Diffusion Coefficients Of Cations Research Articles

A deep insight into the dynamic crystallization of coal slags and the correlation with melt microstructure

In liquid discharging furnace, crystallization can occur in slags during the cooling process. The presence of crystal changes the structure, flow, heat transfer, especially the viscosity with a sharp increase. A deep understanding of the crystal kinetics is significant to optimize the flow of liquid slag. Crystal kinetics varies significantly with different slags due to the complexity and variability of the multi components in slags. In this paper, a high-temperature microscopy with high-resolution is used to clearly observe the in situ precipitation of different crystals and the kinetic parameters of different crystals are analyzed. Microscopic structure analysis of both the melt and the precipitated crystal shows that the proportion of basic oxygen structure and the diffusion coefficient of the basic cation in the melt have a direct correlation with the growth of crystal. A structural parameter St of the melt is developed, which has a positive correlation with the crystal growth rate. This is a new discovery in bridging the gap between the melt and precipitated crystals and it provides a way to control the crystal growth during slag cooling.

Diffusion kinetics of ionic charge carriers across Ti3C2Tx MXene-aqueous electrochemical interfaces

Titanium carbide MXene -Ti3C2Tx - is a representative and well-employed electrode material for a variety of electrochemical energy storage applications. Despite exhibiting metal-like electronic conductivity, restacked Ti3C2Tx films show mediocre rate performance. Here, we report on the estimation of diffusion coefficients of aqueous cations (H+, Li+, Na+and Ca+2) across Ti3C2Tx MXene interlayer spaces by employing electrochemical impedance spectroscopy. Proton diffusivity across 2D slits is found to be 2.2 * 10−8 ± 0.4 * 10−8 cm2/s, which is three orders of magnitude slower than in bulk water. Such a sluggish diffusive behavior is attributed to interfacial confinement effects in the 2D space. It is found that protons diffuse three times faster than hydrated metal cations (Li+, Na+and Ca+2) through Ti3C2Tx slits and exhibit strong potential dependent behavior. Therefore, the rate performance of Ti3C2Tx MXene electrodes is limited by the ionic transportation in the 2D slits rather than electronic transport. This study provides insights into the engineering of open 2D electrode architectures for better ionic diffusive pathways towards the design of high rate MXene based energy storage devices.

Insights into the structure and Ion transport of pectin-[BMIM][PF6] electrolytes.

We investigate the effect of pectin on the structure and ion transport properties of the room-temperature ionic liquid electrolyte 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) using molecular dynamics simulations. We find that pectin induces intriguing structural changes in the electrolyte that disrupt large ionic aggregates and promote the formation of smaller ionic clusters, which is a promising finding for ionic conductivity. Due to pectin in [BMIM][PF6] electrolytes, the diffusion coefficient of cations and anions is observed to decrease by a factor of four for a loading of 25 wt. % of pectin in [BMIM][PF6] electrolyte. A strong correlation between the ionic diffusivities (D) and ion-pair relaxation timescales (τc) is observed such that D ∼ τc-0.75 for cations and D ∼ τc-0.82 for anions. The relaxation timescale exponents indicate that the ion transport mechanisms in pectin-[BMIM][PF6] electrolytes are slightly distinct from those found in neat [BMIM][PF6] electrolytes (D∼τc-1). Since pectin marginally affects ionic diffusivities at the gain of smaller ionic aggregates and viscosity, our results suggest that pectin-ionic liquid electrolytes offer improved properties for battery applications, including ionic conductivity, mechanical stability, and biodegradability.

