Pressure Dependence Of Ionic Conductivity Research Articles

Inflection Point in Pressure Dependence of Ionic Conductivity as a Fingerprint of Local Structure Formation.

In this study, we employed dielectric spectroscopy to investigate the effect of temperature and pressure on the ion dynamics of phosphonium ionic liquids (ILs) differing by the length of an alkyl chain, [P666,n][TFSI] (n = 2, 6, 8, 12). We found that both temperature and pressure dependence of dc-conductivity (σdc) determined for all examined ILs herein exhibit unique characteristics, unusual for aprotic ILs. Two regions differing by ion self-organization have been identified from the derivative analysis of σdc(T-1) data. On the other hand, isothermal measurements performed at elevated pressure revealed a unique concave-convex character of σdc(P) dependences, resulting in a clear minimum in the pressure behavior of activation volume. Such an inflection point characterizing the pressure dependence of σdc in [P666,n][TFSI] ILs can be considered an inherent feature of ion dynamics governed by structural self-assembly. Our results offer a unique perspective to link the ion mobility at various T-P conditions to the nanostructural organization of ionic systems.

Super Mg2+ Conductivity around 10-3 S cm-1 Observed in a Porous Metal-Organic Framework.

We first report a solid-state crystalline “Mg2+ conductor” showing a superionic conductivity of around 10–3 S cm–1 at ambient temperature, which was obtained using the pores of a metal–organic framework (MOF), MIL-101, as ion-conducting pathways. The MOF, MIL-101⊃{Mg(TFSI)2}1.6 (TFSI– = bis(trifluoromethanesulfonyl)imide), containing Mg2+ inside its pores, showed a superionic conductivity of 1.9 × 10–3 S cm–1 at room temperature (RT) (25 °C) under the optimal guest vapor (MeCN), which is the highest value among all Mg2+-containing crystalline compounds. The Mg2+ conductivity in the MOF was estimated to be 0.8 × 10–3 S cm–1 at RT, by determining the transport number of Mg2+ (tMg2+ = 0.41), which is the level as high as practical use for secondary battery. Measurements of adsorption isotherms, pressure dependence of ionic conductivity, and in situ Fourier transform infrared measurements revealed that the “super Mg2+ conductivity” is caused by the efficient migration of the Mg2+ carrier with the help of adsorbed guest molecules.

Influence of Stress Distribution on the Ionic Conductivity of a Sulfide All-Solid-State Lithium-Ion Battery

Introduction, Objective, MethodsIt is widely known that the ionic conductivity of a sulfide based solid electrolyte (SE) is enhanced with pressurization[1]. In order to take advantage of this property, sulfide all-solid-state lithium-ion battery (SASSLiB) is expected to be pressurized. The electrodes of SASSLiB are composite material constituted by different substances such as SE and active material (AM). Hence, when pressurized, a non-uniform stress distribution forms within the electrodes due to the unique Young’s moduli of the constituting substances[2]. Considering the pressure dependence of ionic conductivity observed in SE, a non-uniform stress distribution is likely to affect the net ionic conductivity of the electrodes.Objective of this study is to elucidate the effect of stress distribution on the ion transportation characteristics of an electrode layer in SASSLiB by combining experiment and numerical calculation. The experiment includes internal visualization by x-ray CT and electrochemical measurement of a simplified electrode consisting SE (Li10GeP2S12)[3] and electrochemically stable zirconia. To separate the effect of stress distribution from morphological factors such as volumetric fraction and tortuosity, structural data collected from x-ray CT was used to quantify the volume of non-SE regions and used to quantify tortuosity by numerical calculation employing random-walk algorithm. Additionally, voxel-finite-element-stress calculation and electric field calculation was combined to perform ion transportation simulation. To elaborate, stress distribution was expressed in two different calculation models: isotropic and anisotropic. The isotropic model assumes that the ionic conductivity at a particular voxel is dependent on the minimum principal stress value in all x, y, z directions. The anisotropic model assumes that the ionic conduction occurs anisotropically with different ionic conductivity corresponding to the voxel’s perpendicular stresses in x, y, z direction. Furthermore, the ion transportation simulation was conducted for cases with SE’s Young’s modulus set to 0.3 GPa and 21.7 GPa. The former value is experimentally measured under low pressure condition, and the latter is the value obtained by first principle calculation[4]. We have assumed that the true value of unknown and pressure dependent Young’s modulus of SE lies in between these two values.Result and DiscussionThe ionic conductivity of a composite made by SE and zirconia was significantly lower than that of SE-only sample. Comparing the ionic conductivities obtained experimentally and numerically, the discrepancy was large when only void ratio and tortuosity were considered. When stress distribution was additionally considered, discrepancy drastically decreased. Fig.1 and Fig.2 show the magnitude of discrepancy between experimental and numerical results when composite was under 100 MPa of pressure, and are results obtained by isotropic model and anisotropic model respectively. The points plotted are relative conductivity with respect to a mixture with 0% zirconia, and the points represented are results from the composite with zirconia fraction varying from 19% to 67% from left to right. The dotted line represents a complete alignment of experimental and numerical results, and closer a value is to this line, smaller the discrepancy. It is apparent from the two figures that in both models that there is a significant offset between the line of complete alignment and the case stress distribution was not considered. On the other hand, the line of complete alignment lies in between the results from numerical models obtained with respective Young’s modulus. This validates our assumption that the true Young’s modulus lies in between the values mentioned earlier. Additionally, this implies that by accounting for the non-linearity of stress-strain relationship of SE, the discrepancy between the line of complete alignment and the numerical results can further be decreased.The discrepancy between the line of complete alignment and the results excluding stress distribution increases as the volumetric fraction of zirconia increases. Additionally, the discrepancy between the numerical results of two different Young’s moduli increases as zirconia fraction increases. These increasing discrepancies with the increased zirconia fraction can be attributed to the decreasing stress in SE, instigating decrease of ionic conductivity, and in turn, widening the discrepancy.ConclusionPresence of a material with different Young’s modulus than that of SE alters the ion transportation characteristics of an electrode layer by forming a non-uniform stress distribution. And the impact of such stress distribution is much greater as the volumetric ratio of materials with high Young’s modulus increase. Therefore, stress distribution is an important factor to consider to accurately evaluate the ion transportation characteristics in the electrode layers of SASSLiB, especially as volume of non-SE materials, such as AM, increases. Figure 1

