Thermal Stability of Ionic Liquids: Current Status and Prospects for Future Development

: Ionic liquids (ILs) are the safest solvent in various high-temperature applications due to their non-flammable properties. In order to obtain their thermal stability properties, thermogravimetric analysis (TGA) is extensively used to analyze the kinetics of the thermal decomposition process. This review summarizes the different kinetics analysis methods and finds the isoconversional methods are superior to the Arrhenius methods in calculating the activation energy, and two tools — the compensation effect and master plots — are suggested for the calculation of the pre-exponential factor. With both parameters, the maximum operating temperature (MOT) can be calculated to predict the thermal stability in long-term runnings. The collection of thermal stability data of ILs with divergent cations and anions shows the structure of cations such as alkyl side chains, functional groups, and alkyl substituents will affect the thermal stability, but their influence is less than that of anions. To develop ILs with superior thermal stability, dicationic ILs (DILs) are recommended, and typically, [C 4 (MIM) 2 ][NTf 2 ] 2 has a decomposition temperature as high as 468.1 °C. For the convenience of application, thermal stability on the decomposition temperature and thermal decomposition activation energy of 130 ILs are summarized at the end of this manuscript.

So far, the thermal stability of ILs has been studied by many techniques, such as UV (ultraviolet)-vis spectroscopy [36], flame ionization detection (FID) [37,38], and mass spectrometry (MS) [39][40][41]. Moreover, in some studies, ILs are heated isothermally in a furnace and open to air to simulate the real environment [42,43]. In order to quantitatively evaluate the thermal stability of ILs, parameters accounting for short-term and long-term thermal stability are obtained by thermogravimetric analysis (TGA), and the most representative ones are the onset decomposition temperature Tonset and Tz/y (decomposition degree z in a selected time y), respectively. Although many studies have indicated Tonset obtained by dynamic TGA overestimates the thermal stability [36,39,44], it is still widely used to express the stability of ILs from divergent papers [45,46]. Despite the long-term isothermal experiments can more accurately reflect the real stability of ILs [44,[47][48][49], but it should be noted that the time in these experiments is still far less than the heat exposure time in many real applications. To provide the method in predicting long-term thermal stability, different models have been developed [50][51][52].
To solve the problem when the entire sample cannot be decomposed [47,48], nonisothermal TGA is developed as the most popular method to determine the kinetic parameters in the thermal decomposition process, and the combination of TGA data allows researchers to calculate the activation energy and pre-exponential factor in this process. Arrhenius methods are widely used in kinetics analysis due to their simple calculation process. However, some recent studies have used the isoconversional methods to calculate the activation energy [44,53,54], while the methods of compensation effect and master plots provide the pre-exponential factor calculation [55][56][57].
The thermal stability of ILs is mainly determined by the structure of anions and cations, and anions usually play a major role [46,58,59]. However, more studies are focused on the modification of cations, including alkyl chain length, functional groups, and alkyl substituents to improve the thermal stability of ILs [60,61]. Other conditions, such as gas atmosphere, heating rates, and impurities, also influence the thermal stability measurements [45,62]. Among which, the heating rate has the most significant impact on the TGA results, and the difference in Tonset obtained at 1 °C /min and 20 °C /min is even up to 100 °C [44]. Finally, ILs mixtures and dicationic ILs (DILs) are introduced as potential applications for the further development of high-temperature ILs.

