Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications

Daniel Rauber,Andreas Hofmann,Frederik Philippi,Thomas Hanemann,Rolf Hempelmann,Tatiana Zinkevich,Christopher W M Kay

doi:10.3390/app11125679

Abstract

Ionic liquids are attractive and safe electrolytes for diverse electrochemical applications such as advanced rechargeable batteries with high energy densities. Their properties that are beneficial for energy storage and conversion include negligible vapor-pressure, intrinsic conductivity as well as high stability. To explore the suitability of a series of ionic liquids with small ammonium cations for potential battery applications, we investigated their thermal and transport properties. We studied the influence of the symmetrical imide-type anions bis(trifluoromethanesulfonyl)imide ([TFSI]−) and bis(fluorosulfonyl)imide ([FSI]−), side chain length and functionalization, as well as lithium salt content on the properties of the electrolytes. Many of the samples are liquid at ambient temperature, but their solidification temperatures show disparate behavior. The transport properties showed clear trends: the dynamics are accelerated for samples with the [FSI]− anion, shorter side chains, ether functionalization and lower amounts of lithium salts. Detailed insight was obtained from the diffusion coefficients of the different ions in the electrolytes, which revealed the formation of aggregates of lithium cations coordinated by anions. The ionic liquid electrolytes exhibit sufficient stability in NMC/Li half-cells at elevated temperatures with small current rates without the need of additional liquid electrolytes, although Li-plating was observed. Electrolytes containing [TFSI]− anions showed superior stability compared to those with [FSI]− anions in battery tests.

Highlights

The use of ammonium ionic liquids with the incorporation of an ether functionalization into the cation and the use of the [FSI]− anion was motivated by beneficial effects for the electrolyte dynamics, e.g., lower viscosity and higher conductivity
The structure–property relationships of various ionic liquid electrolytes with the trimethyl ammonium cation were investigated with particular focus on their applicability in rechargeable lithium batteries
The transport properties are comparable to other ionic liquid subclasses with accelerated dynamics for short side chains, ether functionalization, the use of the [FSI]− anion and low concentrations of lithium salt

Summary

Introduction

Ionic liquids are organic salts that melt below ambient temperatures. They are well established as a class of liquid materials with an exceptional combination of favorable properties. General characteristics of ionic liquids include ultralow vapor-pressure, intrinsic electric conductivity, relatively high ionic concentration as well as high thermal, chemical and electrochemical stabilities. As result of these features, ionic liquids are in development or implemented in many technical applications ranging from small-scale niche uses to industrial processes on a production scale [1,2]. Many of the ionic liquids’ technical implementations thereby lie in the field of electrochemical energy storage and conversion [3]

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Results

Conclusion