Reviewing electrochemical stability of ionic liquids-/deep eutectic solvents-based electrolytes in lithium-ion, lithium-metal and post-lithium-ion batteries for green and safe energy

Abstract

Sustainable energy is the key issue for the environment protection, human activity and economic development. Ionic liquids (ILs) and deep eutectic solvents (DESs) are dogmatically regarded as green and sustainable electrolytes in lithium-ion, lithium-metal (e.g., lithium-sulphur, lithium-oxygen) and post-lithium-ion (e.g., sodium-ion, magnesium-ion, and aluminum-ion) batteries. High electrochemical stability of ILs/ DESs is one of the prerequisites for green, sustainable and safe energy; while easy electrochemical decomposition of ILs/DESs would be contradictory to the concept of green chemistry by adding the cost, releasing volatile/hazardous by-products and hindering the recyclability. However, (1) are ILs/DESs-based electrolytes really electrochemically stable when they are not used in batteries? (2) are ILs/DESs-based electrolytes really electrochemically stable in real batteries? (3) how to design ILs/DESs-based electrolytes with high electrochemical stability for batteries to achieve sustainability and green development? Up to now, there is no summary on this topic, to the best of our knowledge. Here, we review the effect of chemical structure and non-structural factors on the electrochemical stability of ILs/DESs in simulated conditions. More importantly, electrochemical stability of ILs/DESs in real lithium-ion, lithium-metal and post-lithium-ion batteries is concluded and compared. Finally, the strategies to improve the electrochemical stability of ILs/DESs in lithium-ion, lithium-metal and post-lithium-ion batteries are proposed. This review would provide a guide to design ILs/DESs with high electrochemical stability for lithium-ion, lithium-metal and postlithium-ion batteries to achieve sustainable and green energy.

Headnote

Received 23 January 2023; revised 3 April 2023; accepted 6 May 2023

Available online 9 May 2023

Abstract

Sustainable energy is the key issue for the environment protection, human activity and economic development. Ionic liquids (ILs) and deep eutectic solvents (DESs) are dogmatically regarded as green and sustainable electrolytes in lithium-ion, lithium-metal (e.g., lithium-sulphur, lithium-oxygen) and post-lithium-ion (e.g., sodium-ion, magnesium-ion, and aluminum-ion) batteries. High electrochemical stability of ILs/ DESs is one of the prerequisites for green, sustainable and safe energy; while easy electrochemical decomposition of ILs/DESs would be contradictory to the concept of green chemistry by adding the cost, releasing volatile/hazardous by-products and hindering the recyclability. However, (1) are ILs/DESs-based electrolytes really electrochemically stable when they are not used in batteries? (2) are ILs/DESs-based electrolytes really electrochemically stable in real batteries? (3) how to design ILs/DESs-based electrolytes with high electrochemical stability for batteries to achieve sustainability and green development? Up to now, there is no summary on this topic, to the best of our knowledge. Here, we review the effect of chemical structure and non-structural factors on the electrochemical stability of ILs/DESs in simulated conditions. More importantly, electrochemical stability of ILs/DESs in real lithium-ion, lithium-metal and post-lithium-ion batteries is concluded and compared. Finally, the strategies to improve the electrochemical stability of ILs/DESs in lithium-ion, lithium-metal and post-lithium-ion batteries are proposed. This review would provide a guide to design ILs/DESs with high electrochemical stability for lithium-ion, lithium-metal and postlithium-ion batteries to achieve sustainable and green energy.

Keywords: Green solvents; Decomposition; Sustainable chemistry; Lithium-oxygen batteries; Lithium-sulphur batteries; Sodium-ion batteries

(ProQuest: ... denotes formulae omitted.)

1. Introduction

Ionic liquids (ILs) are deemed as green solvents for many years [1]. ILs are composed of cation and anion via ionic bond. The selection of various cation and anion makes ILs highly tunable and designable [2,3]. The favorable properties of ILs (e.g., less volatility, no flammability, high stability, high electrochemical conductivity, and low viscosity) make the application of ILs in many fields, such as separation [4-8], liquid robot [9], conductor [10], energy storage [11], catalysis [12-15]. Particularly, ILs have attracted much attention in lithium-ion, lithium-metal and post-lithium-ion batteries by acting as electrolytes [16-18]. Deep eutectic solvents (DESs) are also one kind of green solvents [1]. DESs have also attracted much attention due to the high atomic efficiency and easy synthesis procedure [19-27].

Lithium-ion batteries have been widely applied in energy storage and electronic devices [21,28-31]. ILs restrain water activity in aqueous lithium-ion batteries [32]. ILs could make lithium-ion batteries safer and more non· flammable [33,34]. The performance of lithium-ion batteries could be significantly improved by using ILs-based electrolytes [28]. ILs are helpful for constructing flexible and wearable lithium-ion batteries [35,36]. The performance of lithium-ion batteries is enhanced after using ILs as electrolytes [37].

Lithium-metal batteries (e.g., lithium-sulphur, lithium-oxygen batteries) are deemed as the next generation of rechargeable batteries that own high specific energy, high conductivity and low cost [16,30,31,38-43]. ILs/DESs-based electrolytes in lithium-metal batteries could avoid the usage of volatile organic compounds (VOCs) with high volatility [16]. More importantly, ILs-based electrolytes in lithium-metal batteries could delay the corrosion of aluminum [44,45], favor the deposition of lithium, suppress/avoid the formation of lithium dendrite [46-48]. ILs confined in covalent organic frameworks are able to enhance the safety of lithium-metal batteries [49].

Post-lithium-ion batteries is the new era of rechargeable batteries with low cost and high theoretical energy density, including sodium-ion, potassium-ion, magnesium-ion, aluminum-ion batteries, zinc-ion and calcium-ion batteries [50-53]. Instead, some other reports deem lithium-air, lithiumsulphur, lithium-metal, solid-state batteries or other kinds of batteries as post-lithium-ion batteries [54,55]. ILs-based electrolytes show promising prospect for the safe, non-flammable [56], long-lived [57], stable [58] and high-performance [59,60] post-lithium-ion batteries.

Most of the attention is paid to the enhancement of efficiency and safety of lithium-ion, lithium-metal and postlithium-ion batteries by using ILs-/DESs-based electrolytes; however, are ILs-/DESs-based electrolytes electrochemically stable when used in the above batteries? The concept of green and sustainable chemistry proposes the principles such as waste prevention, safer chemicals, safe solvents/auxiliaries, renewable feedstock, energy efficiency and inherently safer chemistry [61]. If ILs/DESs are easy to decompose electrochemically, (1) much waste would be produced; (2) the decomposed products would be flammable, explosive, corrosive and toxic; (3) the hard regeneration and purification of ILs/DESs would be energy-consuming and expensive; (4) ILs/DESs could not act as renewable feedstock. Therefore, high electrochemical stability of ILs/DESs is very important for developing green, sustainable and safe energy. Previous reports concluded that ILs as well as other so-called green solvents (e.g., DESs and supercritical fluids) are not so green [21,62-67]. Several reviews also summarize the electrochemical stability of ILs [63,65,68-70]. Specifically, Lan et al. only concluded the electrochemical reduction mechanisms of cation in ILs in 2012 [68]. De Vos et al. summarized the common factors and electrochemical decomposition mechanism of cation and anion in ILs in 2014 [69]. Xue et al. simply revealed the electrochemical stability of ILs in 2018, where thermal and radiolytic stability were also emphatically reviewed [65]. Further, Chen et al. reviewed the thermal stability and volatility of ILs and proposed strategy for reliable evaluation of ILs and DESs [71,72]. Recently, we reviewed the electrochemical stability of DESs while the section of electrochemical stability of ILs is only simply mentioned in this review [63]. Meanwhile, this review comprehensively summarized the thermal, chemical, radiolytic and biologic stability of ILs and DESs besides electrochemical stability [63]. As for the electrochemical stability of ILs/DESs in lithium-ion, lithium-metal and postlithium-ion batteries, there is still no review reported to the best of our knowledge. Due to the wide application of lithium-ion, lithium-metal and post-lithium-ion batteries and the high promising aspect of ILs/DESs applied in these fields, it is necessary to review the electrochemical stability of ILs/DESs in lithium-ion, lithium-metal and post-lithiumion batteries.

