Salt concentration polarization of liquid electrolytes and determination of transport properties of cations, anions, ion pairs and ion triples

"Soggy-Sand" Chemistry for High-Voltage Aqueous Zinc-Ion Batteries

The narrow electrochemical stability window, deleterious side reactions, and zinc dendrites prevent the use of aqueous zinc-ion batteries. Here, aqueous "soggy-sand" electrolytes (synergistic electrolyte-insulator dispersions) are developed for achieving high-voltage Zn-ion batteries. How these electrolytes bring a unique combination of benefits, synergizing the advantages of solid and liquid electrolytes is revealed. The oxide additions adsorb water molecules and trap anions, causing a network of space charge layers with increased Zn2+ transference number and reduced interfacial resistance. They beneficially modify the hydrogen bond network and solvation structures, thereby influencing the mechanical and electrochemical properties, and causing the Mn2+ in the solution to be oxidized. As a result, the best performing Al2 O3 -based "soggy-sand" electrolyte exhibits a long life of 2500 h in Zn||Zn cells. Furthermore, it increases the charging cut-off voltage for Zn/MnO2 cells to 2 V, achieving higher specific capacities. Even with amass loading of 10 mgMnO2 cm-2 , it yields a promising specific capacity of 189 mAh g-1 at 1 A g-1 after 500 cycles. The concept of "soggy-sand" chemistry provides a new approach to design powerful and universal electrolytes for aqueous batteries.

Spectral deconvolution in electrophoretic NMR to investigate the migration of neutral molecules in electrolytes

Electrophoretic nuclear magnetic resonance (eNMR) is a powerful tool in studies of nonaqueous electrolytes, such as ionic liquids. It delivers electrophoretic mobilities of the ionic constituents and thus sheds light on ion correlations. In applications of liquid electrolytes, uncharged additives are often employed, detectable via ¹ H NMR. Characterizing their mobility and coordination to charged entities is desirable; however, it is often hampered by small intensities and ¹ H signals overlapping with major constituents of the electrolyte. In this work, we evaluate methods of phase analysis of overlapping resonances to yield electrophoretic mobilities even for minor constituents. We use phase-sensitive spectral deconvolution via a set of Lorentz distributions for the investigation of the migration behavior of additives in two different ionic liquid-based lithium salt electrolytes. For vinylene carbonate as an additive, no field-induced drift is observed; thus, its coordination to the Li⁺ ion does not induce a correlated drift with Li⁺ . On the other hand, in a solvate ionic liquid with tetraglyme (G4) as an additive, a correlated migration of tetraglyme with lithium as a complex solvate cation is directly proven by eNMR. The phase evaluation procedure of superimposed resonances thus broadens the applicability of eNMR to application-relevant complex electrolyte mixtures containing neutral additives with superimposed resonances.

High Lithium Transference Number Electrolytes Containing Tetratriflylpropene's Lithium Salt

Electrolytes with a high lithium transference number linked with high ionic conductivity are urgently needed for high power battery operation. In this work, we present newly synthesized lithium tetra(trifluoromethanesulfonyl)propene as a salt-in-glyme-based "salt-in-solvent" electrolyte. We employ impedance spectroscopy in symmetric Li/electrolyte/Li cells and pulsed field gradient nuclear magnetic resonance spectroscopy to investigate the lithium conduction mechanism. We find predominant lithium conductivity with very high lithium transference numbers (∼70% from the polarization experiments) and three times higher ionic conductivity compared to well-known lithium triflate in diglyme electrolyte. This is a consequence of the reduced mobilities of large anions linked with improved ionic dissociation.

Negative effective Li transference numbers in Li salt/ionic liquid mixtures: does Li drift in the “Wrong” direction?

Due to association with anions and an inverted drift direction in an electric field, Li⁺ cations have negative effective transference numbers.