## 2 Topical Research Areas Related to Ionic Liquids and the Contribution of Spiroammonium Compounds
### Green Chemistry and the IL Concepts
#### 2.2.1 The State of the Art
ILs based on imidazolium dicationic salts can act as lubricants with high temperature stability and can be used as lubricants in severe conditions, such as spacecraft applications with ultra-high vacuums and extreme temperatures (from cryogenic conditions up to 500 \({}^{\circ}\)C) at degradation temperature (Td \(>\) 415 \({}^{\circ}\)C) [35; 36; 37]. ILs are used as proton carriers in the proton exchange membranes of fuel cells (PEM-FSs) at higher temperatures under non-humidified conditions [38]. An anion exchange membrane (AEM) forms the core of AEM-FCs. AEM-FCs are a promising technology that can solve the high-cost problem of PEM-FCs [39]. Moving toward safer and more environmentally friendly electrolytes, a fluorine-free class of ILs based on dicyanamide and tricyanomethanide anions has been proposed, which have shown promising results when used as the electrolyte for Li metal electrodes [40]. While ILs have attracted great interest in the patent literature, only a limited number of processes (over 50) are known to have been commercialized for large-scale productions [41]. The synthesis of all ionic liquids is a multi-step process. It starts with the 'neutralization' of a Lewis base, typically alkylimidazoles; trialkylamines, including pyrolles and piperidines, trialkylphophosphines, and pyridines are the most frequently used [42]. For the synthesis of protic ILs, the Lewis base (represented as methylimidazole) can be neutralized directly by the addition of a Bronsted acid. This step can occur in conventional organic solvents in which Lewis bases are soluble. Anion metathesis then occurs to the desired ionic liquid. For many ILs, the preparation is strongly associated with the considerations about the required purity of the ILs post-synthesis. At this technological stage, a water washing stage must be carried out to remove inorganic salts with cations Li\({}^{+}\), Na\({}^{+}\), or NH\({}_{4}\)\({}^{+}\). In the case of a hydrophilic ionic liquid, its extraction will require a large amount of conventional volatile organic solvents that are immiscible with water. Thus, it is obvious that considering ionic liquids as trivial solvents cannot be justified, primarily for economic reasons. And only the unique uses of ILs, which are discussed below (for processing biomass into cellulose, as catalysts, high-temperature lubricants, materials for ion-exchange membranes, pharmaceutical additives, etc.) inspire researchers to create modern materials with useful properties. Using a design approach to the construction of ionic liquids, it is possible to change their properties, such as viscosity, melting point, density, and hydrophobicity, by variation in the cations and anions and cross the conventional boundary established by the requirement of a 'true ionic liquid' existing as a melt at r.t. Considering the basic structural similarity of 'true ionic liquids' and all other compounds consisting of ions, one or two of which are organic, the ionic liquids approach can be applied to all of them. The huge potential and abilities of such liquids, as well as the restrictive definition limiting melting points to below 100 \({}^{\circ}\)C, likely steered research away from higher Tm materials in these families of salts that have applications above 100 \({}^{\circ}\)C [43]. While most of the ionic liquid community is focused on low melting temperature salts, their solid analogues are likely to offer similar benefits such as low vapor pressure, high chemical and thermal stability, nonflammability, and scope to tailor the properties as per demand [44]. These features can be well utilized in the rapidly emerging field of phase change materials (PCM) for thermal management of electrical devices and most importantly in solar-thermal energy storage [45].