Ki-Tae Kim

Effects of Nanosized Adsorbing Material on Electrochemical Properties of Sulfur Cathodes for Li/S Secondary Batteries

In order to prevent polysulfide dissolution into liquid electrolytes and to promote the Li/S redox reaction (16Li + S 8 ↔ Li 2 Sn ↔ Li 2 S), nanosized Mg 0.6 Ni 0.4 O, which has the catalytic effect of chemical bond dissociating and is expected to have an adsorbing effect due to the effect of retaining liquid electrolyte of MgO in a Li/iron sulfide secondary battery, 16 was prepared by the sol-gel method as an electrochemically inactive additive for an elemental sulfur cathode for Li/S rechargeable batteries. The Li/S battery using an elemental sulfur cathode with a nanosized Mg 0.6 Ni 0.4 O added showed the improvement of not only the discharge capacity but also cycle durability (maximum discharge capacity: 1185 mAh/g sulfur, C 50 /C 1 = 85%).The rate capability of the sulfur cathode was also increased with the addition of the nanosized Mg 0.6 Ni 0.4 O. From the msults. it is confirmad that the nanosized Mg 0.6 Ni 0.4 O had the polysulfide adsorbing effect and the catalytic elfect of promoting Lt/S redox reaction. Furthermore, it is found that the nanosized Mg 0.6 Ni 0.4 O also increased the porosity of the sulfur cathode.

Improvement of the rate capability of LiMn2O4 by surface coating with LiCoO2

Abstract In order to use LiMn 2 O 4 as a cathode material of lithium-secondary battery for an electric vehicle (EV), its rate capability should be improved. To enhance the rate capability of LiMn 2 O 4 in this work, the surface of LiMn 2 O 4 particle was coated with LiCoO 2 by a sol–gel method. Because LiCoO 2 has a higher electric conductivity than LiMn 2 O 4 , it is possible to improve the rate capability of LiMn 2 O 4 . After the surface coating, LiCoO 2 -coated LiMn 2 O 4 showed a higher discharge capacity of 120xa0mAh/g than as-received LiMn 2 O 4 (115xa0mAh/g) because LiCoO 2 has a higher capacity than LiMn 2 O 4 . The rate capability of the coated LiMn 2 O 4 improved significantly. While as-received LiMn 2 O 4 maintained only 50% of its maximum capacity at a 20 C rate (2400xa0mA/g), the LiCoO 2 -coated LiMn 2 O 4 maintained more than 80% of maximum capacity. LiCoO 2 -coated LiMn 2 O 4 with 3xa0wt.% conducting agent (acetylene black) showed the higher rate capability than as-received LiMn 2 O 4 with 20xa0wt.% conducting agent. From electrochemical impedance spectroscopy (EIS) result that the first and second semicircles of coated LiMn 2 O 4 were reduced, the improvement of rate capability is attributed to a decrease of passivation film that acts as an electronic insulating layer and a reduced inter-particle contact resistance. Accordingly, It is proposed that the surface coating of LiMn 2 O 4 with LiCoO 2 improve the rate capability as well as the specific and volumetric energy density due to the decrease of conducting agent.

Improvement of Initial Coulombic Efficiency of Co3 O 4 by Ballmilling Using Ni as an Additive

Graphite has been widely used as the anode material of commercial Li-ion secondary batteries. With the growing demands of high-capacity secondary batteries, the low capacity of graphite has been looked upon as the limiting factor in wide applications and new anode materials with high capacity have been sought. Among many materials, Co 3 O 4 showed the best anode performance. It has high capacity of about 700 mAh/g and good cycle life (93.4% of initial capacity is maintained after 100 cycles). However, because its irreversible capacity comes to about 30% of initial capacity, the initial coulombic efficiency needs improvement in order to commercialize Co 3 O 4 as the anode material of Li secondary batteries. Because it was reported that Co 3 O 4 is cycled through the formation and decomposition of Li 2 O, a poor decomposition of Li 2 O at the first charge can be regarded as the cause of high initial irreversible capacity. Therefore, at first, addition of Ni to Co 3 O 4 was selected as a method to decrease the initial irreversible capacity, but it didnt have any effect. The ballmilling method was known to be very effective, connecting original material with additive material. As a result of ballmilling with Ni, Co 3 O 4 on which Ni is well dispersed was obtained, and as expected, its irreversible capacity was decreased from 30 to 20%.

Formation of nanosized organic molecular crystals on engineered surfaces.