Negative activation volume of oxygen self-diffusion in forsterite

Oxygen self-diffusion coefficients (DOx) were measured in single crystals of dry synthetic iron-free olivine (forsterite, Mg2SiO4) at a temperature of 1600 K and under pressures in the range 10−4 to 13 GPa, using a Kawai-type multi-anvil apparatus and an ambient pressure furnace. Diffusion profiles were obtained by secondary ion mass spectrometry operating in depth profiling mode. DOx in forsterite increases with increasing pressure with an activation volume of −3.9 ± 1.2 cm3/mol. Although Mg is the fastest diffusing species in forsterite under low-pressure conditions, O is the fastest diffusing species at pressures greater than ∼10 GPa. Si is the slowest throughout the stable pressure range of forsterite. Based on the observed positive and negative pressure dependence of DOx and DMg (Mg self-diffusion coefficient), respectively, DOx + DMg in forsterite decreases with increasing pressure, and then increases slightly at pressures greater than 10 GPa. This behavior is in agreement with the pressure dependence of ionic conductivity in forsterite based on conductivity measurements (Yoshino et al., 2017), and can be used to explain the conductivity increase from ∼300 km depth to the bottom of the asthenosphere.

A molecular dynamics study and molecular level explanation of pressure dependence of ionic conductivity of potassium chloride in water

Experimental ionic conductivity of different alkali ions in water shows markedly different dependences on pressure. Existing theories such as that of Hubbard-Onsager are unable to explain these dependences on pressure of the ionic conductivity for all ions. We report molecular dynamics investigation of potassium chloride solution at low dilution in water at several pressures between 1 bar and 2 kbar. Two different potential models have been employed. One of the models successfully reproduces the experimentally observed trend in ionic conductivity of K(+) ions in water over the 0.001-2 kbar range. We also propose a theoretical explanation, albeit at a qualitative level, to account for the dependence of ionic conductivity on pressure in terms of the previously studied Levitation Effect. It also provides a microscopic picture in terms of the pore network in liquid water.

Pressure dependence of ionic conductivity of hydrated and dehydrated zeolites A

Hydrated and dehydrated zeolites MA (where M=Li, Na and K) with LTA structure have been studied by impedance spectroscopy with scanning frequency from 1 Hz to 1 MHz at high pressure up to 4.5 GPa and high temperature up to 250 °C. Anomalous increase in electrical AC conductivity at about 1.5–2 GPa observed in hydrated zeolites is associated with changes in crystalline structure leading to the formation of high-diffusion state of cation and water stuffing of the channels. In dehydrated zeolites, electrical conductivity is controlled by diffusion of cations (Li +, Na + and K +), which is determined by cation sites and aluminosilicate ring windows. LiA and NaA zeolites show normal decrease of conductivity with pressure, whereas KA zeolite exhibits the anomalous dependence with considerable increase and then fast decrease of conductivity. The behaviour of KA zeolite is associated with nearly central location of cation site in 8-membered ring, different from that in LiA and NaA zeolites.

Mode-coupling calculation of the anomalous pressure effect on the ionic conductivity of molten LiBr

The pressure dependence of the ionic conductivity of molten lithium bromide is calculated by the mode-coupling theory. The required equilibrium structure is obtained by the hypernetted-chain integral equation. The pressure dependence of the molar ionic conductivity shows a maximum value, which is consistent with an experiment. The conductivity slightly exceeds the value predicted by the Nernst–Einstein equation, which is also consistent with a molecular dynamics simulation and an experiment.