Short-Term Thermal Stability
Dynamic TGA is applied in the study for short-term thermal stability measurement, and the most common heating rate is 10 °C /min [36,39,42,[63][64][65][66]. Effects of different heating rates, at 5, 10, 15, 20 °C /min are available in previous studies, and it should be noted that the faster the heating rate is, the more overestimated the thermal stability of ILs is, and the corresponding dynamic TGA curve will move to the right accordingly. [18,39,49,53,57].
Tonset, which is basically known as the short-term thermal stability, is determined by dynamic TGA. Moreover, it is a value calculated by the thermal analysis software, which is defined as the intersection of the baseline of zero weight loss and the tangent of the weight versus temperature curve as decomposition occurs [45]. Therefore, the temperature at which the sample begins to decompose is lower than Tonset. Tz (decomposition degree z) is also a parameter to characterize the short-term thermal stability, which directly shows the temperatures at different decomposition degrees [55,63,[67][68][69]. Both parameters are illustrated clearly in Figure 1. In fact, the difference between Tonset and T50% or T10% is a measure of the decomposition rate, and the lower the temperature difference between Tonset and T50% or T10%, the lower the stability of the ionic liquid [70].
Another method to investigate the short-term thermal stability called derivative thermogravimetry (DTG) is shown in Figure 2. It determines the temperature of maximum degradation Tpeak [42,64,[71][72][73], i.e., if there is one more peak in the DTG curve, the temperature corresponding to the highest peak will be selected [74]. The number of peaks in a DTG curve is also an important parameter. Most ILs have only one peak, indicating the decomposition process is a simple one-step process [44], and two peaks in some DTG curves correspond to two different degradation processes in the sample [63].

Long-Term Thermal Stability
Although Tonset is widely used to describe the thermal stability of ILs, it is not a suitable parameter for long-term industrial applications. To establish the correlation between the operating temperature and time, it is necessary to investigate the long-term thermal stability of ILs through isothermal TGA [36,75,76]. In this method, samples are heated at a fixed temperature varying from an hour to tens of hours, then Tz/y is determined [49,57,[77][78][79]. As shown in Figure 3, more than three temperatures are usually used in the analysis [75,78], and the interval between different temperatures is determined by Tonset [74]. However, it is much shorter than the cycle of industrial runnings. Neither extrapolating experimental data nor extending the heating time of isothermal TGA seems to be the best solution. Therefore, some means predicting the long-term thermal stability of ILs have been proposed. Seeberger et al. [50] proposed using the maximum operating temperature (MOT) to measure the long-term thermal stability at 1% decomposition degree, which is where denotes the pre-exponential factor, is the activation energy, is the universal gas constant, and is the maximum operation time. MOT has been used to predict the long-term thermal stability successfully in several recent studies [56,71,73]. Moreover, MOT is also applied in the prediction of the thermal stability of ILs mixtures [80].
To estimate the maximum time for ILs used under a specific temperature, Salgado et al. [51] proposed an exponential function of the temperature, similar to Cao and Mu [74], which is where is the time in minutes, and are the fitting parameters, and is the scanning temperature in K. From isothermal TGA, , that each IL takes to decompose to a certain percentage of mass, is determined at different . This equation can quantitatively describe the relationship between the decomposition temperature and the decomposition time of ILs under a certain degree of decomposition. With this method, T0.01/10h, T0.05/10h, and T0.1/10h have been correlated, and T0.01/10h was given by Wooster et al. [52] according to where ( / ≠0) is the temperature at which the first appreciable weight loss occurs. The results of 0.01/10ℎ calculated by Equation (2) is higher than that by Equation (3), which attributes to the different experimental conditions [51]. In another research, the above three methods are used to calculate 0.01/10ℎ of some aprotic ILs [63]. The experiments prove that the results obtained by Wooster's method (Equation (3)) are the highest, followed by Salgado's method (Equation (2)), and those calculated by Seeberger's method (Equation (1)) are the lowest.

Heating Rates
In dynamic TGA, Tonset varies significantly with different heating rates, since when the temperature rises rapidly, it will easily exceed the onset decomposition temperature, resulting in a mass loss that cannot be measured correctly [50]. As shown in Figure 4, compared with a slower heating rate, the higher Tonset is obtained at a faster heating rate [57]. For [C2MIM][NTf2] and [C3MIM][NTf2], it is found that the difference of Tonset obtained at 1 °C /min and 20 °C /min is 100 °C [44]. This trend has also been found in more investigations [39,54]. Therefore, it is necessary to pay attention to the heating rate when comparing Tonset by different authors. As a criterion, most of the literature known to date gives the Tonset value at 10 °C /min.