Here, in this review, we mainly (1) conclude the chemical structure and non-structural factors affecting the electrochemical stability of ILs/DESs not in batteries; (2) review the electrochemical stability of ILs/DESs in real batteries, i.e., lithium-ion, lithium-metal and post-lithium-ion batteries; (3) propose promising strategies to enhance electrochemical stability for their better application in lithium-ion, lithium-metal and post-lithium-ion batteries. As shown in Scheme 1, the key stability findings for the lithium-ion, lithium-metal and postlithium-ion batteries include thermal, chemical, electrochemical, radiolytic and biological stability; meanwhile, stability for each composition of batteries (e.g., cathode, anode, electrolytes, separator) is very equally important. This review only focuses the electrochemical stability of ILs-/DESs-based electrolytes in or not in lithium-ion, lithium-metal and postlithium-ion batteries by changing the factors of chemical structure (e.g., cation, alkyl chain length, C2 methylation, functional group, substituent position, anion) and non-structural factors (e.g., working electrode, reference electrode, conductivity, viscosity, melting point, cut-off current density, scan rate, AC/DC, AC frequency, temperature, water, oxygen, other compounds, pH, ultrasound). Particularly, we classify lithium-ion batteries into three subsections by electrolytes, namely pure ILs or pure DESs as electrolytes, ILs/DESs-based hybrid electrolytes and (quasi) solid-state ILs/DESs electrolytes. Lithium-metal batteries are divided into lithium-sulphur batteries and lithium-oxygen or lithium-air batteries. Postlithium-ion batteries include sodium-ion, potassium-ion, magnesium-ion, aluminium-ion, zinc-ion, calcium-ion types. The chemical structure of cation and anion of ILs is shown in Scheme 2, Table 1 and Table 2.

2. Electrochemical stability of ILs/DESs not in batteries

Electrochemical stability is commonly evaluated by the electrochemical window (EW), which is the difference between anodic limit and cathodic limit from the cyclic voltammetry or linear sweep voltammetry at a specific cut-off current. High value of anodic limit of ILs/DESs means the high difficulty to be oxidized. Low value of cathodic limit means the high difficulty to be reduced. High value of EW of ILs/DESs implies the high electrochemical stability of ILs/DESs. The electrochemical stability of ILs/DESs not in batteries is easy to be measured; furthermore, it is very important for the advanced prediction and estimation of electrochemical oxidation and reduction ability before the application in batteries [17]. We select cation type, alkyl chain length, C2 methylation, functional group, substituent group and anion to discuss the effect of chemical structure on the electrochemical stability of ILs. However, we select component to summary the effect of chemical structure on the electrochemical stability of DESs mainly due to the following two reasons: (1) DESs are mainly composed of two components, hydrogen-bonding acceptor (HBA) and hydrogen-bonding donor (HBD); therefore, it is commonly to analyze the factors from the aspects of components rather than anion, cation or functional group. Furthermore, there are no cation, anion for some DESs, such as Type V DESs [73-75]. (2) There are only very few papers reporting the electrochemical stability of DESs.

2.7. Effect of chemical structure

2.1.1. Effect of cation type

Inorganic cation ILs based on binary mixtures of alkali bis(fluorosulfonyl)amides show the wide EW of 5.0 V vs. Na⁺/ Na ([Na][Tf₂N]-[K][Tf₂N]) and 6.0 V vs. Li⁺/Li ([Li][Tf₂N][K][Tf₂N]) [76].

For vacuum-dried ILs with Tf₂N anion, the E W is ordered as [P₁₄₆₆₆ > [BmPyrr] > [Bmim] > [N₆₂₂₂], which suggests that phosphonium cation is the most electrochemically stable cation [77]. However, the cathodic limit is ordered as [P₁₄₆₆₆ > [N₆₂₂₂] > [BmPyrr] > [Bmim]; instead, the anodic limit is ordered as ... > [N6222] > [Bmim] ~ [BmPyrr] [78]. Computational study gives a different order of EW, i.e., phosphonium > pyrrolidinium > ammonium > imidazolium [79], which is slightly different from the order from the experiment above ... > [BmPyrr] > [Bmim] > [N₆₂₂₂ [77]). The difference could be ascribed to the impurities or anion type.

Slight modification of cation type would result in a different EW order of Tf₂N-based ILs, i.e., [N 1,1,2,102] > [HmPyrr] > [MOEmMor] > [MOPmPip] > [HPy] > [S₂₂₂] [80]. The order of cathodic stability is slightly different, i.e., ... > [HmPyrr] > [MOPmPip] > [MOEmMor] > [S₂₂₂] > [HPy] [80]. The order of anodic stability is very different, i.e., [MOEmMor] > [HPy] > [HmPyrr] > ... > [MOPmPip] > [S₂₂₂] [80]. It suggests that the electrochemical stability of ILs affected by cation type is closely related to the specific groups in the cation. We can also conclude that the orders for EW, cathodic stability and anodic stability affected by the cation type are not the same. The highest anodic stability of [MOEmMor] could ascribed to the presence of oxygen in both the six-membered ring and the alkyl chain. The second anodic stability of [HPy] is caused by the conjugated effect of the pyridinium ring. The cation without conjugated effect or stable ring would be easier to be oxidized, hence a lower anodic stability (e.g., [Nļ j 2,102] [MOPmPip][S₂₂₂]).

For [Li][Tf₂N]+Tf₂N-based ILs mixtures, pyrrolidiniumand piperidinium-based ILs are much more cathodically stable than imidazolium-based ILs at room temperature and elevated temperatures [81]. The cathodic limit of pyrrolidinium- and piperidinium-based ILs is comparable with that of ethylene carbonate/diethyl carbonate (EC/DEC) mixture [81]. The lower reductive stability of imidazolium-based ILs is due to the C2-H in the imidazolium ring; however, there is only alkyl hydrogen atoms in the pyrrolidinium- and piperidinium-based ILs. Similarly, EC/DEC mixture only own alkyl hydrogen atoms rather than C2-H, thus, their cathodic stability is similar to that of pyrrolidinium- and piperidinium-based ILs.

EW of ILs (i.e., [BmAzp][Tf₂N] [BmmPip][Tf₂N] and [BmPyrr] [Tf₂N]) varying in cation is ordered as azepanium > piperidinium > pyrrolidinium; the anodic limit is ordered as azepanium ~ piperidinium > pyrrolidinium; the cathodic limit is ordered as azepan ium < piperidinium < pyrrolidinium [82]. It suggests that electrochemical stability of ILs cation is related to the carbon number in the cation ring. Specifically, seven-membered ring is the most cathodically and anodic stable (e.g., azepanium); six-membered ring is the second most cathodically and anodic stable (e.g., piperidinium); the poorest stable ring is the fivemembered ring (e.g., pyrrolidinium). It is anticipated that the cathodic and anodic stability of four- and three-membered ring would be poorer. It should be noted that the comparison of electrochemical stability requires the similar substituted group in the cation.

After functionalization, the electrochemical stability of ILs with different cations could be different. The comparison of ether-containing ILs ([MOEmAzp][Tf₂N] [MOEmmPip] [Tf₂N] and [MOEmPyrr][Tf₂N]) suggests the order of piperidinium > azepanium ~ pyrrolidinium (EW), piperidinium > azepanium > pyrrolidinium (anodic limit), and pyrrolidinium ~ piperidinium < azepanium (cathodic limit) [82]. Functionalized ILs with a different anion (e.g., [MOEmAzp][TFA] and [MOEmmPip] [TFA]) own different order of electrochemical stability, such as piperidinium < azepanium (EW), piperidinium < azepanium (anodic limit), and piperidinium ~ azepanium (cathodic limit) [82]. It could be concluded that if the anion is different, the electrochemical stability order affected by the cation would be different. Furthermore, after functionalization, the electrochemical stability order of azepanium > piperidinium > pyrrolidinium is also changed.

EW order (vs. Pt) of ILs affected by the cation type is also related to the water or cut-off current density. For example, EW is ordered as [BmPyrr][Tf₂N] 5.9 V = [N₄in][Tf₂N] 5.9 V > [S₂₂₁][Tf₂N] 5.0 V > [Bmim][Tf₂N] 4.7 V (1.9 mA cm"2, <1 RH%) and [BmPyrr][Tf₂N] 5.6 V > [N_4m] [Tf₂N] 4.1 V > [Bmim][Tf₂N] 3.8 V > [S₂₂₁][Tf₂N] 3.5 V (0.19 mA cm^-2, <1 RH%) [83]. When the RH% increases as in Fig. 1, the EW (vs. Pt) is ordered as [BmPyrr] [Tf₂N] 2.3 V = [N₄₁₁₁][Tf₂N] 2.3 V > [Bmim][Tf₂N] 2.2 V > [S₂₂₁] [Tf₂N] 2.1 V (0.19 mA cm^-2) [83]. It could also be concluded that the EW order is slightly changed in the presence of RH. As can be seen from Fig. 1, the anodic limit decreases and the cathodic limit increases when RH increases for all the ILs investigated (Fig. 1) [83].

2.1.2. Effect of alkyl chain length

Longer alkyl chain length in the imidazolium cation could slightly increase the EW. For example [Bmim][Tf₂N] and [Emim][Tf₂N] own the EW of 5.0 V and 4.6 V vs. Fc⁺/Fc at 25 °C, respectively (Pt electrode, 5 mA cm^-2 cut-off current density) [77]. However, for anodic and cathodic stability, ILs with longer alkyl chain are not necessarily more stable. For example [Bmim][Tf₂N] owns a lower anodic limit than [Emim][Tf₂N], indicating that ILs with longer alkyl chain length is easy to be oxidized [78]. Cathodic limit of [Bmim] [Tf₂N] with longer alkyl chain could be either higher or lower than that of [Emim][Tf₂N] depending on the cut-off current density, which implies the complex reduction mechanism for [Bmim][Tf₂N] and [Emim][Tf₂N] [78].