Relevance of diffuse-layer, Stern-layer and interlayers for diffusion in clays: A new model and its application to Na, Sr, and Cs data in bentonite

The diffusive flux of cations is enhanced and that of anions is decreased compared to that of water tracers in bentonite or other clays, as a result of electrostatic interactions between ions and the charged clay surfaces. Clays are often used or foreseen as barriers in waste disposal in order to protect the environment from hazardous materials. A consistent description of diffusion of various ions and uncharged species is important in this context, especially if long-term interactions between clays and other materials shall be predicted by reactive transport simulations. Here, diffusion of a suite of tracers (HTO, Cl, Na, Sr and Cs) in bentonite was investigated. Experimental through-diffusion data at different bentonite dry densities were described with models of increasing complexity. First, ‘standard’ empirical single ion transport models (uncoupled, simple Fickian diffusion) were applied for each density. These models served as reference cases for comparisons. For sorbing tracers (Na, Sr, Cs), surface diffusion models were used in a next step, where average surface mobilities for the cations were determined based on comparisons with transport parameters from HTO diffusion. Finally, a more complex model was developed in order to describe anion and cation diffusion in a coupled way. This model accounts for locally parallel diffusion in different environments, namely in ‘free’ water unaffected by surface charges, in diffuse (Donnan) layer water, within the Stern layer, and within interlayer water (D-S-I model). This coupled model requires additional parameters related to the bentonite microstructure as well as to cation mobilities in the Stern layer and in the interlayer. The latter were taken from literature. Microstructural parameters were constrained in a manner that overall anion exclusion matches anion accessible porosities found by the simple Fickian diffusion model. This was possible with a reasonable choice of microstructure parameters that are consistent with literature values. A good agreement between the experimental data and the simulated cation diffusion coefficients of the D-S-I model was found, using constraints by the HTO and Cl diffusion data. The interlayer pathway was found to be most important for diffusion of Na, Sr and Cs through bentonite. Stern layer diffusion was significant and more important than diffusion through the diffuse layer for Cs and Sr, while for Na the two pathways were of equal importance.

Cation–polymer interactions and local heterogeneity determine the relative order of alkali cation diffusion coefficients in PEGDA hydrogels

Current research efforts are focused on endowing polymer membranes with ion–ion selectivity by incorporating ion–polymer interactions into materials to bias the selective partitioning and or diffusivity of one species over another. However, little is known about the impact of such interactions on the mechanisms of ion transport. In this study, we probe the influence of cation–polymer interactions on cation, anion, and salt diffusivity in a model membrane material, poly(ethylene glycol) diacrylate (PEGDA) by modeling concentrated polyethylene oxide solutions via molecular dynamics simulations. These results are compared to published experimental data for LiCl, NaCl, and KCl diffusion in PEGDA. Experimentally, the order of salt and cation diffusion coefficients for LiCl, NaCl, and KCl deviate from the order in aqueous solutions. Simulations identify these deviations to arise from cation–polymer coordination in the membrane. Both the fraction of bound cations and the average binding lifetime increases with decreasing cation hydration free energy (moving down the alkali series), leading to different diffusivity trends in the membrane compared to solution. However, to recover the experimentally observed order of diffusivities cations and salt in our simulations, we needed to incorporate membrane heterogeneity explicitly via a polymer charge scaling procedure. Together, our results indicate that cation–polymer interactions, as well as spatial heterogeneity within the membrane, play a critical role in dictating the observed order of alkali cation and salt diffusion coefficients in membranes.

Corrosion of Low-Carbon Steel in a Flow of Phosphoric Acid Solution Containing Iron(III) Phosphate

