Byoung-Koo Choi

Ionic conduction in PEO–PAN blend polymer electrolytes

Polymer electrolyte films consisting of polyethylene oxide (PEO), polyacrylonitrile (PAN), LiClO 4 and a mixture of ethylene carbonate (EC) and γ-butyrolactone (BL) were examined in order to obtain the best compromise between high conductivity, homogeneity, dimensional and electrochemical stability. Measurements of electrical conductivity. differential scanning calorimetry and linear sweep voltammetry have been carried out for films of (50 x)PEO xPAN-7LiClO 4 -43EC/BL and (50-x)PEO-xPAN-12LiClO 4 -38EC/BL with x = 10, 20, 30, 40 and 50. When the EO/AN ratio is lower, the PEO-rich crystalline domains are nearly absent. The PEO- PAN blend film is more likely to be a gel-electrolyte than a plasticized PEO-salt electrolyte. The Li + ions in these films seem to migrate primarily through the solvent domains as in the gel electrolytes. The highest room temperature conductivity of 1.2 × 10 -3 S cm -1 is found for a film of 10PEO-40PAN-12LiClO 4 -38EC/BL. The beneficial effects of PEO blending with PAN-based electrolytes are in comparable ionic conductivity, better dimensional stability and wider electrochemical stability.

Relation between glass transition and melting of PEO: salt complexes

Abstract Ionic conductivity and thermal properties on (PEO) 16 LiClO 4 complexes mixed with 29 different inert fillers have been studied. The ionic conductivity at 100°C, where the polymer electrolytes are mostly amorphous, does not change markedly, revealing that ceramic particles are almost inert. At room temperature, the ionic conductivity was more or less enhanced or reduced according to filler species, indicating that the ceramic particles give rise to morphological changes, thus modifying the crystallinity and PEO chain stiffness. We have examined whether any relations exist between various quantities such as the conductivity, particle size, glass transition temperature ( T g ), melting temperature ( T m ) of PEO, and heat of melting. For 30 different species, we found a correlation only between T g and T m . Among various factors, the size and morphology of spherulite and nonspherulitic amorphous materials were regarded to be of primary importance on the occurrence of the correlation.

Humidity-sensitive properties of a cross-linked polyelectrolyte prepared from mutually reactive copolymers

Two mutually reactive three component copolymers were synthesized to utilize as humidity-sensitive membranes. The major ingredient of a humid membrane is the cross-linked polyelectrolyte obtained from the copolymers 4-vinylpyridine (VP)–(methacryloxyethyl)trimethylammonium chloride (METAC)–n-butyl acrylate (BA) = 1 ∶ 2 ∶ 1, 2 ∶ 2 ∶ 1, 2 ∶ 3 ∶ 1 and 3 ∶ 2 ∶ 1, and 2-chloroethyl acrylate (CEA)–METAC–BA = 1 ∶ 2 ∶ 1, 2 ∶ 2 ∶ 1, 2 ∶ 3 ∶ 1 and 3 ∶ 2 ∶ 1. The humidity sensor prepared from the reaction of VP–METAC–BA = 3 ∶ 2 ∶ 1 with CEA–METAC–BA = 3 ∶ 2 ∶ 1 showed average impedance of 620, 41.9 and 3.38 kΩ at 30, 60 and 90% relative humidity humidity (RH), respectively. Temperature dependence, frequency dependence and response time were measured. The variation of humidity on temperature between 5 and 35 °C was −0.62 to −0.67% RH °C−1 and the hygrometric hysteresis between humidification and desiccation processes were less than ±2% RH. The humidity sensors using polyelectrolyte cross-linked were quite resistant to water.

Effects of SiC fillers on the electrical and mechanical properties of (PEO)16LiClO4 electrolytes

Abstract The influence of fine SiC fillers (1 μm) on ionic conductivity of (PEO) 16 LiClO 4 electrolytes has been studied using impedance spectroscopy and differential scanning calorimetry. Small enhancement in the conductivity is observed for samples having SiC concentrations below 15 wt.%. Two antagonistic effects on the conductivity are observed, i.e. the decrease of the glass transition temperature, which enhances polymer chain motion, and the increase of the volume fraction of the crystalline phase, which hinders the transport process. This observation suggests that it is the enhancement of the segmental motions of PEO that principally determines the ionic conductivity, while the increase in the conductivity is not large due to the increase of the crystalline phase. Measuring the thickness of specimens, the composite electrolyte is found to have a much better stiffness.

Humidity sensitive properties of alkoxysilane-crosslinked polyelectrolyte using sol-gel process

New trialkoxysilyl group-containing copolymers for humidity-sensitive polyelectrolytes were prepared by copolymerization of [2-(methacryloyloxy)ethyl]trimethyl ammonium chloride (METAC), 3-(trimethoxysilyl)propyl methacrylate (TSPM) and 2-ethylhexyl acrylate (2-EHA). They were self-crosslinkable copolymers composed of different contents of METAC/TSPM/2-EHA = 4/1/5 and 4/2/4. The resistance varied from 10(7) to 10(3)[capital Omega] between 20% RH and 95% RH, which was required for a humidity sensor operating at ambient humidity. Temperature dependence, hysteresis, response time, water durability and long-term stability at high temperature and humidity were also measured and estimated.