Comparison of ILs [C_nmim][BF₄] with alkyl chain length from ethyl to octyl would make the order of electrochemical stability more complicated. EW of [C_nmim] [BF₄] is ordered as [Emim] > [Bmim] < [Hmim] < [Ornim (or [Omim > [Emim > [Hmim] > [Bmim]); anodic limit of [C_nmim][BF₄] is ordered as [Emim] < [Bmim > [Hmim] < [Ornim] (or [Ornim > [Bmim] > [Hmim] > [Emim]); cathodic limit of [C_nmim] [BF₄] is ordered as [Emim < [Bmim] > [Hmim] < [Omim] (or [Bmim] = [Omim] > [Hmim] > [Emim]) [67]. It seems that [Bmim] owns the narrowest EW while longer or shorter alkyl chain length would widen the EW. The electrochemical stability of ILs is not linearly correlated with the alkyl chain length although it is claimed that increasing alkyl chain length increases the EW width [67]. Correspondingly, Bmim owns the widest EW when comparing the EW of [Mmim][Tf₂N] [Emim][Tf₂N] [Bmim][Tf₂N] and [Hmim][Tf₂N] [67].

For pyrrolidinium-based ILs, longer alkyl chain length leads to a higher cathodic, anodic and electrochemical stability. For example [BmPyrr][Tf₂N] with butyl has the cathodic, anodic limit and EW of -2.92, 2.28 and 5.20 V vs. Ag/AgCl, respectively [HmPyrr][Tf₂N] with hexyl has the cathodic, anodic limit and EW of -3.06, 2.61 and 5.67 V vs. Ag/AgCl, respectively [80].

For ILs with other cation type, longer alkyl chain length could decrease or keep the electrochemical stability. For example, cathodic limit of [BmAzp][Tf₂N] and [HmAzp] [Tf₂N] is the same; instead, EW and anodic limit of [HmAzp] [Tf₂N] with hexyl group is lower than that of [BmAzp][Tf₂N] with butyl group [82]. The order of [BmmPip][Tf₂N] and [HmmPip][Tf₂N] is similar to that of [BmAzp][Tf₂N] and [HmAzp] [Tf₂N] [82].

2.1.3. Effect of C2 methylation

C2 methylation in ILs would increase or decrease the electrochemical stability depending on the ILs type and the working electrode.

[Bmim][Tf₂N] and [Bmmim][Tf₂N] own the EW of 5.0 V and 5.2 V vs. Fc⁺/Fc at 25 °C, respectively (Pt electrode, 5 mA cm^-2 cut-off current density) [77]. It indicates that C2 methylation could widen the EW in these conditions.

The EW of [Bmmim][BF₄] with C2 methylation is the same as that of [Bmim] [BF₄] by using Au as the working electrode, higher than that of [Bmim][BF₄] by using Ta working electrode, while lower than that of [Bmim][BF₄] by using GC/Pt working electrode [84].

The tendency of anodic and cathodic limit affected by C2 methylation by using different working electrode is also different from that of EW. For example, anodic limit of [Bmim][BF₄] and [Bmmim][BF₄] at Ta working electrode is the same, while anodic limit of [Bmim][BF₄] is higher than that of [Bmmim][BF₄] at Au and Pt working electrode. However, anodic limit of [Bmim][BF₄] is lower than that of [Bmmim][BF₄] at GC working electrode [84].

2.1.4. Effect of functional group

Benzyl-substituted cation is less electrochemically stable than alkyl-substituted cation due to the easier leaving ability of benzyl group [85].

Ether functionalization on azepanium cation (e.g., [BmAzp][Tf₂N] and [MOEmAzp][Tf₂N]) would increase the cathodic limit, decrease the anodic limit, and thus narrow the EW [82]. Instead, ether functionalization on piperidinium cation (e.g., [BmmPip][Tf₂N] and [MOEmmPip][Tf₂N]) would not change the cathodic limit; however, both anodic limit and EW are decreased after ether functionalization [82]. Ether functionalization on pyrrolidinium cation (e.g., [BmPyrr][Tf₂N] and [MOEmPyrr][Tf₂N]) would not change the EW while decrease both cathodic limit and anodic limit [82]. Ether functionalization on ammonium cation (e.g., [Nin₄][Tf₂N] and [NiiiioJ[Tf₂N]) would decrease the cathodic, anodic and electrochemical stability [86]. The order of electrochemical stability for [2MBenBim][Tf₂N] and [2MOBenBim][Tf₂N] or [4MBenBim][Tf₂N] and [4MOBenBim][Tf₂N] is complex [87].

Hydroxyl group would make the anodic, cathodic and electrochemical stability of ILs lower. For example [HPy] [Tf₂N] without hydroxyl group has the cathodic limit, anodic limit and EW of -1.22, 2.74 and 3.96 V vs. Ag/AgCl, respectively at GC working electrode. However, the cathodic limit, anodic limit and EW of [OHpPy][Tf₂N] with hydroxyl group are -1.12, 2.69 and 3.81 V vs. Ag/AgCl, respectively [80].

Alkenyl groups in ammonium cation (e.g., [Nn3₃][Tf₂N] and [N_113A][Tf₂N]) would improve the cathodic stability and the overall electrochemical stability, while the anodic stability is deteriorated [69]. Incorporation of unsaturated bonds in phosphonium cation would lead to a lower anodic stability while the cathodic stability is unchanged [69].

Cyano, ether, ester functionalization in sulfonium-based ILs would decrease the cathodic stability of ILs due to the easy reduction and the increased acidity of functional group. However, the anodic stability is enhanced after incorporating cyano and ester groups in sulfonium cation [69].

Electrochemical stability of ILs could be controlled by the functional group. The incorporation of cyclic alkene, ether, amine group in ILs is less electrochemically stable than linear alkane group in cyclic phosphonium-based ILs [88]. It means that linear alkane group would be a potential strategy to achieve the electrochemically stable electrolytes for lithiumion, lithium-metal and post-lithium-ion batteries.

The incorporation of CF₃ group in the chain of ILs cation would reduce the electrochemical stability of ILs compared with the alkyl group [89]. It could be ascribed to the withdrawing effect of CF₃ group.

2.1.5. Effect of substituent position

Even though the substituent group is the same for ILs, the electrochemical stability would be different if the substituent position is not the same [2MBenBim][Tf₂N] and [4MBenBim] [Tf₂N] have the methyl group substituted at 2 and 4 position, respectively [87]. Cathodic, anodic and electrochemical stability of [2MBenBim][Tf₂N] is higher than that of [4MBenBim][Tf₂N]. The order is also the same if methyl is replaced by methoxyl (i.e., [2MOBenBim][Tf₂N] > [4MOBenBim] [Tf₂N]) [87].

For different anions, the effect of substituent position on electrochemical stability of ILs is different. The anodic and electrochemical stability of [2MOBenBim][DCA] is higher than that of [4MOBenBim][DCA]; however, the cathodic stability of [2MOBenBim][DCA] is less than that of [4MOBenBim][DCA] [87]. This tendency is different from the order for [2MOBenBim][Tf₂N] and [4MOBenBim][Tf₂N] [87].

2.1.6. Effect of anion

Anion plays an important role in determining the electrochemical stability of ILs. EW order for [Bmim]-based ILs is FAP > PF₆ > Tf₂N > BF₄ at 1.9 mA cm^-2 (Fig. 1) [83]. The order of EW of Bmim-based ILs is PF₆ > BF₄ > Tf₂N > TFO > I (Fig. 2) [77]. It indicates that halide-based ILs is very electrochemically unstable, which is not suitable for application in electrochemical analysis, catalysis and batteries. A different EW order of Bmim-based ILs (PF₆ > Tf₂N > BF₄) is observed by other reports [90].

The cathodic limit of [Bmim]-based ILs is ordered as FAP ~ Tf₂N > BF₄ > PF₆ > TFO > MeSO₄ > NO₃ at mercury electrode; the anodic limit of [Bmim]-based ILs is ordered as FAP = Tf₂N > BF₄ > PF₆ > MeSO₄ > NO₃ > TFO [78].

Anodic limit at GC working electrode is reported to follow the sequence of Tf₂N > FAP > TFO > DCA > TFA [80], where the sequence could be improved or refined because the cation is not the same. In detail, for ILs with [BmPyrr] cation, the cathodic limited is ordered as

TFO ~ DCA < Tf₂N < TFA; the anodic limit and EW is ordered as Tf₂N > TFO > DCA > TFA [80]. However, the comparison of electrochemical stability for FAP and Tf₂N is complex. For ILs with the same [MOEmMor] cation, Tf₂N is more cathodically stable than FAP, while Tf₂N is less anodically stable than FAP; however, the EW of [MOEmMor] [Tf₂N] is wider than that of [MOEmMor][FAP]. For ILs with the same [MOPmPip] cation, cathodic, anodic and electrochemical stability of Tf₂N is higher than that of FAP [80]. It means that the types of cation influence the order of electrochemical stability for ILs with different anion.

