# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 1 Introduction
In addition, the electrochemical stability of ammonium cations is among the highest reported and usually significantly higher than for ionic liquids based on the more common imidazolium cation. Hence, quaternary ammonium cations are also stable at the harsh cathodic conditions of the reactive lithium metal, which is usually not the case for organic electrolytes [11]. The anions [FSI]\({}^{-}\) and [TFSI]\({}^{-}\) result in ionic liquids with a combination of excellent properties for the use as electrolytes [12]. Ionic liquid incorporating the [FSI]\({}^{-}\) anion in general show higher fluidities, lower temperatures of solidification [13] and formation of a robust solid-electrolyte interphase (SEI) [14], whereas the [TFSI]\({}^{-}\) based molten salts have higher thermal and electrochemical stabilities. Although [TFSI]\({}^{-}\) ionic liquids have slower transport properties than the [FSI]\({}^{-}\) ones with the same cation, they still have lower viscosities and higher conductivities than most ionic liquids with the same cation, but other anions [15]. Some benefits of the two anions can be combined using the asymmetric (fluoroosulfonyl)(trifluormethylsulfonyl)imide anion [13]. However, this anion is currently only sparsely commercialized. Among the most widespread and mature battery technologies are those based on lithium [16]. These batteries combine high energy densities, high power and low weight with longevity and flexibility in shape and size. Rechargeable batteries based on lithium are the dominant type of energy storage devices for portable electronics and in the field of electric mobility. Lithium batteries and lithium-ion batteries contain an electrolyte in liquid state which is able to transport lithium ions between the electrodes. Commercial lithium batteries containing lithium electrodes are used as primary (non-rechargeable) batteries since depositing and re-dissolving lithium from the electrolyte forms dendrites on the lithium surface as result of repeated charging and discharging [17]. The growth of these dendrites may result in short-circuiting of the battery resulting in thermal runaway [18]. However, rechargeable batteries based on lithium metal remain attractive due the extremely high specific capacity and lowest electrochemical potential allowing high amounts of energy to be stored [19]. Furthermore, lithium-ion batteries, which--with correct use--do not suffer from lithium dendrite formation, are close to their theoretical capacity, making the use of still unknown alternative anode materials inevitable if the energy density is to be increased [20]. By simply adjusting the composition and properties of the electrolyte the formation of dendrites may be sufficiently suppressed [21]. Another problem is the thermodynamic instability of many electrolytes towards metallic lithium resulting in parasitic reactions and low cycle stability [22]. The main disadvantages of batteries using liquid electrolytes are concerns in terms of safety due to the use of highly flammable organic solvents and lithium salts with low thermal stability in the electrolyte. A further method for the increase of energy density in lithium-based batteries is the use of high voltage cathode materials. The use of high voltage cathode materials in turn demands electrolytes with increased electrochemical stabilities [23]. These different factors show that the optimization of the electrolyte is an important factor in the search for rechargeable batteries with increased energy density, cycle-stability and safety. Therefore, research into the characteristics of novel electrolyte systems for existing, established battery technologies also have implications on the search for advanced electrolytes to be used in potential batteries utilizing new cell chemistries. Here, we have studied the use of quaternary, aprotic ammonium ionic liquids as potential electrolytes for lithium batteries.