Humidity-sensitive properties of phosphonium salt-containing polyelectrolytes

New phosphonium salt-containing monomer, (vinylbenzyl) tributyl phosphonium chloride (1), was prepared and polymerized for a humidity-sensing film. The humidity-sensitive membranes were composed of copolymers with different contents of 1 and styrene (2) or n-butyl acrylate (3) (1/2 or 1/3 = 1/0, 4/1, 2/1 and 1/1). When the impedance dependencies on the relative humidity of the copolymers were measured, the impedance increased with an increase in the content of 2 or 3. The impedance ranged from 107 to 104 Ω between 20%RH and 95%RH, which was required for a humidity sensor operating at ambient humidity. Temperature dependence, frequency dependence, hysteresis and response time were also measured and reliability was evaluated.

Characterization of new polyacrylonitrile-co-bis[2-(2-methoxyethoxy)ethyl]itaconate based gel polymer electrolytes

Abstract Since the gel polymer electrolytes based on polyacrylonitrile (PAN) host have not been sufficient to fulfil the requirements as a separator of current lithium polymer battery, a new polymer host copolymerized with PAN, polyacrylonitrile-co-bis[2-(2-methoxyethoxy)ethyl]itaconate (abbreviated as PANI), was synthesized in expectation of enhanced trapping ability of liquid electrolytes. Electrical, electrochemical, thermal and mechanical studies have been carried out on PAN and PANI blended gel polymer electrolytes, complexed with ethylene carbonate (EC) and γ-butyrolactone (BL) containing LiClO 4 salt. The addition of PANI as a host polymer in the PAN-based gel polymer electrolytes has beneficial effects such as higher ionic conductivity, better thermal and electrochemical stabilities and enhanced ability of trapping organic solvent, possibly due to ion chelating ability of itaconate unit, though it shows less mechanical rigidity caused by amorphization of the PAN matrix.

High temperature phase transitions and thermal decomposition of KH2PO4 crystals

Abstract The Raman spectrum of KH 2 PO 4 has been measured from room temperature to 250 °C to obtain better microscopic understanding of the high-temperature phase transitions and the thermal decomposition, where the existence and nature of both effects have yet to be firmly established. We found that after the onset of the thermal decomposition near 175 °C, two successive high-temperature phase transitions occur at around 195 and 215 °C. Clear evidence is given that information about the onset and progress of the thermal decomposition can be obtained from additional Raman lines corresponding to vibrations of (PO 3 ) n polyanions. The thermal decomposition could be much suppressed by using a glass seal or by controlling heating conditions and, even in these cases, high-temperature transitions are also observed to occur, revealing that it can occur independently irrespective of the partial dehydration. The high-temperature phases could be stabilized at room temperature in vacuum, but they revert slowly to tetragonal KH 2 PO 4 in the open air within several days. Electrical conductivity measurements have also been carried out for several heating cycles with various heating rates.

Electrical conductivity and phase transition studies on K2SO4 crystals

Abstract The electrical conductivity measurements on single crystals of K 2 SO 4 have been carried out in the temperature range from 300 to 800°C. The bulk ionic conductivity along the three principal crystallographic axes has been determined by means of complex impedance formalism. The ionic conductivity in the low-temperature β phase follows closely the classical Arrhenius law up to around 540°C, which is some 50°C below the β → α transition temperature ( T c ). From 540°C, the slope of the conductivity curve increases up to T c and is attributed to the existence of a pretransitional region, in which significant rotational disorder of sulfate ions is present. The thermal hysteresis of theα ↔ β transition and the conductivity anisotropies are discussed. The dielectric properties and the conductivity relaxation studies by means of electric modulus formalism are also discussed.

Peculiarities of the structural phase transitions in Na2SO4(V): a Raman scattering study

The structural phase transitions occurring at the first heating of as-grown Na2SO4(V) are so peculiar that there have been many controversial reports about them, especially with regard to the possible existence of several intermediate phases (II–IV) between the room-temperature (V) and high-temperature (I) phases. From this first detailed study of polarized Raman spectra based on oriented single crystals, we have obtained new results regarding the existence and nature of intermediate phases between phases V and I. We normally found that single-crystal Na2SO4(V) transforms directly to phase I at 242 °C without any occurrence of an intermediate phase (type-1 transition), but occasionally it transforms to a mixed phase of II and III near 234 °C, and then the phase II regions transform to phase I near 240 °C, and finally the phase III regions transform to phase I near 250 °C (type-2 transition). These results were confirmed from single-crystal ionic conductivity measurements and thermal calorimetry studies. For polycrystalline powder, however, both of these two types of transition sequences occur simultaneously, resulting in a complex and sluggish behaviour with temperature change. Hence it is now evident for powdered samples why many authors have hitherto reported so many diverse and contradictory results regarding the structural transitions of phase V.