Gas Atmosphere
N2 is often chosen as the gas atmosphere in TGA, while it is just a special case in industrial applications. Therefore, it is necessary to figure out the influence of different gas atmospheres on the thermal stability of ILs. As shown by Götz et al., the TGA results of [P14, 6,6,6][NTf2] indicate the mass loss in H2 is much higher than that in N2 at the same temperature, i.e., H2 accelerates the decomposition of [P14,6,6,6][NTf2] [6]. Figure 5 shows some Tonset and T10% obtained in both N2 and O2 atmospheres. The results reveal that the Tonset and T10% of [NTf2] -ILs obtained in O2 are lower by 38 °C to 97 °C than those obtained in nitrogen, indicating the reactive atmosphere reduces their thermal stability [49]. The effect of air on thermal stability is similar to that of O2. As shown in Figure 6, [P4, 4,4,8][BScB] and [P4, 4,4,8][BMB] are more thermally stable in N2 than in air [81], and the same results are observed in another study [55]. However, according to Figure 7, the Tonset of some choline, piperidine, and phosphonium ILs are not significantly affected by gas atmospheres, and the maximum temperature difference is only 26 °C [40,73]. -ILs in N2 and O2 at a heating rate of 10 °C /min [49].    [82]. On the other hand, the Tonset of the quaternary phosphonium carboxylate ILs decreases by 19.8 °C at most when the water content is saturated, as shown in Figure 8 [83]. Moreover, the TGA curves of two protic ILs, [ 6,6,14][(C2F5)3PF3], differences of Tonset between water saturation and supply conditions are lower than the expanded uncertainties of the apparatus [63].
In addition to water, the influence of inorganic salts and metal oxides on thermal stability has also been studied. Adding CuO to [C2MIM][Ac] results in an exothermic reaction and lowers the decomposition temperature. It is assumed that CuO decomposes ester compounds produced from [C2MIM][Ac] as it might accelerate the decomposition of organic compounds [85]. In addition, results summarized in Figure 9 show Tonset of bulk ILs are significantly higher than the Tonset of γ-Al2O3-supported ILs, indicating the interactions of ILs with γ-Al2O3 control their thermal stability limits [58]. Putting 20% NH4Cl into [BMMIM][Cl] reduces the thermal stability [86], and Tonset of commercial [P6, 6,6,14][Cl] is 8 °C lower than its pure counterpart [40]. Both studies mean the existence of impurities alters the thermal stability of ILs.  Tonset of Bulk and γ-Al2O3-supported ILs at a heating rate of 10 °C /min in nitrogen atmosphere [58].

Kinetics of Thermal Decomposition
The kinetic data in the thermal decomposition process can be obtained based on isothermal and non-isothermal TGAs. Generally, the rate of thermal decomposition can be expressed by the following formula: where ( ) is the rate constant, ( ) is determined by the kinetic models as shown in Table 1, and 0 , , and 1 are the initial mass, mass at certain time, and terminal mass of the sample, respectively. is defined as the fraction of the total mass loss in the process, ranging from 0 (no mass loss) to 1 (complete mass loss). Table 1. Some of the models used in thermal decomposition kinetics of ILs. (n ≠ 1).

Reaction Model
Code The correlation between the rate constant and the temperature is defined by the Arrhenius equation as where denotes the pre-exponential factor, is the activation energy, is the universal gas constant, and refers to the temperature. Combining Equation (4) and Equation (6) yields If the heating rate is constant, then Equation (7) can be transformed into Equation (8) as follows: The calculation of Arrhenius parameters, E and A, will be introduced in the following sections.