Ionic Conductivity of Quasi-One-Dimensional Ionic Conductor KTiOPO4(KTP) Single Crystal at High Pressure

The pressure dependence of ionic conductivity was carried out in a single crystal of KTiOPO 4 (KTP) to 2 GPa at room temperature in the frequency range from 100 Hz to 10 MHz. The samples were pressed by a piston-cylinder-type high-pressure vessel, and the pressure was monitored by using a manganin gage. The ionic conductivity decreased nonlinearly and monotonously with increasing pressure. The decrease of ionic conductivity would be mainly due to the increase of an activation energy for the ionic conduction. The increase of the activation energy was estimated to be 0.01 eV at 2 GPa. It is considered that the increase of the activation energy is due to the decrease of the size of “bottle necks” which are formed by oxygen atoms in the framework.

LITHIUM IONIC CONDUCTIVITY AT HIGH HYDROSTATIC PRESSURE

Ionic conductivity and activation volume of bulk sample and comprssed powder disc of amorphous lithium ionic conductor B2O3-0.7Li2O-0.7LiCl-xAl2O3 (x= 0.05, 0.15) in the hydros-tatie pressure range of 0.0001-1.23 GPa were investigated. We found that the peak of conductivity of compressed powder disc consisted of contact conductivity and bulk conductivity. For hydrostatic pressure dependence of ionic conductivity of the bulk sample, a preliminary microscopic explanation was obtained by considering the physical picture of ionic transport passages. In addition, we observed that decrease of the content of alumina lowers the pressure of conductivity maximum. Variation of ionic conductivity with pressure was measured after heating samples at different temperature or at 300℃ for 4-20 hours. The behaviors were due to the amorphous phase separation and successive crystallization process of the two amorphous phases.

Pressure Dependence of Ionic Conductivity in KCl, NaCl, KBr, and NaBr

The pressure dependence of the ionic conductivity of KCl, NaCl, KBr, and NaBr has been measured to 6 kbar in the intrinsic and extrinsic temperature regions. In the intrinsic region the conductivity deviates markedly from a simple exponential decrease with pressure. A detailed analysis, including contributions from anions and extrinsic cation vacancies, shows that pressure can suppress the intrinsic vacancies sufficiently to cause such a deviation. The analysis also shows that the uncertainties in the estimated activation volumes are greater than those determined in previous work. For KCl, NaCl, KBr, and NaBr crystals, respectively, the estimated activation volumes for motion of cation vacancies are 8 \ifmmode\pm\else\textpm\fi{} 1, 7 \ifmmode\pm\else\textpm\fi{} 1, 11 \ifmmode\pm\else\textpm\fi{} 1, and 8 \ifmmode\pm\else\textpm\fi{} 1 ${\mathrm{cm}}^{3}$/mole, and the activation volumes for formation of Schottky defects are 61 \ifmmode\pm\else\textpm\fi{} 9, 55 \ifmmode\pm\else\textpm\fi{} 9, 54 \ifmmode\pm\else\textpm\fi{} 9, and 44 \ifmmode\pm\else\textpm\fi{} 9 ${\mathrm{cm}}^{3}$/mole. The values for activation volumes of motion agree with predictions of the strain-energy model of Keyes. Schottky-defect-formation volumes are substantially larger than the molar volumes in KCl and NaCl, but in KBr and NaBr they are approximately equal.

The effect of hydrostatic pressure on the structure sensitive ionic conductivity in KCl

The carriers of the ionic current in alkali halides in the structure sensitive range are cation vacancies predominantly due to the presence of divalent cationic impurities dissolved substitutionally in the crystal. Conductivity of ‘pure’ (Harshaw) and ‘doped’ (10 −4 mole fraction of Sr ++) KCl has been measured in this range (50–300°C) at pressures up to 4 kbars. Measurements were made using argon in a conventional pressure system and applying standard electrometer techniques. The pressure vessel had electrical leads of high electrical resistance and low capacitance. Results obtained using Harshaw KCl show three distinct regions which can be identified as extrinsic unassociated, extrinsic associated, and precipitation regions. The volume of motion of free cation vacancies in KCl is found to be 8.6 (cm 3/mole) and the volume of association of a (divalent impurity-cation vacancy) pair 3.6 (cm 3/mole). The volume of solution of divalent impurities as obtained from measurements in the precipitation region increases from 6 to 10 (cm 3/mole) with decreasing temperature. The pressure dependence of the conductivity is in agreement with the qualitative features of the Stasiw-Teltow association theory, which should be applicable for the low concentration (about 3 ppm) of divalent impurities (in Harshaw KCl). Results on KCl ‘doped’ with 10 −4 mole fraction of Sr ++ show none of the structure seen in ‘pure’ KCl in the same temperature range. The behavior of conductivity in this material can be related to precipitation of the Sr ++ impurity. The results are compared with similar measurements by Pierce and Biermann on the same material at higher temperatures. The pressure dependence of ionic conductivity is discussed as a means of understanding processes operating at various temperatures and impurity concentrations. Finally, theoretical calculations of the activation volumes, especially the volume of motion, are proposed as a possible indication of the suitability of a particular form of the repulsive interionic potential.