately 850 mAh g-1 with enhanced initial coulombic efficiency of 66%. This performance of the composite electrode was achieved not through carbon incorporation, but through the formation of Si-embedded nanocomposites.

Surface-controlled Nb2O5 nanoparticle networks for fast Li transport and storage

Hybrid supercapacitors are successfully introduced to reduce the gap between high-capacity battery electrodes and high-power capacitor electrodes in case of electrochemical energy storage devices. Niobium pentoxide (Nb2O5) has attracted great interest for hybrid supercapacitors because of its moderate capacity and excellent cycle performance. However, its low electronic conductivity is still a major problem. Carbon is usually incorporated to address this limitation. Here, we report the Nb2O5 nanoparticle networks to facilitate electronic transport via continuous connection of materials. Additionally, the high surface area of the nanoparticles is maintained. The Nb2O5 nanoparticle network was synthesized using a simple solvothermal reaction in organic media. The materials characterization was performed using X-ray diffraction analysis, and scanning and transmission electron microscopies. The charge storage mechanism of the synthesized Nb2O5 material was investigated by cyclic voltammetry. In galvanostatic charge–discharge tests, the synthesized Nb2O5 nanoparticle network electrode exhibited stable cycle performance and remarkable rate capability without carbon incorporation.

Bottom-up self-assembly of nano-netting cluster microspheres as high-performance lithium storage materials

We report an elaborately designed cluster nanostructure comprising a niobium pentoxide (Nb2O5) and germanium/germanium dioxide (Ge/GeO2) network for electrochemical Li storage devices. The three-dimensional cluster microspheres were prepared by a bottom-up self-assembly using a facile one-pot solvothermal synthesis method. Nb2O5 materials with a diameter of a few tens of nanometers were produced as the core structure in netting Ge/GeO2 nanomaterials, resulting in the formation of inverse close-packed cluster microspheres. In this structure, during repeated Li insertion and extraction cycles, the core Nb2O5 nanoparticles acted as a buffer medium for the volume changes of Li alloying-type Ge-based materials as well as stable Li storage materials with rapid Li+ transport kinetics. The netting Ge-based materials provided a high capacity and a pathway for rapid Li+ and electron transport. As a result, the microsphere electrode delivers a high specific capacity of ∼600 mA h g−1 and an excellent rate capability, with a long-term cycling stability of up to 1000 cycles. This material design concept can be applied to various other applications and could be used to fabricate electrochemical Li storage devices, e.g., Li-ion batteries and hybrid supercapacitors.