Isoconversional Methods
The isoconversional methods are based on the isoconversional principle, meaning that the reaction rate / is only a function of temperature at a constant extent of conversion [41] and the reaction model ( ) is independent of temperature [87]. In addition, these methods require that the decomposition process can be approximated as a single-step kinetic process, that is to say, activation energy calculated by isoconversional methods does not vary significantly with [88]. According to this principle, the isoconversional values of activation energy can be evaluated without assuming or determining any particular form of the reaction model, so the isoconversional methods are frequently called "model-free" methods. Currently, these methods have been classified into differential isoconversional methods and integral isoconversional methods.

Differential Isoconversional Methods
Taking the logarithm on both sides of Equation (8), the most representative differential isoconversional method is obtained according to Friedman [89] where the subscript denotes individual heating rate, and , is the temperature at which the extent of conversion is reached under th heating rate. The activation energy corresponding to each conversion can be obtained easily by using DTG and TGA data. As shown in Figure 10, in order to better characterize the thermal decomposition process of [C4MIM][PF6] in the nitrogen atmosphere, the thermal decomposition is divided into two steps. The activation energy value calculated by the Friedman method suggests that the linearity of the result is relatively poor in the fitting process and the activation energy varies significantly with [55]. In addition, a similar conclusion has also been drawn in other investigations [90]. In the non-isothermal kinetic analysis of imidazolium [NTf2] ILs, it is found that the activation energy calculated by the Friedman method has the largest variation range [44].

Integral isoconversional methods
By integrating Equation (7), the following equation can be obtained as In the case of constant heating rate, Equation (10) can be converted into According to the isoconversional principle, Equation (10) can be converted into Equation (12) under the isothermal condition as follows: where indicates isothermal temperature, and t , is the time to reach a given extent of conversion in the th isothermal experiment. Apart from the study by Williams et al. that used this method to calculate the activation energy and pre-exponential factor values of 1-alkyl-3-methylimidazolium chloride ILs [57], no other literature is known using this method to analyze the thermal degradation kinetics of ILs.
Compared with isothermal experiments, the constant heating rate ( =constant) is more popular in isoconversional kinetic analysis [18,39,49,53,56,57]. There is no analytical solution to the integral of Equation (11), so a series of integral isoconversional methods using different approximations have appeared. Many integral isoconversional methods can be expressed in the following form [91]: Flynn-Wall-Ozawa (FWO) method [92,93], Kissinger-Akahira-Sunose (KAS) [94] method, which is also known as Coats-Redfern (CR) method [95], and Starink method [96] are all derived based on Equation (14), Equation (15), and Equation (16), respectively. Then the activation energy values can be calculated from the slope of plots.  [18]. The calculated results are in line with many other studies [44,53,54]. Therefore, as recommended by ICTAC (the International Confederation for Thermal Analysis and Calorimetry), there is no need to perform kinetic analysis in multiple forms of integral isoconversional methods [88].  [97][98][99]. For a series of different heating rates, the activation energy values can be obtained by minimizing the following function: where the subscript denotes different heating programs, and the subscript is used to denote all heating rates other than . In order to reduce error as much as possible, the temperature integral is calculated over a small segment [87] as Table 3 shows that the activation energy values calculated by the Vyazovkin method are similar to the results of the KAS and Starink methods, indicating that the latter two simple integration methods are precise enough in most cases [55,57], and the Vyazovkin method is a better choice when the activation energy values vary with obviously [88]. When the maximum rate of decomposition of ILs is achieved, in Equation (13) can be replaced by [91]. This class of methods will be indicated by adding "max rate" before the name. In the max-rate Starink method, the activation energy of 1-alkyl-3methylimidazolium chloride can be calculated, which is similar to that calculated by the Vyazovkin method and Starink method, as shown in Table 4 [57]. However, the activation energy of [C4MPy][NTf2] calculated by is about 20 kJ/mol higher than that calculated by [53].