Theoretical aspects of low-carbon steel corrosion in H3PO4 solutions containing FePO4 are considered. In the system under study, reactions of iron with the acid solution and Fe(III) salt are thermodynamically allowed. The oxidizing power of this medium, characterized by the Fe(III)/Fe(II) couple redox potential, is mainly determined by its anionic composition. Phosphate anions of a corrosive medium bind Fe(III) cations into complex compounds, reducing their oxidizing ability. In H3PO4 solutions containing FePO4 and Fe3(PO4)2, the dependence of the system’s redox potential on the Fe(III) and Fe(II) cation relative content is poorly described by the Nernst equation, which is due to the nonequivalent complex formation of these cations with phosphate anions. Analysis of the effect of the studied media convection on the low-carbon steel electrode reactions allowed revealing some of their features. In a FePO4-containing H3PO4 solution, kinetically controlled partial reactions of iron anodic ionization and H+ cathodic reduction, as well as diffusion-controlled Fe(III) cation cathodic reduction, occur on the steel. The FePO4 accelerating effect on the steel corrosion in H3PO4 solution is due only to the Fe(III) reduction but does not affect the H+ reduction and the iron ionization. The value of the Fe(III)-cation diffusion coefficient in the studied corrosive medium was experimentally determined from the data of cyclic voltammetry of the Pt electrode therein and the results of the studying of the cathodic reaction of a steel disk electrode at different rotation velocities. The data on the low-carbon steel corrosion in the flow of the studied media, obtained from the metal samples mass loss, are in full agreement with the results of the study of the electrode partial reactions. An accelerating effect of FePO4 on the steel corrosion in H3PO4 solutions is observed. In this environment, steel corrosion is determined by the convective factor, which is typical of processes with diffusion control. The empirical dependence of the steel corrosion rate on the medium flow intensity is described by the linear dependence k = kst + λw1/2, where kst is the steel corrosion rate in a static medium, w is the rotation velocity of the propeller stirrer that creates the medium flow, λ is the empirical coefficient.

Al2O3 interfacial layer derived hybrid conductive filament for the reliability enhancement of Ta2O5-based resistive random access memory

This study used an Al2O3 interfacial layer to improve the reliability characteristics of Ag/Ta2O5/Au resistive random access memory (RRAM), such as endurance and retention. The Ag/Ta2O5/Au device without the Al2O3 interfacial layer exhibited a high on/off ratio, low endurance, and poor retention. In Ag/Ta2O5/Au devices, it is difficult to precisely control the formation and rupture of conductive filaments (CFs) owing to the high diffusion coefficient of Ag cations. However, in the RS mechanism of Ag/Ta2O5/Au with an Al2O3 interfacial layer, a hybrid CF composed of Ag cations in Ta2O5 and oxygen vacancies in the Al2O3 layer was formed during the set process. During the reset process, the hybrid CF was stably ruptured by the recombination of oxygen vacancies and oxygen ions at the Al2O3 interfacial layer. This RS mechanism induces stable switching characteristics, improving endurance and retention. Therefore, Ag/Ta2O5/Au with an Al2O3 interfacial layer exhibits relatively high endurance (up to 700 cycles) and excellent retention (up to 1.0 × 104 s) while maintaining a high on/off ratio (>106) compared to those of the Ag/Ta2O5/Au device.

Li+ intercalation current generation in amorphous and crystalline MoS2: Experiment and theory

In this work, crystalline and amorphous nano-MoS2 materials for effective Li+-intercalation current generation have been synthesized. The effect of disordering on the structural and electrochemical properties of nano-MoS2 has been systematically investigated by multiple characterizations: transmission electron microscopy (TEM), X-ray diffraction method and electrochemical measurements. Thermodynamic and kinetic peculiarities of intercalation processes have been studied. Dependences of the change in Gibbs’ free energy of the intercalation reaction on the extent of ‘guest’ lithium loading are analyzed. The distinctive feature of disordered structures is their ability to show colossal lithium ‘guest’ load, which ensures the specific capacity of the material in cathode processes up to ~2500 mAh·g-1 under the discharge not less than 2.6 V relative to lithium. The diffusion coefficient of lithium cations in the structure of disordered nano-MoS2 is two orders of magnitude higher than that in the crystalline MoS2. A quantum-mechanical model of the observed phenomena is suggested.

Impedance response of electrochemical interfaces. III. Fingerprints of couplings between interfacial electron transfer reaction and electrolyte-phase ion transport.

