LiNO3-Based Deep Eutectic Mixtures Comprising Organic Additives As Promising Electrolytes for Stable Li-Metal Batteries
Authors: Lin Cian-Ping , Pan Chun-Jern , Lee Chun-I 
Published: August 2024

Deep eutectic electrolytes have emerged as a novel energy storage solution, presenting distinct advantages over traditional ionic liquid electrolytes. These advantages include a high ion concentration, cost-effective, wide electrochemical window, and inherent safety.

In this study LiNO3, which is valuable additive but difficult to melt in carbonate electrolytes, was used as the lithium salt for a deep eutectic solvent (DES) electrolyte. LiNO3 demonstrated its capability to form deep eutectic solutions with N-methyl acetamide (N-MAc) and Methyl carbamate (MCB) in a 1:4 molar ratio. An suitable additives was introduced to study the effect on electrochemical behaviors.

Li//Cu asymmetric cells was utilized to evaluate the coulombic efficiency (CE) of lithium plating/stripping with various electrolytes. The lithium plating/stripping stability is investigated by evaluating the cycle life of Li//Li symmetric cells. The electrochemical window is determined by the Li//Pt asymmetric cells underwent linear sweep voltammetry (LSV). A commercial LiMn2O4 (LMO) is used as cathode to assemble the Li//LMO lithium-metal cells to evaluate cycle stability, rate capabilities, and AC impedance. The physical properties of the electrolytes underwent systematic analysis using Fourier Transform Infrared Spectroscopy (FT-IR), Raman spectroscopy, Differential Scanning Calorimetry (DSC), conductivity, and viscosity measurements. Post-analysis of Li//Cu asymmetric cells revealed additive-induced differences. SEM and EDS analyses unveiled elemental variations on copper foil, and X-ray Photoelectron Spectroscopy (XPS) provided surface insights.

For the first system, the LiNO3 and N-MAc electrolyte exhibited an extended plating/stripping cycle life of reaching 450 hours with the addition of 10 wt% VC in Li//Li symmetrical cell. Addition VC as additive significantly improved conductivity and viscosity. Surprisingly, in Li//Cu asymmetrical cells, 5 wt% VC demonstrated better stability. The Li//Pt asymmetrical cells shows the electrolyte decomposition is occurs at 5V, which is high compared to organic electrolyte. In Li//LMO lithium-metal cells, an impressive 2000 cycles were achieved, with a remarkable 80% capacity retention in rate capabilities testing at high current density and sustained cycling up to 1000 cycles at lower current density. Raman and FT-IR analysis revealed blue shifts in characteristic peaks of C=O and N-H, indicating lithium ions from LiNO3 indeed interact with N-MAc. SEM and EDS testing confirmed that the presence or absence of VC influenced lithium-ion deposition. In the presence of VC, there was a notable increase in oxygen, nitrogen, and carbon content, imply that the enhanced lithium-ion deposition and dissolution capabilities.

For the second system, the LiNO3 and MCB electrolyte system, 8 wt% FEC addition in Li//Li symmetrical cells resulted in a cycle life of only 60 hours. Li//Cu asymmetrical cells showed improved lithium-ion deposition and dissolution with FEC for 12 cycled. Li//Pt asymmetrical cells displayed a decomposition at 4.6V. Li//LMO lithium-metal cells with 5 wt% FEC exhibited stability, cycling up to 120 cycles. Raman and FT-IR analysis indicated blue shifts in the characteristic peaks of C=O and N-H stretching vibrations, signifying deep eutectic interactions between LiNO3 and MCB. Red shifts were observed in N-H bending vibrations. FEC addition improved conductivity and viscosity. SEM and EDS testing demonstrated that the presence or absence of FEC affected lithium-ion deposition. In the presence of FEC, components such as oxygen, nitrogen, carbon, and fluorine were detected, with fluorine constituting a significant 26.70%, indicating improved lithium-ion deposition and dissolution capabilities. XPS peaks corresponding to Li 1s, F 1s, and N 1s, affirming the efficacy of FEC in promoting lithium-ion deposition.

In conclusion, the deep eutectic electrolyte formed by LiNO3 and N-MAc in a 1:4 molar ratio demonstrated superior stability. Additives proved effective in improving electrolyte properties without compromising coordination. Both electrolytes exhibited comparable cycle life, emphasizing their suitability for positive electrode materials like LMO. Moreover, sustained high current density in rate capabilities testing further highlighted their potential for practical applications.

Figure 1