Arrhenius Methods
According to the data of isothermal TGA, Arrhenius methods directly use Equation (6) to calculate the values of activation energy and pre-exponential factor. Due to the simplicity of the calculation process, Arrhenius methods have been used extensively [39,44,71,100,101]. By assuming the decomposition is of zero-order or first-order, the rate constant ( ) can be calculated.
For zero-order reactions Equation (19) can be transformed into where is sample mass, and / is obtained as the slope of a linear fitting of mass loss versus time for every isothermal TGA. The activation energy and pre-exponential factor can be derived from Equation (20). Several studies have applied this method to calculate the activation energy and pre-exponential factor [47,74,75,78,101]. As shown in Table 5, Parajó et al. used both the isoconversional methods and the zero-order Arrhenius method to calculate the activation energies of several imidazolium [NTf2] ILs, and they found some activation energies calculated by the non-isothermal methods are about 20 kJ/mol lower than those calculated by the isothermal method [44]. Table 5. Activation energies for the selected ILs by the different methods in the air atmosphere [44]. For the first-order reaction

ILs
Equation (21) can be transformed into Table 6 shows the values of activation energy and pre-exponential factor calculated by the first-order Arrhenius method, and these values are quite different from other calculation results. In the studies of Efimova et al., the activation energy values are calculated using the isoconversional methods, and then Equation (22) is used to calculate the pre-exponential factor values [39,71].

Determining the Pre-Exponential Factor
Despite the isoconversional methods are convenient to calculate the values of activation energy without the reaction model, it is difficult to obtain the values of the preexponential factor. For single-step reactions, the pre-exponential factors can be determined by the following means when using model-free methods [88].

Using Compensation Effect
Different pairs of the Arrhenius parameters and can be obtained by substituting different ( ) and experimental data into Equation (8). Although the Arrhenius parameters vary widely with ( ), according to the compensation effect, they conform to the following correlation [88]: where and are the parameters of the compensation effect and are determined by fitting the pairs of and at different into Equation (23). Then for the singlestep reaction, the average pre-exponential factor 0 can be obtained by substituting the average activation energy 0 determined in non-isothermal experiments into Equation (23

Using Master Plots
Using a non-isothermal TGA, after determining the activation energy values of the reaction, the reaction mechanism can be simply and accurately determined by plotting master plots ( ) or ( ) [102,103]. The ( ) function has the following form: After determining ( / ) and at different , it is possible to plot the experimental values of ( ) against . Because is an unknown constant, the shape of the theoretical master plots ( ) is the same as ( ). The experimental master plots ( ) are normalized to vary from 0 to 1, then compared with the theoretical shape of ( ) in different kinetic models in Table 1, and the reaction model of thermal decomposition can be determined. Finally, can be easily obtained from Equation (24). Another function ( ) has the following form: The temperature integral in ( ) can be approximately expressed by the following formula [97]: = .
where ( ) = 3 + 2 + 88 + 96 4 + 20 3 + 120 2 + 240 + 120 Combining Equations (7) and (26) followed by some rearrangement, the following equation is obtained: where ( / ) 2 [ ( ) ] is an experimental value. It has been proved [ ( ) ] has a negligible influence on the shape of experimental master plots ( ) [102,103]. So the correlation between experimental ( ) and can be determined from the data of nonisothermal TGA and DTG. Theoretical master plots ( ) can be drawn according to different kinetic models in Table 1, and the suitable reaction models can be determined by normalizing experimental ( ) and the theoretical ones in different kinetic models from 0 to 1 and comparing the normalized results.
After the reaction model has been established, Williams et al. calculated the preexponential factor values from Equation (24). By comparing the pre-exponential factor values of [C4MIM][Cl] calculated by several different methods in Table 7, it can be found that results calculated by master plots [57] are close to those calculated by the zero-order Arrhenius method [74], and those calculated by the first-order Arrhenius method [39] are much lower. * A is in min −1.

