Sebastian Menne

The beneficial effect of protic ionic liquids on the lithium environment in electrolytes for battery applications

This work reports the improved rate performance of ionic-liquid based lithium-ion batteries by replacement of the conventional aprotic ionic liquid (AIL) N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) by protic ionic liquids (PILs). Two model pyrrolidinium-TFSI PILs are synthesized and their mixtures with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are characterized in terms of conductivity, viscosity and self-diffusion coefficients. Raman measurements show pronounced differences in terms of TFSI− coordination to Li+ between the AIL and the PILs. Li+ is coordinated by significantly fewer TFSI− anions in the investigated PILs, which is discussed as the likely cause for the much improved rate performance of lithium vanadium phosphate-based electrodes in these electrolytes.

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

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.