Regulating the Solvation Shell Structure of Eutectic Electrolytes for Stable and High Performance Anode-Free Zn-Li Hybrid Batteries
Authors: Lin Shih-Che , Pan Chun-Jern , Lee Chun-I 
Published: December 2024

With the development of technology, various related policies have emerged wherein the application of batteries plays a crucial role. Presently, lithium-ion batteries widely used in the market suffer from poor safety, high cost, and temperature usage limitations. To solve these challenges, this study employs Zinc ion batteries (ZIBs) as the electrolyte. Zinc-based electrolytes exhibit high energy density, low cost, high safety, and abundant resources. However, the application of Zinc ion batteries (ZIBs) is accompanied by specific electrochemical issues, such as hydrogen evolution reaction (HER), dendritic growth, and the formation of passivation layers, which subsequently affect battery lifespan, Coulombic efficiency (CE), and energy efficiency (EE). Therefore, addressing these side reactions can lead to superior performance and benefits across various aspects of the electrolyte.

Deep Eutectic Solvent (DES), formed through intricate hydrogen bond networks, divides salt compositions into hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD), thereby altering the total melting point. DES electrolytes can form unique structures, thus improving electrochemical side reactions. Leveraging these characteristics, this experiment designs a novel deep electronic solution (DES) electrolyte and assembles a pouch cell to measure its electrochemical performance.

This electrolyte is prepared by initially heating and stirring a mixture of 1 mol of zinc chloride and 4 mol of acetamide until fully dissolved, followed by adding 1 mol of lithium acetate. Due to the high viscosity of the resulting electrolyte, 5 mol of acetonitrile is added after cooling to reduce viscosity. Additionally, the impact of adding water to the electrolyte on battery lifespan, overpotential, Coulombic efficiency, and energy efficiency is investigated. Therefore, another electrolyte is prepared for comparison, maintaining the same primary salt concentration but including 1 mol of water and 4 mol of acetonitrile.

Using the positive electrode material lithium manganese oxide (LiMn2O4, LMO), a Zn//LMO anode-free cell is constructed for electrochemical testing. Initially, the electrolyte is tested for the electrochemical performance of Zn//Zn symmetric cells, Zn//Cu asymmetric cells, and Zn//LMO anode-free cells (with or without co-solvent). Various electrochemical tests are then conducted, including Electrochemical Impedance Spectroscopy (EIS), cyclic voltammetry (CV), linear sweep voltammetry (LSV), Tafel plots, and C-Rate cycling. Furthermore, Raman spectroscopy and Fourier Transform Infrared (FTIR) spectroscopy techniques are utilized to investigate the solvation structure of the electrolyte. Lastly, viscosity and conductivity measurements are performed.

The results indicate that in Zn//Zn symmetric cells, electrolytes with added co-solvent exhibit a 0.36V reduction in overpotential during Zn plating/stripping compared to those without co-solvent. Additionally, in Zn//Cu asymmetric cells, electrolytes with added co-solvent demonstrate relatively stable Coulombic efficiency, maintaining an average of over 99.7% efficiency at a current density of 0.5 mA/cm2. In Zn//LMO anode-free cell testing, it is observed that electrolytes with added 1 mol of water and 4 mol of acetonitrile initially exhibit the highest specific capacity (75mAh/g), maintaining good specific capacity even after cycling at a current of 100mA/g. In C-Rate testing, the Zn//Cu asymmetric cell with added co-solvent shows better stability, with Coulombic efficiency maintaining above 99.9% when cycling back to low current density. the electrolyte with added 1 mol of water and 4 mol of acetonitrile in the Zn//LMO half-cell maintains a Coulombic efficiency of 99.4% and a specific capacity of 50 mAh/g when cycling back to low current density.

Electrolyte analysis reveals the presence of [ZnCl(C2H5NO)2(CH3COO-)]+ at the peak of 282 cm-1 in the Raman spectrum. Upon adding acetonitrile, a characteristic peak of C-C≡N at 381 cm-1 appears, which decreases with adding water. Additionally, at the peak of 944 cm-1, a characteristic peak of lithium acetate C-C is observed, which shifts upon adding acetonitrile. This suggests a change in the coordination of zinc ions in the solvation shell, from one chloride ion and three acetamide molecules to one chloride ion, one acetate ion, and two acetamide molecules upon adding acetonitrile. In the FTIR spectrum, a blue shift is observed in the C=O characteristic peaks between 1640 cm-1~1670 cm-1 upon adding acetonitrile, indicating the penetration of acetonitrile into the solvation shell. Additionally, a blue shift is observed between 1360 cm-1~1420 cm-1, attributed to the C=O characteristic peaks of acetonitrile and acetate ions, confirming the penetration of acetate ions into the solvation shell, forming a unique Deep Eutectic Solvent (DES) structure.

Figure 1