Electron transfer (ET), electric double layer (EDL) charging, and ion transport (IT) are three elementary physicochemical processes in electrochemistry. These processes are coupled with each other in the way that the local reaction environment for the ET is shaped by EDL charging, which is nothing but IT in a nanoscale nonelectroneutral region. Herein, we investigate fingerprints of the coupling between these processes in electrochemical impedance spectroscopy. EDL charging and IT are described uniformly using the Poisson-Nernst-Planck theory, and interfacial ET is described using the Frumkin-Butler-Volmer theory. Different diffusion coefficients for cations and anions (D+ ≠ D-) are considered. Exact analytical expressions are obtained when the potential of zero charge (Epzc), the equilibrium potential of the reaction (Eeq), and electrode potential (EM) are equal. The analytical solution shows that a decoupling treatment is valid only for the case of D+ = D-. Using a new scheme of calculating impedance response at any electrode potential, we observe an inductive loop in the low frequency range, which is a clear impedance fingerprint of the coupling effects.

(Digital Presentation) Progress Towards the Key Factors Governing Pit Stability

The pit stability product—defined as the product of pit depth and dissolution current density (x·i)—was first introduced by Galvele as the criterion indicating the conditions that sustain a critically acidic solution within the pit. The one-dimensional (1D) electrode is the most commonly used experimental configuration for determining the pit solution chemistry and pit stability product. In our current works, a ‘sandwich’ like 1D pit electrode was developed, which enabled the in-situ and ex-situ visualization of pit depth. A methodology that avoids lacy cover formation was proposed to pre-dissolve a Type 316L stainless steel (SS) 1D electrode to specific depths. Based on the new methodology and mathematical models, it was found that 1D Fick’s law of diffusion could not be used to estimate the pit stability product under a salt film because electro-migration had a measurable contribution (i.e., 67%) to the dissolution current. Although the diffusion coefficient of metal cations decreased with an increasing concentration inside the pit, it could be replaced by a constant diffusion coefficient to estimate the pit stability product, defined as an equivalent diffusion coefficient in our work. Later, a more comprehensive mathematical model, which included hydrolysis reactions and activity, was built to predict the local chemistry within the pit cavity. Recently, a galvanostatic method was developed to estimate the critical pit stability product of Type 316L SS, which was between 0.8 and 0.9 A/m in 0.6 M NaCl at 25°C, findings that were also confirmed by mass transport modelling. Here, we will present the proposed improved theoretical framework and discuss the next steps in modelling the propagation of pits in stainless steels.

Study of tetraalkylammonium salts in acetonitrile solutions: Transport properties, density, thermal expansion and phase transitions

The electrical conductivity of binary solutions of tetraethylammonium trifluoromethanesulfonate ([TEA][OTf]), tetraethylammonium tetrafluoroborate ([TEA][BF4]) and spiro-(1,1′)-bipyrrolidinium tetrafluoroborate ([SBP][BF4]) in acetonitrile was measured at atmospheric pressure in the temperature range of 293.15–348.15 K for different salt mole fractions. The highest electrical conductivities of the acetonitrile solutions of [TEA][BF4], [SBP][BF4] and [TEA][OTf] were 96 ± 7, 94.5 ± 0.5 and 71.6 ± 0.5 mS·cm−1, respectively, at 348.15 K. The activation energy of the conductivity for nonassociating electrolytes was calculated using the Arrhenius equation, and the activation energy of the ionic conductivity of glass-forming liquids was calculated using the Vogel-Fulcher-Tamman equation. The cation and anion diffusion coefficients in the [TEA][OTf], [TEA][BF4] and [SBP][BF4] solutions in acetonitrile were determined by the robust pulsed-gradient stimulated echo sequence technique. Experimental density data were used to calculate the isobaric thermal expansion coefficients in the temperature range of 293.15–348.15 K. The thermophysical properties (the temperatures and heats of the phase transitions and the melting temperatures) of the pure salts and their binary solutions in acetonitrile were determined by differential scanning calorimetry.

