Zinc-Lithium Dual Ion Deep Eutectic Mixtures As Electrolyte for Stable Zinc-Based Hybrid Batteries
Authors: Syu Sin-Yi , Pan Chun-Jern , Lee Chun-I 
Published: August 2024

Anode-free zinc batteries (AFZBs) are proposed as promising energy storage systems due to their high energy density, low cost, high safety, and easy production characteristics. However, the high reactivity of water molecules can induce passivation and dendritic growth within zinc-based aqueous batteries, thereby affecting the durability of the battery.

A deep eutectic solvent (DES) is a solvent created through a suitable combination of salts or zwitterions, acting as hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD). Its melting point is considerably lower than the combined melting points of its components, attributed to its engagement in a complex network of hydrogen bonds. Due to the distinctive properties of deep eutectic solvents, they possess a notable ability to decrease water activity substantially Consequently, we design a novel deep eutectic electrolyte and examine its performance on zinc-based batteries.

This electrolyte design composed of 1 mole of zinc chloride combing with 1 mole of lithium acetate as the ion sources. Subsequently, 4 moles of N-methyl acetamide are added to the mixture, and the resulting mixture is heated to 353 K with continuous stirring until a liquid state of deep eutectic electrolyte is achieved. Due to the high viscosity of the electrolyte, different cosolvents, namely 2 moles of methanol and 2 moles of acetonitrile, are introduced after cooling to improve the viscosity. The positive electrode material, lithium manganese oxide (LiMn2O4, LMO), is employed to construct a Zn//LMO half-cell and conduct electrochemical tests.

The study commences by investigating the electrochemical performance of Zn//Zn symmetric cells, Zn//Cu asymmetric cells, Zn//LMO half-cell, and Cu//LMO anode-free cells, both with and without the addition of cosolvents (methanol, acetonitrile). Various electrochemical tests, including AC impedance, cyclic voltammetry (CV), and linear sweep voltammetry (LSV), are then performed. In addition, Raman spectroscopy and Fourier-transform infrared spectroscopy techniques will be utilized to glean the solvation structure of the electrolyte. Lastly, viscosity, conductivity, and differential scanning calorimetry (DCS) tests will be also performed.

The results demonstrate that in the Zn//Zn symmetric cell, the addition of cosolvent reduces the overpotential of Zn plating/stripping by 0.6 V compared to the case without cosolvent. Furthermore, in the Zn//Cu asymmetric cell, the inclusion of cosolvent exhibits a remarkable Coulombic efficiency of 99.9% after 450 cycles at a current density of 0.5 mA/cm². In the Zn//LMO half-cell, the best cyclic performance is observed with 2 moles of methanol, maintaining a specific capacity of 80 mAh/g after 900 cycles at a current density of 100 mA/g, with a capacitance retention of 97.5%. For the Cu//LMO anode-free cell, a specific capacity of 70 mAh/g is retained with a capacitance retention of 96.5% after 900 cycles at a current density of 100 mA/g. Additionally, a Coulombic efficiency of 96.5% is maintained after 900 cycles. Compared to methanol cosolvent, acetonitrile as cosolvent in the Zn//LMO half-cell and Cu//LMO anode-free cell exhibit inferior performance. After 900 cycles at a current density of 100 mA/g, exhibits a significantly reduced specific capacity of only 20 mAh/g, with a capacitance retention of only 62.5%.

The electrolyte analysis reveals that the Raman peak at 870 cm-1 and the FTIR peak at 990 cm-1, corresponding to N-CH3 vibrations, for electrolytes showing blue shift compared to pure N-methyl acetamide. Additionally, a blue shift is also observed in the Raman peak at 1340 cm-1 and the FTIR peak at 1160 cm-1 in, corresponding to N-C vibrations. These shifts suggest that N-methyl acetamide coordinates with zinc ion and lithium ion, forming a deep eutectic electrolyte and altering the structure of the solvated shell as results. The addition of 2 moles of methanol, the conductivity is improved and the viscosity reduced significantly. The DCS shows the freezing point of the deep eutectic electrolyte is decreased compared to N-methyl acetamide, reaching a temperature of around 214 K.

This study demonstrate that deep eutectic solvents can alter the structure of the solvated shell within the electrolyte, significantly reduce the activity of free water, and mitigate passivation and dendrite formation on the zinc anode surface. This leads to enhanced stability in the electrochemical performance of the battery.

Figure 1