The anodic limit of [Emim]-based ILs is ordered as B(CN)₄ > DCA > C(CN)₃ [91], which means that B(CN)₄ owns the highest anodic electrochemical stability while C(CN)₃ anion is the least stable.

A more systematic order for the EW of Bmim-based ILs is BF₄ > Tf₂N > TEO > C1O₄ > HSO₄ > Cl > Br > I > NO₃ > Ac > DCA (GC working electrode, 100 mV s^-1). The anodic limit of Bmim-based ILs follows a different order of BF₄ > Tf₂N > C1O₄ > TEO > HSO₄ > Cl > Br > I > NO₃ > DCA > Ac. The cathodic stability of Bmim-based ILs is also different [67].

Tf₂N is the anion with high electrochemical stability. EW of Tf₂N is wider than that of f₂N [92], suggesting that CF₃ group (in Tf₂N) enhances the electrochemical stability when compared with F group (in f₂N).

When the cation type is different, the order of electrochemical stability is different. For example [4MBenBim] [DCA] 5.41 (-2.45, 2.96) is more anodically, cathodically and electrochemically stable than [4MBenBim][Tf₂N] 3.39 (-2.13, 1.26) [87]. It means that DCA anion is more stable than Tf₂N in this condition, which is not consistent with other reports with common cations (e.g., [Brnim]) [67,80].

Five anions are identified as the most stable anion among 42 anions by theoretical calculation, i.e., B(CN)₄ > PF₆ > BF₄ > BOB > Tf₂N [93]. It shows the general order of electrochemical stability affected by anion; however, the order of anion stability is related to the cation type and the functionalization, which is beyond the consideration of theoretical calculation.

2.1.7. Effect of component

Chemical structures affecting the electrochemical stability of DESs include HBD, HBA and mole ratio [63,67]. The EW order affected by HBD is methyl urea > xylitol > urea > malonic acid > butanediol > EG > GL > oxalic acid (ChCl as HBA), xylitol > GL > urea > methyl urea > malonic acid > EG > butanediol (ChBr as HBA), oxalic acid > EG > GL > methyl urea > urea (Chi as HBA) [67]. The order of cathodic limit for ChCl-based DESs is methyl urea < urea < xylitol < butanediol < malonic acid < EG < GL < oxalic acid, while that of anodic limit is malonic acid > methyl urea ~ xylitol > urea > GL > butanediol > EG > oxalic acid [67]. Effect of HBA on the EW is ChCl > ChBr > Chi (urea as HBD), ChCl > ChClO₄ > ChBF₄ > ChBr > ChNO₃ > Chi (methyl urea as HBD) [67].

2.2. Effect of non-structural factor

2.2.1. Effect of working electrode

The composition of working electrode directly influences the electrochemical stability. For different ILs, the effect of working electrode on the electrochemical stability is different. For example, the EW follows the order of Ta > Pt > Au > GC for [Bmim][BF₄] while the order becomes Ta > Au > Pt > GC for [Bmmim][BF₄] [84]. This particular order could be explained by the working function of the working electrode [84].

The order is different in other reports. For example, the order of EW is GC > Pt > Au for [Emim][BF₄] [Bmim] [BF₄] [Bmim][PF₆] [Bmim][Tf₂N] and [S₄][DCA] in both alternating current (AC) and direct current (DC) mode [90]. For [N₂₂₂₂][Ac], the EW order follows GC > Au > Pt; however, the order could also be Au > Pt > GC for [Pmim][Tf₂N] [67].

By using Pt as working electrode, Pt-Tf₂N radicals are produced from ILs [BmPyrr] [Tf₂N] at anaerobic condition [94]. The formation of Pt-Tf₂N radicals suggests that Tf₂N is not electrochemically stable by using Pt as working electrode.

2.2.2. Effect of cut-off current density

Cut-off current density is the key parameter to determine the EW, cathodic limit and anodic limit. For better and reliable comparison, the cut-off current density should be the same. Cut-off current density is commonly selected as 0.5, 1, 2, 5 mA cm^-2. High (low) cut-off current density would lead to a wide (narrow) EW. For example, EW of [Bmim][Tf₂N] by 1 and 5 mA cm^-2 cut-off current density are 4.3 and 5 V vs. Fc⁺/Fc, respectively [77].

Sometimes, cut-off current density could change the order of potential limit for different ILs. For example, cathodic limit of [Bmim][Tf₂N] with longer alkyl chain could be higher than that of [Emim][Tf₂N] at high cut-off current density (e.g., 320 pA cm^-2), and lower than that of [Emim][Tf₂N] at low cut-off current density (e.g., 250 and 275 pA cm^-2) [78]. It again corroborates the conclusion that the comparison of ILs electrochemical stability requires the indication of cut-off current density.

The selection of cut-off current density is arbitrary; the determination of EW by cut-off method is not reliable [95]. A new method based on the linear fits of current-voltage curve is proposed to determine the EW without defining the cut-off current density [95]. This new strategy is more reliable and accurate; however, it is complicated and time-consuming. If the linear fit is not available or not favorable, the determination of EW by this novel strategy would be difficult.

2.2.3. Effect of scan rate

Higher scan rate is found to enhance the current density. It means that EW by higher scan rate would be lower [96]. Cathodic limit of [N₄₄₄₄][I] (300 mmol L^-1 in propylene carbonate (PC)) at 1, 10, 100 and 1000 mV s^-1 are -3.295, -3.273, -3.145 and -2.224 V vs. Ag⁺/Ag, respectively [95]. It again corroborates the lower electrochemical stability at higher scan rate.

2.2.4. Effect of AC or DC

AC EW of ILs is much wider than that of DC EW. For example, DC EW of [Emim] [BF₄] at Pt working electrode and 25 °C is 4.2 V, which is much lower than the AC EW (11.918.8 V) [90]. For other ILs by using other working electrode, the wider EW of ILs in AC mode than DC mode is observed [90].

2.2.5. Effect of AC frequency

Interestingly, the AC frequency also has effect on the EW of ILs. For example, EW of [Emim][BF₄] at 20, 50 and 100 Hz are 11.9, 15.2 and 18.8 V, respectively. It implies that higher AC frequency would increase the EW [90].

2.2.6. Effect of temperature

Increasing temperature would make ILs less electrochemically stable, i.e., decrease anodic limit and EW and increase cathodic limit [97,98]. For example, the EW of [Bmim][BF₄] at 30 °C, 40 °C, 50 °C, 60 °C, 70 °C are 3.50 V, 3.45 V, 2.85 V, 2.70 V vs. A1³⁺/A1, respectively [97]. Anodic and cathodic limit correspond to the oxidation of BF₄ anion and the reduction of imidazolium cation, respectively [97]. Johansson et al. attributes the lower electrochemical stability of ILs at higher temperature for the reasons as below: (1) lower viscosity; (2) higher ionic conductivity; (3) faster ionic diffusivity; (4) closer contact with electrode surface; (4) easier dissociation of ions [98].

ILs are hygroscopic; therefore, the investigation on the electrochemical stability of wet ILs is necessary. Increasing the temperature from 25 °C to 45 °C would narrow the EW of ILs, such as [Emim][Tf₂N] [Bmim][TF0] [Bmim][BF₄] and [Bmim][PF₆] (Fig. 2) [77].

At room temperature, imidazolium-based ILs is significantly decomposed at TiO₂-bronze working electrode. When the temperature increases to 120 °C, the formation of surface film is observed from the deposition of imidazolium-based ILs [81]. For pyrrolidinium- and piperidinium-based ILs, only trace of surface film is observed at a high temperature of 120 °C [81].

Some anions are electrochemically stable at room temperature; however, they would undergo significant decomposition at high temperature. For example, Tf₂N is one of the most anodic stable anions among all the anions in ILs. At room temperature, only slight decomposition of Tf₂N in [BmPyrr] [Tf₂N] takes place in the anodic compartment by using Cu sheet as working electrode [99]. Unexpectedly, CuF₂ is formed from copper electrode and ILs [BmPyrr] [Tf₂N] when the temperature increases to 70 °C (Fig. 3) [99]. It means that high temperature would make stable anion unstable.

2.2.7. Effect of waler

Water impurity could not be avoided due to the high hygroscopicity of ILs [13,100]. Thus, effect of water on the electrochemical stability of ILs is very important. Effect of water on the electrochemical stability of ILs is related to the water concentration and the ILs types.

For [Bmim][BF₄] and [Bmim][PF₆], the presence of 3 wt% water increases the cathodic limit and decreases the anodic limit, and thus narrows the EW (as high as 2.0 V) [96]. The presence of atmospheric water in 12 ILs narrows the EW when compared to vacuum-dried ILs [77]. EW of wet ILs could be even lower (Fig. 2) [77]. However, the anodic stability of some ILs would be not higher than ILs containing atmospheric moisture [77]. For example, the EW of [Bmim]PF₆] containing atmospheric water is 3.9 V, which is lower than that of vacuum-dried [Bmim]PF₆] (4.8 V); however, anodic limit of water-containing [Bmim]PF₆] is 2.2 V, which is slightly higher than that of vacuum-dried [Bmim]PF₆] (2.1 V) [77].

