# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 1 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 actually implemented in many technical applications ranging from small-scale niche uses to industrial processes on a production scale [1, 2]. Besides offering a range of completelynovel processes, ionic liquids are also used to improve existing techniques, for instance, in terms of safety, performance or durability. Many of the ionic liquids' technical implementations thereby lie in the field of electrochemical energy storage and conversion [3]. Research in this field is driven by the search for more efficient and safer devices in the context of defossilization and energy supply as the transformation to more sustainable societies accelerates. Ionic liquids generally meet many of the criteria for advanced battery electrolytes by offering a broad electrochemical window and increased safety due to their high thermal stabilities, negligible volatility and low inflammability [4; 5]. These molten salts often have high dissolution capacities for many compounds of different polarities, including conducting salts required for lithium, lithium ion and post-Li batteries. Using mixtures of ionic liquids and lithium salts increases the temperature range in which electrolytes can be employed. At lower temperatures a salting out of the Li-salts is often prevented by formation of eutectic mixtures, whereas at higher temperatures the operating range is extended due to extremely low vapor pressure. Commonly used salts for lithium batteries with high solubility in ionic liquids are lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI). Furthermore, the huge diversity of possible ionic liquid combinations and modifications offers adjustability towards particular desired properties. The main disadvantage of ionic liquids in comparison with commonly used electrolytes based on molecular solvents, such as dialkyl carbonates, is their lower ionic conductivity as a result of the strong, long-ranging Coulombic forces that retard the liquid dynamics [6; 7]. Therefore, batteries using undiluted ionic liquid electrolytes are limited in their cycling rates or must be operated at elevated temperatures. However, the lower charge and discharge rate improves the stability and durability of the battery, thus ionic liquid electrolytes are promising for stationary uses. Using large-scale stationary batteries is regarded as essential to back up the power grid in the future. The demand for these energy storage devices results from the increased use of renewable energy--a characteristic of which is the discontinuous generation of energy. Molecular solvents can be used as additives in ionic liquid electrolytes to increase conductivity, thus providing electrolytes with well-balanced liquid range, safety and performance [8]. Nevertheless, one of the main advantages resulting from the use of undiluted ionic liquids--the significantly increased thermal stability--is thereby partially reduced [9]. It is also possible to use ionic liquids as plasticizers in polymer-electrolytes to improve lithium ion diffusion in this type of battery while preserving benefits of gelled electrolytes, for instance, in the manufacturing process [10]. Ionic liquids based on ammonium cations have a number of benefits. These include high thermal stability as well as commercial availability of a large range of different amine precursors that are cost-efficient and facile in handling, contrary to the phosphines necessary in the synthesis of phosphonium-based ionic liquids.