Reactive flash sintering in a bilayer of zirconia and lanthana: Measurement of the diffusion coefficient in real time

AbstractReactive flash sintering is a process where off‐the‐shelf powders of elemental oxides can be simultaneously sintered and reacted to form a multicomponent oxide in a matter of seconds at low furnace temperatures. The fact that several cation species, each residing within their own particles, can migrate over distances of several micrometers, and mix on the atomic scale to form multicomponent oxides, so quickly, is quite remarkable. The question arises as to the rate of this solid‐state diffusion phenomenon. In this paper, we present measurements of this diffusion coefficient from live flash experiments. The results are obtained from millimeter scale bilayers of yttria‐stabilized zirconia and lanthana where the flash initiates in the zirconia layer and then migrates into the lanthana layer, forming lanthanum zirconates. The velocities of migration of the flash‐front, coupled with measurements of the length scale of the profile of zirconium and lanthanum interdiffusion, across the bilayer interface, provide an estimate of the effective diffusion coefficient. These measurements give a value for the cation diffusion to lie in the range of 2.5 × 10−10 m2 s−1 at 1380°C, with an activation energy of 200–250 kJ mol−1. In comparison, the cation diffusion coefficient in yttria‐stabilized zirconia, at 1350°C, is stated to be 1.1 × 10−20 m2 s−1 with an activation energy of ∼550 kJ mol−1. A pause for reflection.

Thermodynamic and Kinetic Analyses of Ion Intercalation/Deintercalation Using Different Temperatures on NiHCF Electrodes for Battery Electrode Deionization.

Prussian blue analogues are used in electrochemical deionization due to their cation sorption capabilities and ion selectivity properties. Elucidating the fundamental mechanisms underlying intercalation/deintercalation is important for the development of ion-selective electrodes. We examined the thermodynamic and kinetic properties of nickel hexacyanoferrate electrodes by studying different temperatures effects on intercalation/deintercalation with monovalent ions (Li+, Na+, K+, and NH4+) relevant to battery electrode deionization applications. Higher temperatures reduced the interfacial charge transfer resistance and increased the diffusion coefficient of cations in the solid material. Ion transport in the solid material, rather than interfacial charge transfer, was found to be the rate-controlling step, as shown by higher activation energies for ion transport (e.g., 31 ± 3 kJ/mol for K+) than for interfacial charge transfer (5 ± 1 kJ/mol for K+). The largest increase in cation adsorption capacity with temperature was observed for NH4+ (28.1% from 15 to 75 °C) due to its smallest activation energy. These results indicate that ion hydration energy determines the intercalation potential and activation energies of ion transport in solid material control intercalation/deintercalation rate. Together with the endothermic behavior of deintercalation and exothermic behavior of intercalation, the higher operating temperature results in improvement of ion adsorption capacity depending on specific cations.

Stability-Guided Strategies to Mitigate Dendritic Growth in Lithium-Metal Batteries

Dendritic growth is a leading cause of degradation and catastrophic failure of lithium-metal batteries. Deep understanding of this phenomenon would facilitate the design of strategies to reduce, or completely suppress, the instabilities characterizing electrodeposition on the lithium anode. We present a linear-stability analysis, which utilizes the Poisson-Nernst-Planck equations to describe Li-ion transport and, crucially, accounts for the lack of electroneutrality. This allows us to investigate the impact of electric-field gradients near the electrode surface on both ion diffusion and its anisotropy. Our analysis indicates that the use of anisotropic electrolytes (i.e., electrolytes with anisotropic diffusion coefficients of the Li ions) and the control of the local electric field can suppress dendritic growth of lithium metal. Specifically, changes in the local electric field can be used to enhance the longitudinal (perpendicular to the electrode) component of the cation diffusion coefficient tensor, which decreases the maximum growth rate of the dendrites. Electrolytes with electric field-dependent diffusion coefficients would reduce dendritic growth in small batteries, while anisotropic electrolytes (or separators with anisotropic pore structures or columnized membranes) are appropriate for batteries of any size.

Diffusions in Aqueous Solutions with Multivalent Cations and Especially in Cationic First Hydration Shell.

