A link between viscosity and cation-anion contact pairs: Adventure on the concept of structure-making/breaking for concentrated salt solutions

Anionic Effects on Concentrated Aqueous Lithium Ion Dynamics

The dynamics, orientational anisotropy, diffusivity, viscosity, and density were measured for concentrated lithium salt solutions, including lithium chloride (LiCl), lithium bromide (LiBr), lithium nitrite (LiNO2), and lithium nitrate (LiNO3), with methyl thiocyanate as an infrared vibrational probe molecule, using two-dimensional infrared spectroscopy (2D IR), nuclear magnetic resonance (NMR) spectroscopy, and viscometry. The 2D IR, NMR, and viscosity results show that LiNO2 exhibits longer correlation times, lower diffusivity, and nearly 4 times greater viscosity compared to those of the other lithium salt solutions of the same concentration, suggesting that nitrite anions may strongly facilitate structure formation via strengthening water-ion network interactions, directly impacting bulk solution properties at sufficiently high concentrations. Additionally, the LiNO2 and LiNO3 solutions show significantly weakened chemical interactions between the lithium cations and the methyl thiocyanate when compared with those of the lithium halide salts.

Structure-Property Correlations in Aqueous Binary Na+/K+-CH3COO- Highly Concentrated Electrolytes

Highly concentrated aqueous binary solutions of acetate salts are promising systems for different electrochemical applications, for example, energy storage devices. The very high solubility of CH₃COOK allows us to obtain water-in-salt electrolyte concentrations, thus reducing ion activity and extending the cathodic stability of an aqueous electrolyte. At the same time, the presence of Li⁺ or Na⁺ makes these solutions compatible with intercalation materials for the development of rechargeable alkaline-ion batteries. Although there is a growing interest in these systems, a fundamental understanding of their physicochemical properties is still lacking. Here, we report and discuss the physicochemical and electrochemical properties of a series of solutions based on 20 mol kg^-1 CH₃COOK with different concentrations of CH₃COONa. The most concentrated solution, 20 mol kg^-1 CH₃COOK + 7 mol kg^-1 CH₃COONa, gives the best compromise between transport properties and electrochemical stability, displaying a conductivity of 21.2 mS cm^-1 at 25 °C and a stability window of up to 3 V in "ideal" conditions, i.e., using a small surface area and highly electrocatalytic electrode in a flooded cell. Careful Raman spectroscopy analyses help to address the interaction network, the phase evolution with temperature, and the crystallization kinetics.

Crucial roles of ion-specific effects in the flotation of water-soluble KCl and NaCl crystals with fatty acid salts

HYPOTHESIS

Flotation of water-soluble KCl and NaCl minerals in brines is significant for K-fertilizer production, but its mechanism is controversial. Dissolved salt ions are expected to change the physicochemical properties of solvents, interfaces, and collector colloids, thereby affecting flotation significantly.

EXPERIMENTS

Flotation experiments of KCl and NaCl crystals in brines were conducted using potassium and sodium laurates as collectors. Contact angle (CA) and surface tension measurements, X-ray photoelectron spectroscopy (XPS) analysis, and molecular dynamics simulations (MD) were applied to gain a molecular understanding of changing interfacial properties and crystal-collector colloid interactions in the presence of dissolved ions in terms of salt flotation.

FINDINGS

While K⁺ ions activate the NaCl crystal flotation, Na⁺ ions depress the KCl crystal flotation, in agreement with the studies of CA, XPS, and MD results with these crystals. XPS results showed no collector adsorption at crystal surfaces which is a requirement of conventional flotation and presents a new theoretical challenge. We argue the crucial role of ion specificity: Na-laurate colloids adsorb at the bubble surface as a monolayer but solvent-separated from KCl crystals, inhibiting their flotation, or in interactive contact with NaCl crystals, enhancing their flotation. Increasing K⁺ concentration weakens NaCl crystal hydration, increasing Na-laurate colloid attraction with crystals for better flotation. The Contact Interactive Collector Colloid (CICC) and Solvent-separated Interactive Collector Colloid (SICC) hydration states are critical to salt crystal flotation via collector colloid-crystal attraction by dispersion forces.

Flotation surface chemistry of water-soluble salt minerals: from experimental results to new perspectives

The flotation separation of water-soluble salt minerals has to be conducted under the condition of saturation in brines which represents a challenging but exciting topic of colloid and surface chemistry. Despite several proposals on explaining the success of this industrial application for many decades, our understanding of the flotation separation is still far from complete yet, owing to the complexity of the highly selective collection of salt crystals by air bubbles in brines. Here, we thoroughly review the experimental results for halogen, oxyanion, and double salts and match them with the proposed theories on the flotation of soluble salts to identify the agreed and disagreed cases. The experimental results show that the flotation of these salts varies from collectors (surfactants applied to control the crystal hydrophobicity) to collectors and is strongly affected by the brine ion composition and pH conditions. We find some exceptional flotation results that cannot be simply explained by the crystal surface charge and wettability. Furthermore, we outline several disputes and discrepancies between the experiments and the theories when different collectors are applied. Apart from the extensive consideration of surface hydration, the presence of external ion species exhibits ubiquitous effects on the surface properties of salt crystals and the colloidal properties of collectors. We conclude that the interactions between salt ions, water molecules, collectors, and salt crystals must be considered more thoroughly, and the activity of collectors at the air-liquid interface should also be the focus. Advanced techniques such as molecular dynamics simulation, atomic force microscopy, X-ray photoelectron spectroscopy, and sum-frequency generation spectroscopy are expected to be promising research tools for future studies.