For [BPy][chloroaluminate], ca. 100 ppm water would not alter the EW for GC and tungsten working electrode; however, cathodic limit is more positive for Pt working electrode [96]. For [N₁₁₁₃][Tf₂N], even 58 ppm water in ILs could lead to the reduction current of water, which means that the measured cathodic limit is more positive [96]. Even for electrochemically stable anion (e.g., Tf₂N), the presence of low moisture content would significantly reduce the EW [83]. For all the ILs with different cations and anions, higher RH decreases the anodic limit and increases the cathodic limit (Fig. 1) [83]. Interestingly, the EW of ILs approximates that of pure water if the humid moisture content in ILs is high (e.g., saturated) [83].

For TFPB-based hydrophobic ILs/water system, the EW is wider at the ILs/water interface due to the polarization effect [76].

2.2.8. Effect of oxygen

Exposure of ILs to the atmospheric air would lead to the dissolution of trace of oxygen in ILs in addition to water. The reduction current of oxygen at the potential - 1-1.5 V vs. Fc+/Fc is observed when the EW of ILs is measured, which would narrow the EW of ILs [77].

2.2.9. Effect of adding other compounds

The addition of 1 eq HC1 to basic ILs A1C1₃ [Emim] [Cl] (49:51) could form ternary HC1:A1C1₃ [Emim] [Cl]. The EW of ternary HC1:A1C1₃ [Emim] [Cl] is found to be expanded compared to basic ILs A1C1₃ [Emim] [Cl] [101].

Sometimes, ILs are too viscous to measure the electrochemical stability; some ILs even show solid state around room temperature. It is necessary to add solvents into the ILs for the characterization of electrochemical stability of ILs [85,102].

The combination of FAP-based ILs and acetonitrile system affords a higher anodic limit than tetrabutylammonium perchlorate/acetonitrile system and neat ILs, which could be ascribed to the high electrochemical stability of FAP anion [103].

The presence of PC in ILs would change the electrochemical stability of ILs. 300 mmol L^-1 [Ni₄₄₄][Tf₂N] in PC would decrease the cathodic stability of [Ni₄₄₄i[Tf₂N]. However, 300 mmol L^-1 [Emim][Tf₂N] in PC would increase the cathodic stability of [Emim][Tf₂N] [95]. EWof [BPy] [DCA]+ PC (mole fraction = 0.7885) is 2.7 V vs. Pt while EW of mixtures in other mole fractions is ca. 2.5 V [104].

2.2.10. Effect of pH

pH of ILs is determined by the components and mole ratio of ILs, which could affect the cathodic limit, anodic limit and thus the E W of ILs. According to the pH of ILs, ILs could be classified into basic, acidic and neutral ILs. For basic ILs (e.g., mole of [Emim] [Cl] is higher than mole of A1C1₃), anodic and cathodic limit means the oxidation of Cl anion and reduction of imidazolium cation, respectively [105]. For acidic ILs with excess A1C1₃, anodic limit corresponds to the electrooxidation of AICI4 anion to chlorine; cathodic limit corresponds to the electroreduction of A1₂C1₇ anion to yield Al [105]. However, the EW for both basic and acidic ILs is not wide enough. The EW of ILs could be increased by neutral ILs, where anodic and cathodic limit correspond to oxidation of AICI₄ anion and reduction of imidazolium cation [105].

ZnCl₂ [Emim] [Cl] has the same tendency. Cathodic and anodic limit of basic 1:3 ZnCl₂ [Emim][Cl] correspond to reduction of imidazolium cation and oxidation of Cl, respectively. Cathodic and anodic limit of acidic >0.5:1 ZnCl₂ [Emim] [Cl] correspond to deposition of metallic Zn and oxidation of chlorozincate complexes, respectively [106].

2.2.11. Effect of ultrasound

Under the condition of ultrasound, imidazolium-based ILs could be electroreduced to construct C-S bond by forming imidazole-2-thione in the presence of sulphur [107]. Results show that the yield of imidazole-2-thione is affected by temperature, anion of ILs and functional group of ILs [107]. High temperature would enhance the electroreduction of imidazolium-based ILs. The attachment of functional group in imidazolium cation would decrease the electrochemical stability of ILs [107]. Effect of non-structural factor on the electrochemical stability of DESs is also very important, while there is nearly no report.

3. Electrochemical stability of ILs/DESs in batteries

Electrochemical stability of ILs-/DESs-based electrolytes is generally overestimated in common electrode; therefore, evaluating electrochemical stability of ILs-/DESs-based electrolytes in real batteries is necessary and realistic [108-114]. Batteries are classified into three categories: lithium-ion, lithium-metal (e.g., lithium-sulphur, lithium-oxygen or lithium-air) and post-lithium-ion (e.g., sodium-ion, potassiumion, magnesium-ion, aluminum-ion, zinc-ion or calcium-ion) batteries.

3.1. Electrochemical stability in lithium-ion batteries

ILs-base electrolytes are widely used in lithium-ion batteries [115,116]; however, the electrochemical stability are seldom summarized. DESs-based electrolytes are seldom reported in lithium-ion batteries [109-114]. The discussions are divided into three sections based on the types of ILs-/DESsbased electrolytes, i.e., pure ILs/DESs as electrolytes, ILs-/ DESs-based hybrid electrolytes and (quasi)solid-state ILs/ DESs as electrolytes.

3.1.1. Pure ILs/DESs as electrolytes

Cathodic limit of ILs with cation ([Mmim] or [Mim]) and anion (Tf₂N or TSI) ranges from -0.6 V to -0.65 V vs. Ag (platinum: working electrode, silver: pseudo reference electrode, carbon: counter electrode), which could be ascribed to the reduction of proton in ILs cation [117]. Effect of methyl in ILs cation seems to have negligible effect on the cathodic limit; anodic stability of Tf₂N is slightly higher than that of ESI, which makes [Mmim][Tf₂N]+[Li][Tf₂N] as the promising electrolytes for lithium-ion batteries with high capacity retention and high cycle stability [117]. For the electrolytes of [ME_nAmim][Tf₂N]+[Li][Tf₂N], EW increases when n increases [118]. EW of neat solvate ILs (e.g., [Li (G3)][Tf₂N] or [Li (G4)][Tf₂N]) is ca. 4 V vs. Li/Li⁺, which would be narrowed after being diluted with toluene, PC and hydrofluoroether [119]. Kim et al. found that for the electrolytes of ILs+[Li][Tf₂N], cathodic limit of pyrrolinium-based ILs is higher than that of imidazolium-based ILs, which could be ascribed to the reason that C2 proton in imidazolium ring is easier to be reduced [120]. Instead, anodic limit of pyrrolinium-based ILs is comparable to that of imidazolium-based ILs [120].

The anodic limit of protic ILs [N_222H][Tf₂N] (N_222H = triethylammonium)+[Li] [Tf₂N] is comparable to that of Tf₂N-based ILs, while the cathodic limit of protic ILs+ [Li][Tf₂N] is higher than that of normal protic ILs; therefore, lithium-ion batteries with the electrolytes based on [N_222H] [Tf₂N]+[Li][Tf₂N] should be paid much attention to the potential of insertion-extraction process of anodic materials [121].

Neat ILs (e.g., [Bmim][BF₄] or [Emim][BF₄]) own the EW of ca. 4 V vs. Li/Li⁺; instead, the presence of lithium salts (e.g., [Li][BF₄]) in ILs would increase the EW [122]. Possible reasons for the higher EW after adding lithium salts in ILs are: (1) higher viscosity, (2) unfavourable conductivity, (3) low ion self-diffusion coefficient (4) passivation process [122]. Specifically, lithium salt and unavoidable trace water would form passivating films (e.g., LiO₂, LiOH, LiF), on the electrode surface, which would add the cathodic stability of ILs [123]. Anodic limit of 0.8 mol L^-1 [Li][FSI] in [PmPyrr] [ESI] is ca. 5.2 V vs. Li/Li⁺, which is higher than that (ca. 4.2 V vs. Li/ Li⁺) of [Li][PF₆]+EC/DEC by using Pt as electrodes [124].

Dicationic ILs are found to be more electrochemically stable than monocationic ILs. For example, when lithium metal is used as counter/reference electrode and stainless steel is used as working electrode, EW of dicationic ILs [C₆ (mim)₂] [Tf₂N] is ca. 5.3 V vs. Li/Li⁺, which is higher than that of monocationic ILs and other common electrolytes [125]. Due to the high operating voltage window from dicationic ILs [C₆ (mim)₂][Tf₂N], they could be reliably served as electrolytes for high-voltage LiCoO₂/LiMn₂O₄-based lithium-ion batteries [125].

