Solving ZIB challenges: the dynamic role of water in deep eutectic solvents electrolyte†

Abstract

Zinc-ion batteries (ZIBs) emerge as a promising technology in the post-lithium-ion era, offering high theoretical energy density, lower manufacturing costs, and enhanced safety. Zn²⁺ solvation plays a crucial role in the performance and durability of ZIBs, that warrant research since they are still far from industrial standards. As far as Zn²⁺ solvation is concerned, in aqueous electrolytes, two types of water molecules are found: 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 a range of parasitic processes that critically impact durability: hydrogen evolution, passivation, and anode shape changes. Alternative electrolytes such as Deep Eutectic Solvents (DESs) can be used to modulate the Zn solvation shell and limit free water molecules, while still preserving the green and safe characteristics of aqueous-based ones. The electrolyte–electrode interface and zinc solvation structure are effectively tuned by adjusting the hydration percentage of DES, leading to improved Zn plating and stripping processes. This study investigates the electrochemical behavior of zinc in ethaline DES with varying water contents, transitioning from water-in-salt to salt-in-water structures. Electrokinetic and electro-crystallization analyses were performed using cyclic voltammetry and chronoamperometry, complemented by galvanostatic cycling tests of Zn|Zn symmetric cells while ions speciation and DES transitions were followed by Spontaneous Raman, Stimulated Raman Scattering (SRS) and Impulsive Stimulated Raman Scattering (ISRS) spectroscopies. Moreover, in situ Surface-Enhanced Raman spectroscopy (SERS) was used to follow the interface changes and organic component degradation. The results highlight the impact of hydration on the electrochemical stability and zinc deposition mechanisms, providing crucial insights for optimizing ZIB anode performance.