Anti-Freezing Strategies of Electrolyte and their Application in Electrochemical Energy Devices

Wider scenes of human's activities under low temperature demand promising performance of anti-freezing electrochemical energy devices, and the promotion of performance is mainly limited by electrolyte. However, despite many relevant research works reported, there are still few reviews that systematically and comprehensively summarize these modified approaches and applications. In this focus review, we classify the prominent anti-freezing strategies as high concentration aqueous electrolyte, organic additives, organic electrolyte and others. Relevant research works have been put to clarify their anti-freezing mechanisms and exhibit the modification effects. Besides, various energy devices including metal-air batteries, non-gas batteries and supercapacitors which employed aforementioned strategies are discussed and their key low-temperature performance indexes are summarized to exhibit the advanced research progress. Finally, we put forward some remaining challenges of these modification strategies toward practical application and propose prospects on future development of low-temperature electrochemical energy devices.

NaCl aggregation in water at elevated temperatures and pressures: Comparison of classical force fields

The properties of water vary dramatically with temperature and density. This can be exploited to control its effectiveness as a solvent. Thus, supercritical water is of keen interest as solvent in many extraction processes. The low solubility of salts in lower density supercritical water has even been suggested as a means of desalination. The high temperatures and pressures required to reach supercritical conditions can present experimental challenges during collection of required physical property and phase equilibria data, especially in salt-containing systems. Molecular simulations have the potential to be a valuable tool for examining the behavior of solvated ions at these high temperatures and pressures. However, the accuracy of classical force fields under these conditions is unclear. We have, therefore, undertaken a parametric study of NaCl in water, comparing several salt and water models at 200 bar-600 bar and 450 K-750 K for a range of salt concentrations. We report a comparison of structural properties including ion aggregation, hydrogen bonding, density, and static dielectric constants. All of the force fields qualitatively reproduce the trends in the liquid phase density. An increase in ion aggregation with decreasing density holds true for all of the force fields. The propensity to aggregate is primarily determined by the salt force field rather than the water force field. This coincides with a decrease in the water static dielectric constant and reduced charge screening. While a decrease in the static dielectric constant with increasing NaCl concentration is consistent across all model combinations, the salt force fields that exhibit more ionic aggregation yield a slightly smaller dielectric decrement.

Modulating electrolyte structure for ultralow temperature aqueous zinc batteries

Rechargeable aqueous batteries are an up-and-coming system for potential large-scale energy storage due to their high safety and low cost. However, the freeze of aqueous electrolyte limits the low-temperature operation of such batteries. Here, we report the breakage of original hydrogen-bond network in ZnCl₂ solution by modulating electrolyte structure, and thus suppressing the freeze of water and depressing the solid-liquid transition temperature of the aqueous electrolyte from 0 to –114 °C. This ZnCl₂-based low-temperature electrolyte renders polyaniline||Zn batteries available to operate in an ultra-wide temperature range from –90 to +60 °C, which covers the earth surface temperature in record. Such polyaniline||Zn batteries are robust at –70 °C (84.9 mA h g⁻¹) and stable during over 2000 cycles with ~100% capacity retention. This work significantly provides an effective strategy to propel low-temperature aqueous batteries via tuning the electrolyte structure and widens the application range of temperature adaptation of aqueous batteries.

Rechargeable aqueous batteries are promising for potential large-scale energy storage due to their high safety and low cost. Here the authors analyse a zinc chloride based low-temperature electrolyte for improving practicability of the aqueous batteries.

High-Voltage Aqueous Na-Ion Battery Enabled by Inert-Cation-Assisted Water-in-Salt Electrolyte

Water-in-salt (WiS) electrolytes provide a new pathway to widen the electrochemical window of aqueous electrolytes. However, their formulation strongly depends on the solubility of the chosen salts, imposing a stringent restriction on the number of possible WiS systems. This issue becomes more severe for aqueous Na-ion batteries (ANIBs) owing to the relatively lower solubility of sodium salts compared to its alkaline cousins (Li, K, and Cs). A new class of the inert-cation-assisted WiS (IC-WiS) electrolytes containing the tetraethylammonium (TEA⁺ ) inert cation is reported. The Na IC-WiS electrolyte at a superhigh concentration of 31 mol kg^-1 exhibits a wide electrochemical window of 3.3 V, suppresses transition metal dissolution from the cathode, and ensures singular intercalation of Na into both cathode and anode electrodes during cycling, which is often problematic in mixed alkali cation systems such as K-Na and Li-Na. Owing to these unique advantages of the IC-WiS electrolyte, the NaTiOPO₄ anode and Prussian blue analog Na_1.88 Mn[Fe(CN)₆ ]_0.97 ·1.35H₂ O cathode can be coupled to construct a full ANIB, delivering an average voltage of 1.74 V and a high energy density of 71 Wh kg^-1 with a capacity retention of 90% after 200 cycles at 0.25C and of 76% over 800 cycles at 1C.