Solving ZIB Challenges: The Dynamic Role of Water in Deep Eutectic Solvents Electrolyte
Authors: Emanuele Elisa , Mele Claudio , Bozzini Benedetto 
Published: August 2024

Zinc-Ion batteries (ZIBs) are considered one of the most promising technologies in the realm of post-lithium-ion batteries. Rechargeable chemistries with zinc anodes hold significant technological potential due to their high theoretical energy density, lower manufacturing costs, abundant raw materials, and inherent safety. ZIBs typically feature neutral or weakly acidic aqueous electrolytes and exhibit typical drawbacks of Zn anodes: hydrogen evolution, passivation, and shape changes.

In aqueous electrolytes, two types of water molecules can be described: free water molecules and solvated water molecules that participate in Zn2+ solvation structure [Zn(H2O)6]2+. The free water easily reacts with metallic Zn at the electrode/electrolyte interface, leading to the aforementioned irreversibilities. Alternative electrolytes such as Deep Eutectic Solvents (DESs) can be used to modulate Zn solvation shell, preserving the green and safe characteristics of aqueous-based ones. DESs show relevant characteristics for battery applications, such as low vapor pressure, good thermal and chemical stability, non-flammability, easy moldability, wide electrochemical window, and structured interface. However, a main drawback for electrochemical applications is the high viscosity, low diffusivity, and sluggish kinetics.

Water molecules can be added as a cosolvent to eutectic mixtures to increase the diffusion coefficient of electroactive species and reduce the electrolyte viscosity without destroying the eutectic nature by up to 40%. The hydration percentage of DES can modify the thickness of the electrolyte-electrode interface and the Zn2+ solvation structure. Thus, the tuning capability, on one hand, of the solution chemistry of Zn2+ containing electroactive species, and on the other hand, of the electrode–electrolyte interfacial structure and chemistry by the combined use of DES and water provides a flexible tool to control Zn plating and stripping processes.

In this study, we investigate the electrochemistry of Zn in ethaline DES with different percentages of water (0%, 5%, 10%, 25%, 50%, 70%, respectively), transitioning from a water-in-salt structure to a salt-in-water one. Fundamental electrokinetic and electrocrystallization studies based on cyclic voltammetry and chronoamperometry at different cathodic substrates were complemented with galvanostatic cycling tests of Zn|Zn symmetric CR2032 coin cells, SEM imaging of electrodes, and in situ Surface Enhanced Raman spectroscopy (SERS).

Moreover, the Zn solvation structure was investigated using spontaneous Raman spectroscopy . The employed methodology provided valuable insights into the Zn plating reaction. Water molecules are present in a "confined" state rather than a free one, directly interacting with choline cations and being trapped in the eutectic framework if the molar fraction is kept below 40%.

When ZnSO4 is dissolved in anhydrous ethaline, the [ZnCl4]2− species is formed and stabilized by choline. The peculiarities of Zn2+ coordination were found to result in unusual voltammetric behavior, characterized by a cathodic peak in the cathodic branch of the anodic-going scan.

Although tetrachlorozincate is the dominant species, it does not seem to be the complex from which Zn is deposited; indeed, [ZnCl4]2− has a very negative reduction potential. Mechanistic studies of Zn electrodeposition onto different cathodic materials have concluded that the metal is formed by the reduction of an intermediate Zn-containing species. The two organic components of the eutectic solvents can be dehydrogenated at a negative potential, forming RO-, which can replace one or more chloride ligands in [ZnCl4]2−.

Hydrated DES exhibits faster kinetics, which, in turn, leads to a lower nucleation overpotential. This phenomenon has been explained with two different mechanisms occurring at the same time. Firstly, by increasing the hydration percentage in the eutectic framework, the coordination of Zn2+ changes, forming [ZnCl3(H2O)]-. The water molecule in the Zn2+ sheath has a lower energy barrier than chlorine in the step-by-step desolvation of Zn2+. Consequently, the [ZnCl3(H2O)]- complex exhibits a lower dissociation energy than [ZnCl4]2- present in anhydrous ethaline, resulting in a faster desolvation process that can reduce nucleation overpotentials.

Secondly, water molecules hydrogen-bonded to DES components can be reduced at cathodic potential, forming H· radicals that can then participate in the decomposition of the organic component. This apparent side reaction can aid in the formation of RO- ligands that participate in the Zn-species undergoing reduction, thereby increasing the amount of reactants.

This multi-technique approach, also applicable to the study of new electrolytes, leads to the optimization of DES hydration, reducing the presence of free water and addressing the main drawbacks associated with eutectic electrolytes. The nuanced understanding of the interplay between solvent composition, Zn solvation, and electrodeposition mechanisms paves the way for the development of more efficient and sustainable ZIB technologies.

Figure 1