EW of DESs LiNO₃:NMA (1:4) and LiTf₂N:NMA (1:4) is 4.7 V and 5.3 V vs. Li/Li⁺, respectively (0.01 mA cm^-2, Pt as working electrode, Li as reference and counter electrodes) [110]. However, the presence of ca. 600 ppm water in LiNO₃:NMA (1:4) would lead to the reduction peak at ca. 1.2 V vs. Li/Li⁺ [110]. Both cathodic limit and anodic limit are comparable to that of common ILs [110]. Both lithium salts and NMA are solid at room temperature, while DESs are liquid at room temperature with the eutectic temperature ranging from -52 °C to -75 °C. The capacity and efficiency of lithium-ion batteries based on LiNO3:NMA (1:4) electrolytes could reach as high as 160 mA h g 1 and 99% even at the high temperature of 60 °C [110].

ChCkEG (l:3)+0.5 mol L^-1 [Li]]PF₆] or 0.5 mol L^-1 [Li] [Tf₂N] owns the EW of 3.5 V and 3.8 V, respectively (glassy carbon as working electrode, Ag/Ag⁺ as the reference electrode and Pt wire as the counter electrode) [111]. Unfortunately, the authors observe that gas bubbles are generated when lithium metal is inserted into ChCkEG (1:3) or into ChCkEG (l:3)+lithium salt mixture due to the high reactivity of lithium metal [111].

EW of methylsulfonylmethane [Li] [C1O₄]: water (1.8:1:1) with the melting point of -48 °C is ca. 3.3 V, which could act as green and non-flammable electrolytes for favourable lithium-ion batteries (energy density >160 Wh Kg^-1, working voltage = 2.4 V, capacity retention = 72.2% after 1000 cycles) [112].

3.1.2. ILs/DESs-based hybrid electrolytes

Electrolytes based on [Li][PF₆]+EC/dimethyl carbonate (DMC) system owns the anodic limit of 4.18 V vs. Li/Li⁺, which could be enhanced into 4.56 V, 4.68 V and 4.77 V vs. Li/Li⁺ by adding 5 wt% [Hmim][PF6], 10 wt% bidentate phosphonate-functionalized imidazolium ILs and 20 wt% monodentate phosphonate-functionalized imidazolium ILs, respectively [126]. It means that the high electrochemical stability of [Li][PF₆]+EC/DMC+10 wt% bidentate phosphonate-functionalized imidazolium ILs could satisfy the requirement of lithium-ion batteries with LiFePO₄ and even LiCoO₂ cathode [126] [P₄₄₄₁][Tf₂N]+EC/EMC+[Li][PF₆] exhibits a high anodic potential of >4.3 V vs. Li/Li⁺, which is helpful for constructing lithium-ion batteries with high voltage [127]. EW of [BmPyrr][Tf₂N]+[Li][Tf₂N] could be enhanced into as high as >5 V vs. Li/Li⁺ [128]. CF₃ functionalization in the imidazolium cation for ILs+[Li][Tf₂N] is found to increase the anodic stability to 5.7 V vs. Li/Li⁺, which is higher than that of conventional electrolytes; furthermore, both the melting point and viscosity are decreased after the CF₃ functionalization [129]. Cathodic limit of [Amim] [Tf₂N]+PC (Arnim = l-allyl-3-methylimidazolium) is more negative than that of [Pmim][Tf₂N]+PC, where the cations [Amim] and [Pmim] tend to form dimer or polymer via carbene (Fig. 4) [130]. Anodic limit of [Amim] [Tf₂N]+PC is higher than that of [Pmim][Tf₂N]+PC, where Tf₂N tends to be oxidized in different extent due to the different interaction strength with cation [130]. Therefore, lithium-ion batteries based on [Arnim] [Tf₂N]-based hybrid electrolytes have higher rate performance, smaller potential separation and more flat voltage plateau [130].

At 25 °C, both anodic limit of [BmPip] [Tf₂N]+ PC + EC + lithium difluoro (oxalato) borate and [BmPip] [Tf₂N]+PC + fluoroethylene carbonate (FEC) +lithium difluoro (oxalato) borate is higher than 4.6 V vs. Li/Li⁺, which is also similar to that of [Li][PF6]+EC/DEC/EMC [131]. When the temperature increases into 55 °C, both anodic limit of [BmPip] [Tf₂N]+PC + EC + lithium difluoro (oxalato) borate and [BmPip] [Tf₂N]+PC + FEC + lithium difluoro (oxalato) borate is higher than 5.0 V vs. Li/Li⁺, while that of [Li][PF₆]+EC/DEC/EMC is near 4.6 V vs. Li/Li⁺ [131]. Further increasing the temperature into 70 °C [BmPip] [Tf₂N]+PC + FEC + lithium difluoro (oxalato) borate could be still as high as 5.0 V vs. Li/Li⁺, while that of [BmPip] [Tf₂N]+PC + EC + lithium difluoro (oxalato) borate and [Li] [PF6]+EC/DEC/EMC decreases to 4.6 V and 3.7 V vs. Li/Li⁺, respectively [131]. It suggests that [BmPip] [Tf₂N]+ PC + FEC + lithium difluoro (oxalato) borate are highly electrochemically stable electrolytes for lithium-ion batteries.

Anodic limit of [Li][Tf₂N]:TFA (1:4) is 4.95 V vs. Li/Li⁺ (cut-off current = 10 pA, Ni as counter electrode, Pt as working electrode, Ag/AgNO3/CH₃CN as reference electrode) [114]. Decreasing the mole concentration of TFA in [Li] [Tf₂N]:TFA could increase the anodic limit in the same condition. Moreover, addition of 10 wt%, 20 wt% and 30 wt% EC in [Li][Tf₂N]:TFA decreases the anodic limit into 4.93 V, 4.93 V and 4.83 V vs. Li/Li⁺, respectively, which is higher than that of electrolytes based on EC + DMC+1 mol L-1 [Li] [Tf₂N] [114]. By using this kind of DESs-based electrolytes, lithium-ion batteries could achieve 102 mAh g 1 capacity and 84% efficiency [114].

3.13. (Quasi) solid-state ILs/DESs electrolytes

ILs-based organic-inorganic hybrid solid polymer electrolytes show a high EW of 5.0 V and exhibit a high initial discharge capacity (150.7 mAh g^-1) and initial columbic efficiency (100%) when the rate is 0.1 C [132]. Quasi solid electrolytes composed of polymerized 1 -vinyl-3-dodecylimidazolium bis(trifluoromethanesulfonyl) imide and poly (vinylidene fluoride-hexafluoropropylene) show a wide EW of ca. 5.0 V vs. Li/Li⁺ [133].

Electrolytes composed of polymerized ILs (PILs, Fig. 5a), PC and [Li][ClO₄] exhibit high anodic limit of 5.61 V vs. Li/Li⁺ and 4.14 V vs. Li/Li⁺ at 25 °C and 60 °C, respectively (Fig. 5b) [134], which suggests that increasing the temperature would decrease the anodic stability of ILs-based electrolytes. The anodic limit of ILs-based electrolytes is also higher than that of neat ILs when fixing the temperature at 25 °C (Fig. 5b). Moreover, ionic conductivity of ILs-based electrolytes could reach as high as 8.3xl0^-5 S cm^-1 and 2.0xl0^-4 S cm^-1 at 25 °C and 60 °C, respectively (Fig. 5c) [134]. Owing to high electrochemical stability, high conductivity as well as favourable fluidity and low glass transition temperature, lithium-ion batteries by using ILs-based electrolytes show excellent performance [134]. Lu et al. found that PILs + PC electrolytes (Fig. 6) could still keep electrochemically stable over 4.5 V vs. Li/Li⁺ (working electrode: stainless steel), by using which lithium-ion batteries show a favourable initial discharge capacity of 120.4 mAh g^-1 [135].

Anodic limit of gel biodegradable polymer electrolyte [Emim][BF₄]+poly (e-caprolactone)+[Li][BF₄] could reach as high 3.7 V, which could be served as appropriate electrolytes for lithium-ion batteries [136]. Polymer electrolytes based on porous poly (ionic liquid) [Li][PF₆] and poly (vinylidene fluoride-hexafluoropropylene) own a little higher anodic limit of 4.2 vs. Li/Li⁺ [137]. Poly (acrylonitrile-polyhedral oligomeric silsesquioxane)/poly (vinylidene fluoride)+ [Emim][Tf₂N]+[Li][Tf₂N] could keep chemically stable within the potential range of 0.5-5.1 V vs. Lİ/Lİ+ (i.e., EW = 4.6 V) when Li and stainless steel are used as anode and cathode, respectively [138]. Li⁺ de-intercalation (oxidation) and intercalation (reduction) by using [Li][FePO₄] as cathode occur in 2.9 V and 4.0 V vs. Li/Li⁺, respectively [138]. it means that [Li] [FePO₄]-based lithium-ion batteries could be stably performed in this kind of polymer electrolytes [138]. Polymer electrolytes produced from poly (ethylene glycol) diacrylate + tetraglyme+[Li][Tf₂N] are stable from - 1.5 V to 4.5 V vs. Li/Li⁺ [139]. Electrolytes based on polyhedral oligomeric silsesquioxane PILs + poly (vinylidenefluoride-co-hexafluoropropy lene)+[B mim] [T f₂N]+[Li] [Tf₂N] show high electrochemical stability of ~5.0 V vs. Li/ Li⁺ (stainless steel and lithium metal are electrodes); lithiumion batteries by using the electrolytes could achieve a high initial discharge capacity of 151.8 mAh g^-1 at the rate of 0.1 C [140]. EW of poly (N-vinlyimidazole)-co-poly (poly (ethylene glycol) methyl ether methacrylate)+[Li] [Tf₂N] electrolyte and poly (N-(l-vinylimidazolium-3-butyl)-ammonium bis(trifluoromethanesulfonyl)imide)-co-poly (poly (ethylene glycol) methyl ether methacrylate+[Li] [Tf₂N] electrolytes could reach 5.3 V and 5.5 V, respectively, which suggests that ionized electrolyte by ILs would increase the electrochemical stability [141].