The pharmaceutical industry has great interest in organic molecular nanosized crystals because their enhanced solubility and dissolution rate can potentially enhance drug bioavailability. In this work, patterned engineered surfaces were used to crystallize glycine with a lateral dimension below 200 nm in a confined volume while controlling supersaturation. Bifunctional patterned surfaces with hydrophilic islands, as small as 500 nm, surrounded by hydrophobic regions were prepared using lithography and self assembled monolayers. Individual glycine crystals under 200 nm were formed from the confined solutions wetting each hydrophilic island, while supersaturation was controlled by slow antisolvent diffusion. Individual crystals were characterized with AFM and Raman spectroscopy and determined to be the metastable beta form. The solubility enhancement as a function of crystal size was measured, and the solubility of crystals with a radius of 100 nm or less was estimated.

Crystal growth on self-assembled monolayers

This paper illustrates recent work in the field of crystal growth on self-assembled monolayers (SAMs). The ability to functionalize a surface with the desired properties and their highly ordered structure makes self-assembled monolayers attractive templates for nucleation and crystal growth. These properties could be surface chemistry that leads to specific intermolecular interactions with the nucleating plane of crystals thereby controlling their morphology and crystal form, hydrophobicity and hydrophilicity to create patterned bifunctional surfaces or chirality to create chiral surfaces. Our research group has used SAMs to control crystal morphology and for resolution of racemates. We have also used a SAMs based method to study the effect of the supersaturation generation rate on polymorph obtained, the effect of pH on polymorphic outcome, concomitant nucleation and for the creation of nano sized organic crystals.

Polymorphism control of nanosized glycine crystals on engineered surfaces

Crystallization in a constrained environment was used to prepare organic molecular nanocrystals of glycine. Bifunctional patterned surfaces, with hydrophilic islands as small as 1 µm surrounded by hydrophobic regions, were prepared by photolithography. Individual nanosized glycine crystals were formed from the confined solution droplets on each hydrophilic island. Supersaturation was controlled by slow cooling or by slow evaporation. Individual crystals were characterized with AFM and Raman spectroscopy. Slow cooling produced the least stable β-form except for the slowest cooling rate (0.001 °C min−1) which produced both α- and β-form. Slow evaporation (100 hours) resulted in the concomitant nucleation of all three glycine forms.

Solution synthesis of boron substituted LiMn2O4 spinel oxide for use in lithium rechargeable battery

An attempt has been made to synthesize LiMn2O4 spinel and boron substituted LiMn2O4 with atomic concentration of boron ranging from 0.01–0.20 and using glutaric acid as a chelating agent. The spinels have been characterized using PXRD, CV and galvanostatic charge-discharge studies. The precursor obtained from the glutaric acid assisted gel was calcined initially at 300 °C for 4 h to obtain the compound and finally at 800 °C for 4 h so as to obtain homogeneity, high degree of purity and crystallinity for better electrochemical performance. This paper suggests that glutaric acid assisted B3+ doped (LiBxMn2−xO4) spinel was found to be as an apt candidate with good electrochemical performance for use in lithium battery.

Malonic acid assisted sol-gel synthesis and characterization of chromium doped LiMn2O4 spinel

Pristine LiMn2O4 and LiCrxMn2-xO4 (x=0.01−0.20) have been synthesized by sol-gel method using malonic acid as chelating agent. This technique involves less impurities, shorter heat treatment time, sub-micron sized particles, good surface morphology, better homogeneity, good agglomeration and better crystallinity. The synthesized spinel materials have been characterized by XRD, SEM, TEM, EDAX and electrochemical studies like charge-discharge studies, cyclic voltammogram, cycleability studies have also been carried out. All the results exhibit that chromium substitution improves the structural stability of LiMn2O4 spinel upon repeated cycling.

The effects of dislocations on the trapping and transport of hydrogen in 3.3 Ni-1.6 Cr steel during plastic deformation

Abstract In order to make a clear distinction between hydrogen trapping and transport by dislocations, it is necessary to take the apparent diffusivity, Dapp, of hydrogen obtained from specimens involving approximately comparable dislocation structures and to compare them critically. The specimens in question should undergo plastic deformation differences only within a small range of strain to produce similar dislocation structures with different dislocation densities. The unstrained specimen, the 3.5 %-prior strained specimen and the 3.5 %-prior strained specimen, followed by additional straining at different strain rates have been prepared from 3.3 Ni - 1.6 Cr steel. The present static and dynamic hydrogen permeation experiments permitted us to measure the Dapp of hydrogen and correlate the results with the trapping and transport effects.

Hydrogen permeation through 3.3 Ni-1.6 Cr steel during plastic deformation

Determination de la contribution des dislocations au transport de lhydrogene et de la contribution des dislocations au piegeage de lhydrogene par utilisation dune technique de permeation et dune methode de detection electrochimique