Si Hyoung Oh

Screening for Superoxide Reactivity in Li-O2 Batteries: Effect on Li2O2/LiOH Crystallization

Unraveling the fundamentals of Li-O(2) battery chemistry is crucial to develop practical cells with energy densities that could approach their high theoretical values. We report here a straightforward chemical approach that probes the outcome of the superoxide O(2)(-), thought to initiate the electrochemical processes in the cell. We show that this serves as a good measure of electrolyte and binder stability. Superoxide readily dehydrofluorinates polyvinylidene to give byproducts that react with catalysts to produce LiOH. The Li(2)O(2) product morphology is a function of these factors and can affect Li-O(2) cell performance. This methodology is widely applicable as a probe of other potential cell components.

Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide

Zn-ion batteries are emerging energy storage systems eligible for large-scale applications, such as electric vehicles. These batteries consist of totally environmentally-benign electrode materials and potentially manufactured very economically. Although Zn/α-MnO2 systems produce high energy densities of 225 Wh kg−1, larger than those of conventional Mg-ion batteries, they show significant capacity fading during long-term cycling and suffer from poor performance at high current rates. To solve these problems, the concrete reaction mechanism between α-MnO2 and zinc ions that occur on the cathode must be elucidated. Here, we report the intercalation mechanism of zinc ions into α-MnO2 during discharge, which involves a reversible phase transition of MnO2 from tunneled to layered polymorphs by electrochemical reactions. This transition is initiated by the dissolution of manganese from α-MnO2 during discharge process to form layered Zn-birnessite. The original tunneled structure is recovered by the incorporation of manganese ions back into the layers of Zn-birnessite during charge process.

Direct synthesis of electroactive mesoporous hydrous crystalline RuO2 templated by a cationic surfactant

We describe a direct low-temperature, liquid crystal surfactant templating and crystallization route to form quasi-ordered crystalline mesoporous RuO2·0.4H2O. Our method constitutes a direct approach to the crystalline oxide, constructed of nanosized metal oxide building blocks that are assembled to form thin walls. This leads to high surface areas (up to 250 m2 g−1). In our approach, cationic surfactants (i.e., hexadecyl-trimethylammonium chloride, C16TMA+Cl−) serve as pore-directing agents, which can participate in an indirect S+X−I+ interaction mediated by the chloride ion to coordinate a cationic ruthenium nitrosyl precursor. Gentle decomposition of the initially formed mesostructured metal cation–surfactant composite leads to crystallization of the wall structure. The promising electrochemical properties of porous RuO2·xH2O result from a more highly ordered structure and almost tripled surface area (190 m2 g−1) compared to that obtained from the nanocasting method. Crystalline mesoporous RuO2·0.4H2O exhibits high capacitance of 410 F g−1 and good rate capability, whereas the amorphous mesoporous RuO2·1.3H2O displays capacitance over 700 F g−1.

An open-framework iron fluoride and reduced graphene oxide nanocomposite as a high-capacity cathode material for Na-ion batteries

Cathode materials with high capacity and good stability for rechargeable Na-ion batteries (NIBs) are few in number. Here, we report a composite of electrochemically active iron fluoride hydrate and reduced graphene oxide (rGO) as a promising cathode material for NIBs. Phase-pure FeF3·0.5H2O is synthesized by a non-aqueous precipitation method and a composite with rGO is prepared to enhance the electrical conductivity. The encapsulation of FeF3·0.5H2O nanoparticles between the rGO layers results in a lightweight and stable electrode with a three-dimensional network. The composite material delivers a substantially enhanced discharge capacity of 266 mA h g−1 compared to 158 mA h g−1 of the bare FeF3·0.5H2O at a current density of 0.05 C. This composite also shows a stable cycle performance with a high capacity retention of >86% after 100 cycles, demonstrating its potential as a cathode material for NIBs.

Investigation of the Na Intercalation Mechanism into Nanosized V2O5/C Composite Cathode Material for Na-Ion Batteries

There is a significant interest to develop high-performance and cost-effective electrode materials for next-generation sodium ion batteries. Herein, we report a facile synthesis method for nanosized V2O5/C composite cathodes and their electrochemical performance as well as energy storage mechanism. The composite exhibits a discharge capacity of 255 mAh g(-1) at a current density of 0.05 C, which surpasses that of previously reported layered oxide materials. Furthermore, the electrode shows good rate capability; discharge capacity of 160 mAh g(-1) at a current density of 1 C. The reaction mechanism of V2O5 upon sodium insertion/extraction is investigated using ex situ X-ray diffraction (XRD) and synchrotron based near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Ex situ XRD result of the fully discharged state reveals the appearance of NaV2O5 as a major phase with minor Na2V2O5 phase. Upon insertion of sodium into the array of parallel ladders of V2O5, it was confirmed that lattice parameter of c is increased by 9.09%, corresponding to the increase in the unit-cell volume of 9.2%. NEXAFS results suggest that the charge compensation during de/sodiation process accompanied by the reversible changes in the oxidation state of vanadium (V(4+) ↔ V(5+)).