3.2. Electrochemical stability of ILs/DESs in lithiummetal batteries

Common lithium-metal batteries include lithium-sulphur, lithium-oxygen, lithium-air, lithium-metal oxide batteries, where lithium metal is anode and sulphur, oxygen, air or metal oxide is cathode. Lithium-metal batteries are considered as the promising next-generation batteries with high capacity, high greenness, high abundance and low cost [142,143]. By utilizing appropriate ILs-based electrolytes, the performance, safety and sustainability of lithium-metal batteries could be further improved [144,145]; instead, the electrochemical stability of ILs-based electrolytes needs to be investigated for long-term usage of batteries. However, there is no reports on the lithium-sulphur and lithium-oxygen or lithium-air batteries with DESs-based electrolytes, to the best of our knowledge.

3.2.1. Lithium-sulphur batteries

Cyclic voltammetry of [BmPyrr] [Tf₂N]-based gel polymer electrolyte suggests that they are electrochemically stable up to 5 V vs. Li/Li⁺, which could be used for constructing safe and stable lithium-sulphur batteries [146]. The presence of LiNO3 would narrow the electrochemical stability of ILs [147].

3.2.2. Lithium-oxygen or lithium-air batteries

[DTPIm][Tf₂N]+diethylene glycol dimethyl ether+[Li] [Tf₂N]) are promising electrolytes for stable, safe and highperformance lithium-oxygen batteries, where [DTPIm] indicates l,2-dimethyl-3-(4-(2,2,6,6-tetramethyl-l-oxyl-4-piperidoxyl)pentyl) imidazolium [148]. Cyclic voltammetry of the above electrolytes suggests that N-0 radical in ILs is oxidized and reduced at 3.75 V and 3.0 V, respectively (lithium foil: anode, carbon paper: cathode, glass fibre membrane: separator) [148]. When the temperature increases, anodic potential of electrolytes [DEME][Tf₂N]+[Li][Tf₂N] (DEME = N,Ndiethyl-N-(2-methoxyethyl)-N-methylammonium) could keep high electrochemical stability up to 5.0 V vs. Li/Li⁺, 4.87 V vs. Li/Li⁺ and 4.68 V vs. Li/Li⁺ at 30 °C, 40 °C and 60 °C, respectively, which could render lithium-oxygen batteries with high reversibility, low polarization and enhanced energy capacity [149].

Effect of cation type of Tf₂N-based ILs on the cathodic limit of ILs+[Li][Tf₂N] electrolytes for lithium-oxygen batteries is significant, such as [Emim] 1.3 V = [Bmim] 1.3 V > [Pmim] 1.1 V = [Pmmim] 1.1 V > [PmPyrr] 0.3 V > [BmPyrr] 0.2 V > [PmPip] 0.1 V vs. Li/Li⁺ [150]. Instead, anodic limit affected by the cation is limited as demonstrated by the order of [PmPip] 6.9 V > [BmPyrr] 6.8 V > [PmPyrr] 6.6 V > [Bmim] 6.2 V > [Pmmim] 6.0 V > [Emim] 5.4 V = [Pmim] 5.4 V vs. Li/Li⁺ [150]. It suggests that orders of cathodic and anodic limit are not the same when changing the cation of ILs. Furthermore, when compared to mixtures of lithium salts with organic electrolytes (e.g., EC/DEC or tetraethylene glycol dimethyl ether), mixtures of lithium salts with ILs generally own both high cathodic and anodic stability [150]. Polymer electrolytes based on poly (vinylidene fluoride-co-hexafluoropropylene) [Li] [Tf₂N] and [Emim][Tf₂N] show an enhanced EW when the ILs concentration increases (i.e., 3.9 V, 4.1 V, 4.8 V and 4.9 V vs. Li/Li⁺ for the ILs weight composition of 0%, 20%, 40% and 60%, respectively), which are promising electrolytes for practical lithium-air batteries [151]. Similarly, for the system of [Bmim][Tf₂N]+[BmPyrr][Tf₂N]+[Li][Tf₂N], increasing the ratio of [Bmim][Tf₂N] to [BmPyrr] would generally increases the anodic limit; instead, both ionic conductivity and lithium ion transference number decrease correspondingly [152]. Anodic limit of Tf₂N-based ILs+[Li][Tf₂N]+ tetra (ethylene glycol)dimethylether varying in ILs cation is ordered as imidazolium < morpholinium < piperidinium < pyrrolidinium, while cathodic limit is ordered as imidazolium > morpholinium ~ piperidinium ~ pyrrolidinium (platinum and lithium metal are working electrode and counter/reference electrode, respectively) [153]. Electrolytes based on [BmPyrr] [Tf₂N]+[Li] [TEO] + tetra ethylene glycol dimethyl etcher are found to degrade >4.8 V vs. Li/Li⁺, which is suitable for lithium-air batteries [154].

3.3. Electrochemical stability of ILs/DESs in postlithium-ion batteries

ILs/DESs are also widely used as electrolytes in postlithium-ion batteries, including sodium-ion, potas sium-ion, magnesium-ion, aluminium-ion, zinc-ion and calcium-ion batteries [59,155-159]. Electrochemical stability of ILs-/ DESs-based electrolytes in post-lithium-ion batteries is summarized as below.

3.3.1. Sodium-ion batteries

Specific compositions could affect the electrochemical stability of ILs. ILs [Na (giyme)] [C10₄]x (produced from triglyme and [Na][ClO₄]) immobilized in poly (vinylidenefluoride-co-hexafluoropropylene) could construct freestanding and flexible gel polymer electrolyte for quasi-solid-state sodium-ion batteries [160]. Linear sweep voltammetry measurement is measured from simulated sodium-ion batteries to obtain anodic limit by choosing stainless steel foil, Na and [Na (giyme)] [ClO₄]v-based gel polymer as working electrode, reference/counter electrode, and electrolyte, respectively [161]. When x = 0, anodic limit of neat triglyme is 4.0 V vs. Na/Na+; triglyme with ether group would enhance the anodic limit when compared to triethylene glycol with hydroxyl group; oxygen atom in ether group of triglyme tends to lose its electron, which contributes to the oxidation of triglyme at 4.0 V vs. Na/Na+ [161]. When x = 0.1 and 0.3, anodic limit increases into -4.7 V vs. Na/Na+ due to the formation of ILs between triglyme and NaClO₄ via strong interaction, which also means the high oxidation ability [160]. When x = 0.5, 0.6 and 0.8, anodic limit instead decreases into -4.2 V vs. Na/Na+ [160].

Addition of FAP and PF₆ would not enhance the anodic limit of ILs [Na][FSI]-[Emim][FSI] (Pt and Na are working electrode and counter electrode, respectively) [162].

Higher temperature seems to decrease the electrochemical stability of ILs. For example, at 25 °C, anodic current is noticeable for both [Na]_0.2 [Emim]_0.8 [FSI]i and [Na]₀₂ [Emim]o,8 [FSI]o.7 [ЕАР]0.з at 5.1 V vs. Na+/Na; instead, only 4.8 V Na/Na+ is measured at 90 °C (Pt and Na are working electrode and counter electrode, respectively) [162].

When Al is used as the working electrode (Na is still the counter electrode), effect of FAP on the anodic reaction is different. For example, ca. 4 V Na/Na+ is observed for detecting anodic current of [Na]_0.2 [Emim]_0.8 [FSI]1 at both 25 °C and 90 °C, which could be ascribed to the Al corrosion [162]. However, for [Na]_0.2 [Emim]_0.8 [FSI]_0.7 [FAP]_0.3, there is nearly no anodic current even at 6.0 V Na/Na+ while keeping other conditions identical, which could be explained by the Al passivation by FAP anion and the higher anodic limit of ILs by FAP anion [162]. Similarly, addition of PF6 anion into [Na]_0.2 [Emim]_0.8 [FSI]1 would also increase the passivation ability of Al [162], indicating the increased electrochemical stability of ILs in such condition.

Carboxymethyl cellulose-NaAc-30 wt% [Bmim] [Cl] (1butyl-3-methylimidazolium chloride) is electrochemically stable until 2.9 V, which indicates the high suitability of carboxymethyl cellulose -NaAc-30 wt% [Bmim][Cl] as biopolymer electrolytes for sodium-ion batteries [163].