Influence of Alkyl Chain Length
As shown in Figure 11, a method based on the cations exchange of ILs with sodium montmorillonite clay (MMT-Na+) is adopted to estimate thermal stability from [C2MIM] + to [C16MIM] + [104], and the T1% and T50% decrease with the increase of alkyl chain length. The same conclusion is obtained from quantum chemistry calculation [104]. The thermal stability of 1-alkyl-4-methyl-1,2,4-triazolium iodides shows a small but consistent decrease as the chain length is increased from butyl to dodecyl [105]. For the pyrrolidinium [NTf2] ILs in Figure 12, [107]. Generally, longer the chain length usually results in lower thermal stability, which is proved by more and more investigations [46,53]. However, there are some exceptions. Thermal stability of 1-alkyl-4-methyl-1,2,4triazolium [NTf2] ILs in Figure 13 reveals that there is no correlation between Tonset and alkyl chain length, and T0.01/10h instead increases with the increase of alkyl chain length [75]. The same results are obtained for some 3,5-dimethylpyrazolum ILs and some paramagnetic ILs [108,109]. In the study of amino acid ILs, [N1,1,14,2O12][Lys] with the longchain alkyl group on the side of nitrogen has higher thermal stability than [N1,1,6,2O12][Lys] under the isothermal condition, while both longer and shorter alkyl side chains harm the thermal stability under the non-isothermal condition [77]. The influence of alkyl chain length on thermal stability can be explained as follows: 1) Increasing the alkyl chain length weakens the bond between the alkyl chain and the cation such as imidazolium and ammonium, making it more vulnerable to attack and therefore more readily thermally decomposed [53,106]; 2) Carbocations and carbon radicals with longer alkyl chains are more stable and easier to leave on heating, and thus ILs with longer alkyl chains favor the decomposition phenomenon [72]; 3) For choline based amino acid ILs, longer alkyl chains on cation create strong hydrophobic interaction with gradually decreased hydrogen bond interaction due to bulkier cationic size. Upon decreasing alkyl chain length, the reverse effect renders lowering the overall thermal stability of the ILs [77].

Influence of Functionalization and Alkyl Substituents
Comparing the decomposition temperature of some imidazolium ILs and amino acid ILs with different functional groups in Table 8 [Br] respectively. The reduced thermal stability is explained by the introduction of amine to the imidazolium cation, which makes the nearby carbon atom more positively charged and easier to be attacked by the anion [101]. Aromatic functionality also decreases the thermal stability of imidazolium [NTf2] ILs, and the Tonset of IL with two benzyls is higher than that with naphthylmethyl [110].
Hydroxyl functionalization has different effects on the thermal stability of ILs.  [74,112]. It could be speculated that the hydrogen bonding interaction between the hydroxyethyl group and anion results in this trend, and the higher intramolecular hydrogen bond interaction stabilizes ILs and block thermal decomposition reaction to some extent [74].
Moreover, ether in alkyl side chains is favorable for thermal decomposition. As the number of introduced ether increases, the thermal stability tends to decrease [66,[114][115][116] because the introduction of oxygen atoms weakens the interaction between the cation and anion [77,115]. Specifically, Figure 14 demonstrates that the oxygen atom at the β-position significantly decreases the thermal stability of ILs, which can be explained by retroalkylation of piperidinium ILs into 1-methyl piperidine and the corresponding oxocarbenium ion intermediates at elevated temperature [117]. In addition, the length of the O-alkyl chain also influences thermal stability. However, it is interesting that the increase of the length of the oxygen alkyl chain improves the thermal stability in isothermal TGA, while the conclusion obtained by non-isothermal TGA is opposite [77]. This contradiction between the results of non-isothermal TGA and isothermal TGA has also been found in other studies [44,57,75,78,110].  In addition to functional groups, the alkyl substituents also affect thermal stability. Ngo et al. [118] found that the thermal stability of the imidazolium ILs is improved by increasing the degree of substitution of hydrogen by alkyl groups on the imidazolium ring, the potential energy barrier for an attack is increased. Methyl substitution in C2 (the carbon atom between two nitrogen atoms in the imidazolium ring) enhances the thermal stability [45,74,119]. C-2H acidity of the imidazolium ring is one of the structural factors determining short-term thermal stability because the most acidic proton on the imidazolium cation locates at C2 [58].
Moreover, Table 9 shows the bonding of the alkyl chain via tertiary carbon atom decreases the thermal stability of the IL, compared to those isomers, in which the alkyl is connected with the secondary carbon atom to the imidazolium ring [66,120]. The potential energy barrier of the decomposition reaction decreases because the decomposition products originating from tertiary carbon can be easily stabilized [66,72]. Table 9. T5% of several isomeric and quasi-isomeric ILs at a heating rate of 10 °C /min in argon atmosphere [66].

