Paddle-wheel versus percolation mechanism for cation transport in some sulphate phases☆ (original) (raw)
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Journal of Physics: Energy, 2020
The dependence of ionic transport on crystal orientations in NaSICON-type solid electrolytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal x-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along ...
The influence of multicomponent diffusion on crystal growth in electrolyte solutions
Chemical Engineering Science, 2001
Mass transfer phenomena in crystallizing solutes and the concentrations of impurities in solution and in the crystal have been studied based on Maxwell-Stefan's theory of multi-component di usion. The Maxwell-Stefan theory requires data on the driving force, i.e. the concentration gradient for each species. The e ect of the electrolyte mixture on the di usion coe cient of the crystallizing solute and the impurity solute was studied based on the Pitzer theory, which can be used to determine the activity coe cients of a solute in electrolyte mixtures. The studied crystallization systems were the continuous suspension crystallization of potassium sulfate with sodium sulfate as an impurity, and the crystal growth study using potassium nitrate as a single crystal with calcium nitrate as an impurity in the solution. These two crystallization systems have di erent charge types of electrolytes. The proposed model was applied in calculating the mass transfer coe cient between the crystallizing solute and the impurity solute. Also, the molar ux of the speciÿed species for the studied cases was estimated. ?
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ACS nano, 2017
Using molecular dynamics simulations, small-angle neutron scattering, and a variety of spectroscopic techniques, we evaluated the ion solvation and transport behaviors in aqueous electrolytes containing bis(trifluoromethanesulfonyl)imide. We discovered that, at high salt concentrations (from 10 to 21 mol/kg), a disproportion of cation solvation occurs, leading to a liquid structure of heterogeneous domains with a characteristic length scale of 1 to 2 nm. This unusual nano-heterogeneity effectively decouples cations from the Coulombic traps of anions and provides a 3D percolating lithium-water network, via which 40% of the lithium cations are liberated for fast ion transport even in concentration ranges traditionally considered too viscous. Due to such percolation networks, superconcentrated aqueous electrolytes are characterized by a high lithium-transference number (0.73), which is key to supporting an assortment of battery chemistries at high rate. The in-depth understanding of th...
New Insight on the Unusually High Ionic Mobility in Chevrel Phases
Chemistry of Materials, 2009
Chevrel Phases (CPs), M x Mo 6 T 8 (M ) metal, T ) S, Se) are unique materials, which allow for a fast and reversible insertion of various cations at RT. Earlier, CPs were divided into two major types: type I with large immobile cations, which block any ionic transport in the diffusion channels, and type II with small mobile cations. Our analysis of available experimental data shows that the transport behavior in CPs cannot be understood in the framework of the blocking concept; it is much more complex and includes: (i) apparent immobility of the large M cations like Pb 2+ , Sn 2+ , Ag + in the ternary phases, MMo 6 T 8 ; (ii) coupled M + M′ diffusion in the quaternary phases, M x M′ y Mo 6 T 8 , where both large and small cations can assist; (iii) cation trapping in the Mg-Mo 6 S 8 , Cd-Mo 6 S 8 , and Na-Mo 6 T 8 systems; (iv) a combination of low and high rate diffusion kinetics at the first and last intercalation stages, respectively, for the Cu-Mo 6 S 8 , Mn-Mo 6 S 8 , and Cd-Mo 6 Se 8 systems; and (v) a fast ionic transport for small cations like Ni 2+ , Zn 2+ , and Li + . A general structural approach (analysis of the polyhedral linkage in the diffusion channels of CPs and mapping of all the cation sites combined with their bond valence sum values and the distances from the adjacent Mo atoms) used for the first time for a variety of CPs shows two competing diffusion pathways of inserted ions for most of CPs: circular motion within the same cavity between the Mo 6 T 8 blocks with activation energy E c , and progressive diffusion from one cavity to the adjacent one with activation energy E d . The character of the ionic transport depends mostly on the distribution of the repulsive forces for the inserted cations, as well as on the E d /E c ratio, affected in turn by the cation position, its size, cation-Mo interactions, and the anion nature.
Journal of Inorganic and Nuclear Chemistry, 1978
The specific conductance of zirconium phosphate decreases considerably with increase in the degree of crystallinity; however the energy of activation for the conduction apparently does not depend on the degree of crystallinity and is surprisingly low (ll-13KJ/mole). In order to explain these results, different samples of Zr(HPO4)2'H20 of the same degree of crystaUinity but having different specific surface area were obtained by sedimentation and the amount of surface counter-ions per cm ~ of microcrystals and the specific conductance measured. It was found that a large fraction of the total current was transported by the surface counter-ions, their mobility being ~>104 times that of the internal ones. Thus, the low activation energy for ionic conduction in the crystalline material is due essentially to the transport of surface counter-ions. These results demonstrate that the counter-ions present at the surface of microcrystals of zirconium phosphate make an important contribution to the total conduction, to the activation energy, and to the electrochemical properties of membranes consisting of microcystals of zirconium phosphate.
Solid State Ionics, 1991
Ionic conductivity of a-Li2SO4 with up to 4 mol% Li2WO4 has been measured using quartz capillary U-tubes and complex impedance technique. A significant drop in conductivity and increase in activation energy is observed for 2, 2.5, 3 and 3.5 mol% Li2WO4 compositions while 4 tool% LizWO4 composition shows an increased conductivity. Based on a recent phase diagram of the binary system, the results are interpreted as convincing evidence for the "'Paddle Wheel" mechanism of ion transport in ct-Li2SO4.
Novel Aspects of the Conduction Mechanisms of Electrolytes Containing Tetrahedral Moieties
Fuel Cells, 2010
modelling studies on a range of systems containing tetrahedral moieties are presented, including apatite-type La 9.33+x Ge 6 O 26+3x/2 , cuspidine-type La 4 Ga 2-x Ti x O 9+x/2 and La 1-x Ba 1+x GaO 4-x/2 . The type of anion defect (vacancy or interstitial), their location and the factors affecting their incorporation are discussed. In addition, modelling data to help to understand their conduction mechanisms are presented, showing novel aspects including the important role of the tetrahedral moieties in the conduction processes.