Compared with univalent cationic diffusion, little is known about the diffusion behavior of multivalent cations let alone the diffusion of water in their first hydration shell. Here, we show that all published translational diffusion coefficients of multivalent cations and water measured at room temperature exhibit the same concentration dependence when plotted as a function of the mass fraction of free water or of hydrated solute. This behavior is held until only hydration and confined water remain in solutions, wherein their concentration dependences become cationic charge number-dependent. We also found that the iceberg model can well describe the diffusions of multivalent cations with decreasing water content until only hydration water is present. However, 1H-pulsed-field-gradient nuclear magnetic resonance measurements confirmed that 1H in the first hydration shell diffuses faster than Al3+ at room temperature and they have the same diffusion coefficient only with decreasing the temperature down to about 243 K. Therefore, iceberg model only equivalently describes the effect of strong ion-water interaction on multivalent cationic diffusion. These results will also help us reconceive our current understanding of the pathway for hydration water affecting the diffusion behavior of solutes with relatively weak solute-water interactions.

Ion Transport in the EMITFSI/PVDF System at Different Temperatures: A Molecular Dynamics Simulation.

We used all-atom molecular dynamics simulations to study the ion transport in the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/poly(vinylidene fluoride) (EMITFSI/PVDF) system with 40.05 wt % EMITFSI at different temperatures. The glass-transition temperature (Tg = 204 K) of this system shows a good agreement with the experimental value (200 K). With the increase of temperature, the peaks of the pair correlation function show an increasing trend. Interestingly, the coordination numbers of ion pairs and the degree of independent ion motion are mainly affected by the binding energy between ion pairs as the temperature increases. In addition, the ion transport properties with increasing temperature can be studied by the ion-pair relaxation times, ion-pair lifetimes, and diffusion coefficients. The simulation results illustrate that the ion transport is intensified. Especially, the cations can always diffuse faster than the anions. The power law shows that mobilities of anions and cations are seen to exhibit a “superionic” behavior. With the increase of temperature, transference numbers of anions decrease first and then increase and transference numbers of cations show the opposite changes; ionic conductivity increases gradually; and viscosity decreases gradually, indicating that the diffusion resistance of ions decreases. In general, after adding PVDF into the EMITFSI system, the glass-transition temperature and viscosity increase, the ionic conductivity and degree of independent ion motion decrease, and diffusion coefficients of cations decrease faster than those of the anions.

Infiltration of salt solutions through illite particles: Effect of nanochannel size and cation type

Water infiltration through soil particles is of great importance in determining the availability of elements and soil texture. However, process of water infiltrate into the capillary nanochannels and influence of nanochannel size and cation type as well as the underlying mechanism are still lacking, especially at the molecular level. Here, molecular dynamics simulation was used to investigate the infiltration of NaCl and CaCl2 solutions through 1.5, 3, 5, and 10 nm illite channels. At the initial stage, obvious regression of the advanced frontier of salt solution was observed. Water molecules and cations easily accumulated at the entrance of nanochannels, which was clearer in small-sized nanochannels. The infiltration velocity increased with increasing nanochannel size, though time for water saturating the illite nanochannel showed an opposite trend. Both Na+ and Ca2+ exhibited obvious inner-sphere adsorption during the infiltration, but Ca2+ showed more outer-sphere adsorption compared to Na+. Diffusion coefficients of both water and cations were higher in the small- than large-sized nanochannel, which was related to the unsaturated condition, capillary force, and interplay between illite-water interaction and surface tension. By using MD method, this study is the first time, to our knowledge, to investigate the unsaturated capillary infiltration of water in clay soil nanochannels, which is hard to achieve using traditional experimental tools. Results showed that, besides the generally considered interaction between soil particles, cation-clay interactions coupled with nanochannel size were of great importance in affecting the unsaturated infiltration of water and cations. This study provides a new clue in understanding the availability of soil water and stability of soil aggregates.