Influence of Anions and Cations
Anions play a major role in determining thermal stability, which has been confirmed in many studies [46,58,59]. For

ILs Mixtures
Preparing IL mixtures is a simpler and more promising approach compared with developing new ILs. Figure 15 [124]. It can be concluded that the thermal stability of binary mixtures is determined by the ILs which could be reconstituted by all ions in the mixture [125]. In other words, the anions and cations can combine freely in the mixture [111,112]. Figure 15. TGA curves of binary mixtures with different ILs and proportions at a heating rate of 10 °C /min in nitrogen atmosphere [124].
In order to predict the thermal stability of mixtures, Navarro et al. [126] proposed a method to obtain the mass loss of IL mixtures at different temperatures by using pure ILs TGA data as where is the mass that the mixture would lose in an ideal case at each temperature, denotes the experimental mass loss of species in forming the mixture at a certain temperature, refers to the mass fraction of the lth component in the mixture, and is the number of compounds involved in the mixture. This method has successfully predicted the thermal stability of several IL binary mixtures [80,126,127]. However, for the mixture of [4C4MPyr][NTf2] and [C2MIM][EtSO4], the model cannot predict its thermal stability well, and this may be related to the strong interaction between [EtSO4] -anion and the other ions in the mixture [128].
In addition, the study has revealed that mixing certain ILs in an appropriate ratio can improve thermal stability.  [64]. The results of nuclear magnetic resonance (NMR) indicate that the synergistic role of hydrogen bond and electrostatic interactions are proposed as the main reason for the improvement of the thermal stability of the IL mixtures.

Dicationic ILs
Due to their unique properties, dicationic ILs (DILs) have been widely used in thermal storage [67,129], organic synthesis [130], anti-corrosion [131], or as surface-active agents [132], and they are less toxic than their monocationic counterparts [133].  Figure  16. The values of DILs are 30-40 °C higher than their monocationic counterparts [134][135][136]. Other studies have obtained similar conclusions, which are ascribed to higher charges, higher molecular weight, and greater intermolecular interactions of DILs [6,129,137]. Due to their stronger electrostatic interactions, DILs often demonstrate higher melting points than monocationic ILs, which limits their applications [69]. Therefore, numerous methods of lowering melting points have been proposed, such as adjusting alkane linkage chain length, changing cationic head groups and anions, mixing DILs with the same cationic head groups and anion but different alkane linkage chains [37], utilizing branched alkane linkage chains [69] and designing unsymmetrical DILs [138]. As shown in Table 13, when the linkage chain is the same, the thermal stability of DILs with cationic head groups of (MMIM), (C1Py), and (PC3C3C3) is significantly different. Moreover, Figure 17 suggests that the longer the alkyl side chains are, the lower the thermal stability of DILs are. The reduced thermal stability is attributed to the decreasing symmetry of cations with the increase of the number of carbon atoms in the alkyl side chains, which hinders the crystal effective accumulation [129,139]. Table 14 demonstrates that the decomposition temperatures of DILs with some functional groups are higher than those of DILs without corresponding functional groups. This might be explained by the appearance of complex functional groups increasing the strength of intermolecular interaction and the energy of chemical bonds [139]. Table 13. Thermal stability of dicationic ILs (DILs) containing different cationic head groups at a heating rate of 10 °C /min in nitrogen atmosphere [69].   The change of the linkage chain between the two cationic head groups also has an important influence on thermal stability. The linkage chain is usually a straight-chain alkyl, and a longer linkage chain leads to worse thermal stability within a certain range, while a further increase in chain length leads to the decrease of thermal stability [37]. DILs with the branched linkage chain are developed due to reducing the high melting point of DILs with a straight linkage chain. Although the thermal stability of DILs containing the branched alkyl linkage chain is lower than their linear counterparts, it should be noted that this discrepancy is acceptable when the length of the linkage chain reaches C5. Therefore, the thermal stability of DILs can be tuned by branching linkage chains [69]. Recently, DILs with m-xylyl, pyridine functional groups, and dioxane linkage chain have been synthesized, and all of them have a low melting point and high thermal stability [140][141][142]. Similar to monocationic ILs, anions have a major influence in the thermal stability of DILs, and  [129,141]. The [NTf2] -DILs also have high thermal stability and low melting point [37,138,140]. Generally, the influence factors of DILs structure on thermal stability are similar to monocationic ILs. Therefore, the properties of DILs that have not been determined by experiments can be inferred from the known monocationic ILs.

