## 2 Topical Research Areas Related to Ionic Liquids and the Contribution of Spiroammonium Compounds
### Azoniaspiroalkanes with IL Properties Used as Electrolytes  
Owing to their distinctive properties, such as their low volatility, high thermal and electrochemical stability, and better ionic conductivity, ILs are extensively used in a variety of energy applications, particularly in the development of green and sustainable energy storage and conversion devices. Suitable ILs have been designed for specific purposes to be used as electrolytes and/or solvents for fuel cells, lithium-ion batteries, supercapacitors (SCs), and solar cells [99; 100]. In a continued effort to improve the suitability of ionic liquids in applications operating at raised temperatures, novel spirocyclic 'azoniaspiro' salts (with cations derived from five-, six-, seven-, and eight-membered rings) have been prepared and characterized. Azoniaspiroalkanes  
In order to expand the temperature range in which ILs can operate and study the dependence of IL properties on their structure, a comparative study of the thermal stability of a number of alkyl ammonium chloride salts **a-j** was carried out [101]. The inclusion of one or two saturated rings of five, six, seven, or eight atoms into the structure of the studied compounds has been attempted (Scheme 3). The relative thermal stabilities of the chloride species were compared by employing temperature-ramped thermogravimetric analysis (TGA), and trends were elucidated. The thermal decomposition of ILs usually proceeds through an anionic nucleophilic attack on the \(\alpha\)-carbon atom from the ammonium nitrogen of the pendant alkyl chains of the cation. The authors of the cited work suggested that the inclusion of a second cyclic structure would increase the steric interactions around the ammonium nitrogen center, and \(\alpha\)-carbons would be less susceptible to anionic attack. This should provide improved thermal stability to the azoniaspiroammonium chlorides **d-j** compared to the tetraalkylammonium chlorides **a-c**. The authors' hypothesis has been confirmed in practice since the salts **e-j** show higher T\({}_{\text{onset}}\) and T\({}_{\text{start}}\) values than salts **a-c**. For the single-ring salts **a-c**, a slight enhancement in the thermal stability with an increase in the ring size has been observed, from the five-membered pyrrolidinium salt **a** (T\({}_{\text{onset}}\) = 228 \({}^{\circ}\)C) to the seven-membered azepanium salt **c** (T\({}_{\text{onset}}\) = 254 \({}^{\circ}\)C). [T\({}_{\text{onset}}\) parameter represents a profound overestimate of the long-term thermal stability of IL liquids and related compounds; T\({}_{\text{start}}\) quantifies the first point of appreciable weight loss (d\({}_{\text{w}}\)/d\({}_{\text{t}}\) a 0)]. The crystals of azoniaspiro salt **d** were found to be highly hygroscopic, and compound **d** is omitted from the further discussion. Within the investigated series of the azoniaspiro species **d-j**, a significant variation in thermal decomposition behavior was observed--the salts incorporating the five-membered rings **g** and **j** exhibited poorer thermal stabilities than the salts composed of six-, seven-, and eight-membered rings. The melts of the fully inorganic salts are stable at elevated temperatures but they have high melting points that limit their utility at lower temperatures. It has been proposed that azoniaspiro salts may partially bridge this gap with respect to the stable liquid range. The thermal properties of azoniaspiro salts represent promising candidates for IL solvent systems, offering the beneficial properties of high thermal stability and a wide electrochemical window. Therefore, analogous to the reputation of ILs as designer solvents, a broad range of structures and properties via modifications to the size and substituents of the two cyclic moieties is clearly presented.