Structural Studies on the Effects of ZrO2 Coating on LiCoO2 during Cycling Using In Situ X-Ray Diffraction Technique

The effects of ZrO 2 -coating on the surface of LiCoO 2 have been studied using synchrotron-based in situ x-ray diffraction (XRD) technique. Although both ZrO 2 -coated LiCoO 2 and uncoated LiCoO 2 showed capacity fading during high voltage (4.8 V) cycling, the ZrO 2 coated one showed better cycle performance. After observing capacity fade for both samples through 10 charge-discharge cycles, in situ XRD patterns were collected during the 11th charge with reduced current density. With reduced current density, both samples restored part of their lost capacity. However, in situ XRD data shows that the ZrO 2 -coated LiCoO 2 sample has much better structural change behaviors than the uncoated LiCoO 2 . The partial restoration of the lost capacity and the recovered structural variations observed at reduced current density clearly indicates that the lost capacity is very closely related to the increased polarization, which is most likely due to the precipitation of the electrolyte decomposition products on the surface of LiCoO 2 cathode material. ZrO 2 coating layer may provide some protection for the LiCoO 2 cathode surface and help to reduce the electrolyte decomposition at higher voltages.

Critical Role of pH Evolution of Electrolyte in the Reaction Mechanism for Rechargeable Zinc Batteries.

The reaction mechanism of α-MnO2 having 2×2 tunnel structure with zinc ions in a zinc rechargeable battery, employing an aqueous zinc sulfate electrolyte, was investigated by in situ monitoring structural changes and water chemistry alterations during the reaction. Contrary to the conventional belief that zinc ions intercalate into the tunnels of α-MnO2 , we reveal that they actually precipitate in the form of layered zinc hydroxide sulfate (Zn4 (OH)6 (SO4 )⋅5 H2 O) on the α-MnO2 surface. This precipitation occurs because unstable trivalent manganese disproportionates and is dissolved in the electrolyte during the discharge process, resulting in a gradual increase in the pH value of the electrolyte. This causes zinc hydroxide sulfate to crystallize from the electrolyte on the electrode surface. During the charge process, the pH value of the electrolyte decreases due to recombination of manganese on the cathode, leading to dissolution of zinc hydroxide sulfate back into the electrolyte. An analogous phenomenon is also observed in todorokite, a manganese dioxide polymorph with 3×3 tunnel structure that is an indication for the critical role of pH changes of the electrolyte in the reaction mechanism of this battery system.

A conditioning-free magnesium chloride complex electrolyte for rechargeable magnesium batteries

The dissolution of Mg metal in AlCl3/THF using CrCl3 as a “promoter” yields a magnesium aluminum chloride complex electrolyte which shares many common features with MACC, but does not require an onerous conditioning process. This crucial advantage originates from the very high Mg to Al ratio in the new electrolyte, “MaCC”, which promotes 100% coulombic efficiency for Mg in the first cycle.

Sulfur/graphitic hollow carbon sphere nano-composite as a cathode material for high-power lithium-sulfur battery

The intrinsic low conductivity of sulfur which leads to a low performance at a high current rate is one of the most limiting factors for the commercialization of lithium-sulfur battery. Here, we present an easy and convenient method to synthesize a mono-dispersed hollow carbon sphere with a thin graphitic wall which can be utilized as a support with a good electrical conductivity for the preparation of sulfur/carbon nano-composite cathode. The hollow carbon sphere was prepared from the pyrolysis of the homogenous mixture of the mono-dispersed spherical silica and Fe-phthalocyanine powder in elevated temperature. The composite cathode was manufactured by infiltrating sulfur melt into the inner side of the graphitic wall. The electrochemical cycling shows a capacity of 425 mAh g−1 at 3 C current rate which is more than five times larger than that for the sulfur/carbon black nano-composite prepared by simple ball milling.

Evaluation of Thermal Liquid Junction Potential of Water-Filled External Ag/AgCl Reference Electrodes

Pressure-balanced external Ag/AgCl reference electrodes have been extensively used for corrosion monitoring in both pressurized water reactor and boiling water reactor environments. In order to prolong the electrode lifetime, pure water is often employed as the electrode filling solution. Characterization of the potential of the water-filled external Ag/AgCl reference electrode was performed by estimating a thermal liquid junction potential (TLJP) originating from the thermal diffusion of ionic species in the tilling solution. The potential of the thermoelectrochemical cell, Ag/AgCl vs. that of the standard hydrogen electrode at temperature T, was expressed as the sum of the isothermal potentials and TLJP. The TLJP was analyzed for the Soret steady state based on irreversible thermodynamics by calculating the heat of transport after Agar et als theory and estimating the limiting ionic conductance from Quist et als work. Calculated potential of the water-filled external reference electrode was compared with experimental data, showing a qualitative agreement.