Summary
This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, and the experimental conclusions that can be drawn.
The methods measuring the thermal stability of ILs are discussed. Although Tonset overestimates the thermal stability, it is still used as a universal parameter in different investigations. It should be noted that when using Tonset to compare the thermal stability it is necessary to determine whether the experimental conditions (atmosphere type, heating rate, etc.) are the same. Although Tz/y is defined as a parameter to measure the long-term thermal stability of ILs, it is clear that this parameter cannot meet the required time of industrial running. Thus, a series of methods have been proposed to predict the thermal stability of ILs over a longer period. Among them, MOT is considered a highly accurate method, and the activation energy and pre-exponential factor values in the thermal decomposition process are required for calculating MOT.
Different means to study the thermal degradation kinetics of ILs are elaborated. After extensively analyzing the kinetic data from several investigations, it can be concluded that the isoconversional methods have been widely used to calculate the activation energy values, and the results of integral isoconversional methods are more accurate than those of differential methods. In addition, maximum-rate methods and Arrhenius methods are used in many research studies due to their simple calculation process, although both methods are not as precise as isoconversional methods. The Arrhenius method is also used to calculate the pre-exponential factor. However, compensation effect or master plots are seen lately as reliable tools for calculating pre-exponential factor values.
Thermal stability data of divergent ILs are summarized in Table 16 according to the types of anions, and this table reveals many structural factors affecting thermal stability, in which the modification of cations is the prime research object. For cations, a longer alkyl side chain, functionalization, and the bonding of alkyl chain via tertiary carbon atom all reduce the thermal stability of ILs. In the process of etherification, the number and position of oxygen atoms and the length of the O-alkyl chain have evident effects on thermal stability. Moreover, due to the high acidity of the C2 proton, methyl substitution in the C2 position on the imidazolium ring can improve the thermal stability. Although there are many methods to change the thermal stability by tuning cations, it should be stressed that the type of anions plays a major role in determining thermal stability. Generally, some specific anions such as [NTf2] -, [BF4] -, and [PF6] -and cations such as imidazolium, pyrrolidinium, and pyridinium make the ILs more stable at high temperatures.
Finally, the thermal stability of some novel ILs is introduced to provide more choices in high-temperature applications. In ILs mixtures, the anions and cations can combine freely. It is also feasible to predict the thermal stability of ILs mixtures by using TGA data of pure ILs. DILs have better thermal stability than their monocationic counterparts, and the influence factors of DILs structure on thermal stability are similar to monocationic ILs. Moreover, due to the stronger electrostatic interactions, DILs have a higher melting point, while adjusting the length of the linkage chain and the type of anions can obtain DILs with low melting point and high thermal stability.