Facilitating Mg2+ diffusion in high potential LixV2(PO4)3 cathode material with a co-insertion strategy for rechargeable Mg-ion batteries

Rechargeable Mg-ion batteries offer high energy density and good safety. However, the lack of cathode materials with high operating potential and considerable capacity is a great challenge for their practical applications. This is limited by the sluggish diffusion of multivalent Mg 2+ cations in solid lattices. Here, we propose a co-insertion strategy to facilitate cation diffusions and present a polyanion cathode of Li 3 V 2 (PO 4 ) 3 to provide high redox potential. The initial charge of the material is associated with the extraction of two Li + cations, which are co-inserted with Mg 2+ from the 0.5 m Mg(ClO 4 ) 2 /PC electrolyte during the following electrochemical processes. The cation diffusion coefficients are largely enhanced in comparison to the electrode without Li + co-insertion, and higher capacity is obtained. At room temperature, the Li 3 V 2 (PO 4 ) 3 electrode delivers 124 mAh g −1 capacity at 100 mA g −1 with discharge plateaus at 0.46 V and −0.44 V vs. Ag/Ag + . A capacity retention of 80% is realized after 300 cycles at 500 mA g −1 . A rocking-chair Mg-ion full cell coupling the Li 3 V 2 (PO 4 ) 3 cathode with an anthraquinone anode achieves 41 mAh g −1 capacity (based on the mass of both electrodes). The work demonstrates a promising pathway for the design of high performance cathode materials for Mg-ion batteries. • A high potential polyanion cathode of Li 3 V 2 (PO 4 ) 3 is proposed for Mg-ion batteries. • A co-insertion strategy is revealed to enhance cation diffusion in solid lattices. • The Li 3 V 2 (PO 4 ) 3 electrode delivers 124 mAh g −1 capacity at 100 mA g −1 . • High discharge potentials of 0.46 V and −0.44 V vs. Ag/Ag + are achieved. • A capacity retention of 80% is realized over 300 cycles with 99.3% CE.

Effect of Mg Cation Diffusion Coefficient on Mg Dendrite Formation.

Dendrite formation is an important issue for the metal anode-based battery system. The traditional perception that Mg metal anode does not grow dendrite during operation has been challenged recently. Herein, we investigate the Mg electrodeposition behavior in a 0.3 M all-phenyl-complex (APC) electrolyte and confirm that Mg dendrites are readily formed at high current densities. A semiquantitative model indicates that the Mg-ion concentration on the electrode surface, limited by the intrinsic diffusion coefficient of the Mg cation group, decreases with increasing current density, resulting in an extra concentration polarization. However, Mg deposition at the tip of a protrusion on the electrode surface is hardly affected by the concentration polarization, and thus dendrite growth is more prone to occur at the tips. We find that the addition of LiCl in conventional APC electrolytes can suppress the Mg dendrite formation, mainly as a result of the enhanced Mg cation diffusion coefficient due to the influence of the LiCl additive, rather than the less pronounced electrostatic shield effect provided by Li cations.

Ion transport in ionic liquid/poly(vinylidene fluoride) system under electric fields: A molecular dynamics simulation

Molecular dynamics simulations were applied to explore the ion transport in five ionic liquid/poly (vinylidene fluoride) (IL/PVDF) systems under different electric fields where the cation is 1-ethyl-3-methylimidazolium (EMI+) and the anions are Cl−, Br−, BF− 4, PF− 6 and trifluoromethanesulfonate (TfO−) separately, and each system contains about 40 wt% of IL. The structural properties were analyzed by the following calculations as electric fields increase. The pair correlation function reveals that the local spatial structure of ion pairs can be split into two types, which tend to disorder; the dihedral angle and mean square radius of gyration show that PVDF chains tend to straighten. The ion transport properties were studied by the ion-pair intermittent time correlation function and continuous time correlation function, the diffusion coefficient and mobility of anions and cations as electric fields increase. The simulation results illustrate that ion transport is intensified. The transference numbers of anions and cations show the relative contributions of anions to charge transport shows an upward trend as electric fields increase, and under an applied electric field, contribution of anions to charge transport is closely related to anions volume. In addition, the relationship between diffusion coefficient and the electric fields can be fitted into an exponential function, and the fitting parameters are strongly related to the local structure of the ion pairs, and the relationship between ideal conductivity and the electric fields has the same law. Specially, the anions mobility is determined by the anions volume under the stronger electric fields, and the smaller the anion volume, the greater the mobility.