Deep eutectic solvents based on N-methylacetamide and a lithium salt as suitable electrolytes for lithium-ion batteries

Abstract

In this work, we present a study on the physical and electrochemical properties of three new Deep Eutectic Solvents (DESs) based on N-methylacetamide (MAc) and a lithium salt (LiX, with X = bis[(trifluoromethyl)sulfonyl]imide, TFSI; hexafluorophosphate, PF6; or nitrate, NO3). Based on DSC measurements, it appears that these systems are liquid at room temperature for a lithium salt mole fraction ranging from 0.10 to 0.35. The temperature dependences of the ionic conductivity and the viscosity of these DESs are correctly described by using the Vogel-Tammann-Fulcher (VTF) type fitting equation, due to the strong interactions between Li(+), X(-) and MAc in solution. Furthermore, these electrolytes possess quite large electrochemical stability windows up to 4.7-5 V on Pt, and demonstrate also a passivating behavior toward the aluminum collector at room temperature. Based on these interesting electrochemical properties, these selected DESs can be classified as potential and promising electrolytes for lithium-ion batteries (LIBs). For this purpose, a test cell was then constructed and tested at 25 °C, 60 °C and 80 °C by using each selected DES as an electrolyte and LiFePO4 (LFP) material as a cathode. The results show a good compatibility between each DES and LFP electrode material. A capacity of up to 160 mA h g(-1) with a good efficiency (99%) is observed in the DES based on the LiNO3 salt at 60 °C despite the presence of residual water in the electrolyte. Finally preliminary tests using a LFP/DES/LTO (lithium titanate) full cell at room temperature clearly show that LiTFSI-based DES can be successfully introduced into LIBs. Considering the beneficial properties, especially, the cost of these electrolytes, such introduction could represent an important contribution for the realization of safer and environmentally friendly LIBs.

Room-Temperature Molten Salts: Protic Ionic Liquids and Deep Eutectic Solvents as Media for Electrochemical Application

“…The electrochemical stability window of the Lithium-based DESs on Pt is generally from 4.5 to 5.5 V [41]. The cathodic limit shown by these electrolytes was significantly higher, comparable to that observed for aprotic ionic liquid (see Fig.…”

“…Al passivation could be achieved in this PIL electrolyte from the second cyclic voltammogram. Figure 7.17b shows SEM images of an aluminum current collector of the LFP electrode cycled in a cell containing DESs based on Li[N(Tf) 2 ] taken at the end of the cycling [41]. As shown, the current collector displayed a quite uniform morphology, and only some residues were visible.…”

“…In other words, DES viscosity is high and is strongly affected by the selection of the solvent and salt driven by their structures that determine the nature of intermolecular interactions between the molecules and ions in solution as shown in Fig. 7.9 [41].…”

“…The Li[N(Tf) 2 ]-acetamide composite was formerly chosen as the electrolyte applied in the energy storage application by Chen [39]. Thereafter, Anouti et al demonstrated that N-methylacetamide (MAc) was able to decrease the melting point of a complex system containing a molten salt due to its "water-like" physical properties, e.g., a very high-dielectric constant and dipolar moment [40] can be considered as a good H-bond donor for the formulation [40,41]. We also demonstrated that each formulated DES-based electrolyte containing the MAc and a LiX salt (X = [NO 3 ], [N(Tf) 2 ], or [PF 6 ]) allows the realization of energy storage systems as LIBs or EDLC devices that show promising performances even at 80 °C.…”

Investigation of Ammonium- and Phosphonium-Based Deep Eutectic Solvents as Electrolytes for a Non-Aqueous All-Vanadium Redox Cell

“…11,12 A DES is a eutectic mixture of an organic salt (ammonium or phosphonium) and a hydrogen bond donor (HBD), that is made up of different components such as amides, metallic salts, alcohols, carboxylic acids and amines that may be used as complexing agents (typically an H-bond donor). 13,14 DESs have a melting point that is far below that of either individual constituent. The mechanism is that the complexing agent interacts with the anion and increases its effective size.…”