For achieving high-performance and stable sodium-ion batteries, electrochemical stability of ILs is screened by the density functional theory calculation at PBE/def2-TZVP level as suggested by Silva et al. [164]. EW order of ILs cation follows [N₁₁₂₄] > [N_1122Oi] > [Nn2O2Oi] > [Bmim] > [Emim] [164], which indicates that shorter alkyl chain and etherification would narrow EW of ILs. In addition to cation and anion, the intermolecular interaction influences the EW according to the theoretical calculation [164].

Carbonate solvents+[Emim] [Tf₂N]+l mol L-1 [Na][Tf₂N] system (Tf₂N means bis(trifluoromethanesulfonyl)imide)) could be used as safe electrolytes in sodium-ion batteries [165]. When the ILs concentration increases, the oxidation current decreases or the oxidation potential enlarges, which could be ascribed to the intensified interaction between cation and anion, and between ion and molecules [165].

ILs AlCl₃-[Emim][Cl] after being buffered with NaCl, EtAlCl₂ and [Emim] [ESI] (ESI represents bis(fluorosulfonyl) imide) show nearly no anodic current until -4.56 V (carbon fibre paper and Na foil are working electrode and counter/ reference electrode, respectively) [166], which indicates that ILs are high stable electrolytes for high-voltage sodium batteries.

Electrolytes based on [Na][ESI] (0.5 mol L ') dissolved in [BmPyrr] [Tf₂N] is electrochemically stable -4.8 V vs. Na/Na+ [167], indicating the high potential for this electrolyte used for sodium-ion batteries. When [Na] [FSI] or [Na][Tf₂N] in [BmPyrr] [Tf₂N] is used as electrolytes in sodium-ion batteries, Tf₂N anion is decomposed and [BmPyrr] cation is deposited [167]. In this case, N-S bond is cracked by forming SO₂CFT and NSO₂CF^. However, Tf₂N anion shows a different decomposition mechanism by producing NaF when [Na][C10₄] is added in [BmPyrr] [Tf₂N] [167].

Composite polymer electrolytes composed of ILs [PmPyrr] [FSI] (N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl) imide), poly (ethylene oxide) (molecular weight = 600,000) and [Na][ClO₄] could contribute to all-solid-state sodium-ion batteries with good recycle and high performance [168]. Electrochemical stability of composite polymer electrolytes increases with the mass fraction of ILs [PmPyrr][FSI], where anodic limit is 3.70 V, 4.05 V, 4.20 V and 4.44 V vs. Na/Na+ when ILs mass fraction is 10%, 20%, 30% and 40%, respectively (Fig. 7) [168]. It could be ascribe to the higher electrochemical decomposition potential of [PmPyrr][FSI] (5.3 V Na/Na+) than that of poly (ethylene oxide) (3.5-4.0 V Na/ Na+) [168].

ILs-based electrolytes are safe and green electrolytes for sodium-ion batteries. Comparing [Na] [FSI]+[PmPyrr] [FSI] (1:9) and [Na] [FSI]+[BmPyrr] [FSI] (1:9), or [Na][Tf₂N]+ [PmPyrr][FSI] (1:9) and [Na][Tf₂N]+[BmPyrr] [FSI] (1:9) could conclude that ILs with longer alkyl chain would slightly decrease the anodic stability of ILs-based electrolytes (Fig. 8) [108]. Comparing [Na][FSI]+[BmPyrr][FSI] (1:9) and [Na] [FSI]+[BmPyrr] [Tf₂N] (1:9), or [Na] [Tf₂N]+[PmPyrr] [FSI] (1:9) and [Na][Tf₂N]+[BmPyrr][Tf₂N] (1:9) suggests that ILs with Tf₂N is more electrochemical stable than ILs with FSI (Fig. 8) [108]. Comparing [Na] [FSI]+[PmPyrr] [FSI] (1:9) and [Na] [Tf₂N]+[PmPyrr][FSI] (1:9) [Na][FSI]+[BmPyrr] [FSI] (1:9) and [Na][Tf₂N]+[BmPyrr][Tf₂N] (1:9) [Na][FSI]+ [BmPyrr] [Tf₂N] (1:9) and [Na] [Tf₂N]+[BmPyrr] [Tf₂N] (1:9) means sodium salts with Tf₂N is more stable than sodium salts with FSI (Fig. 8) [108] [Na][FSI]+[BmPyrr][Tf₂N] + [BmPyrr] [Tf₂N] (1:4:5) shows higher anodic limit (4.95 V vs. Na/Na+) than that of [Na][Tf₂N]4-[BmPyrr][Tf₂N]+[BmPyrr] [FSI] (1:4:5) (4.84 V vs. Na/Na+), implying that ILs with Tf₂N plays a more important role in enhancing anodic stability than sodium salts with Tf₂N [108]. The cathodic peaks around 0.6 V related to the solid electrolyte interphase for [Na][FSI]and [Na][Tf₂N]-based electrolytes are also different [108].

Fischer et al. found that the physicochemical properties of ILs are suitable for applying as electrolytes in sodium-ion batteries [169]. Glassy carbon and Pt wire are working electrode and counter electrode, respectively. Cut-off current density is 1 mA cm^-2. EW of unfunctionalized ILs is a little wider than that of functionalized ILs; however, conductivity, formation of solid electrolyte interface and batteries performance of functionalized ILs is higher than that of unfunctionalized ILs [169]. Instead, thioether group decreases the EW of ILs (-3.95 V vs. Fc/Fc⁺), which could be ascribed to the easy oxidation of thioether group [169]. Cathodic limit of ether- (-2.63 V vs. Fc/Fc⁺) and thioether-functionalized (-2.76 V vs. Fc/Fc+) pyrrolidinium-based ILs is close to that of unfunctionalized ILs (-2.65 V vs. Fc/Fc⁺), while ester(-2.49 V vs. Fc/Fc+) and nitrile-functionalized (-2.20 V vs. Fc/Fc+) pyrrolidinium-based ILs shows a higher cathodic limit due to the easy reduction of ester and nitrile group [169].

EW of poly (ionic liquid)-based solid electrolytes by polymerizing 1,4-bis [3-(2-acryloyloxyethyl)imidazolium-lyl]butane bis [-bis(trifluoromethanesulfonyl)imide] monomer in [Emim][Tf₂N] is 4.6 V vs. Na/Na+, which is high enough to serve as stable electrolytes for safe sodium-ion batteries [170]. EW of IL-based electrolytes ([Na] [FSI]+[PmPyrr] [FSI], 5.34 V vs. Li/Li⁺) is wider than that of standard carbonate electrolyte for sodium-ion batteries [171].

ILs + sodium salt mixtures with different concentration have been considered as promising safe and non-flammable electrolytes for sodium-ion batteries. For example, when the concentration of [Na][BF₄] increases, EW and anodic limit of [Na][BF₄]+[Emim][BF₄] increases, while cathodic limit of [Na][BF₄]+[Emim][BF₄] decreases [172], which suggests that higher concentration of [Na][BF₄] in [Na][BF₄]+[Emim] [BF₄] increases the electrochemical stability. However, the electrochemical conductivity of [Na][BF₄]+[Emim][BF₄] decreases when increasing the concentration of [Na][BF₄] [172]. It implies that optimal concentration of [Na][BF₄] in [Na] [BF₄]+[Emim][BF₄] for favourable sodium-ion batteries should not be too high or too low.

As the mole ratio of [Na][Tf₂N]:NMA increases, the anodic limit increases (Na as reference and counter electrodes); unfortunately, the ionic conductivity decreases instead [159]. Furthermore [Na][Tf₂N]:NMA tends to react with Na, which could be ascribed to the ionic bond between carbonyl group of NMA and Na particularly when the mole fraction of NMA in [Na][Tf₂N]:NMA is high [159].

The addition of ILs [PmPyrr] [FSI] in hybrid electrolytes could reduce the flammability and increase the conductivity of [PmPyrr] [FSI]+[Na] [FSI]+[Na] [C1O₄]+ PC in sodium-ion batteries [173]. The anodic limits of hybrid electrolytes are ca. 5.7 V and ca. 5.5 V by using glass carbon and platinum electrode, respectively [173]. However, the solvation structure and interfacial properties of hybrid ILs-based electrolytes in sodium-ion batteries require further investigation.

3.3.2. Potas sium-ion batteries

Fig. 9 shows that EW of [K] [ESI]+[Emim] [ESI] (5.19 V vs. Ag/Ag⁺) is higher than that of [Na][ESI]+[Emim][ESI] (4.91 V vs. Ag/Ag⁺) and [Li] [ESI]+[Emim] [ESI] (4.95 V vs. Ag/Ag+) [174]. It suggests that [K] [ESI]+[Emim] [ESI] is the electrolyte with the highest electrochemical stability aiming for potassium-ion batteries. Cathodic limit of [K] [ESI]+[Emim] [ESI] is also the most negative, while anodic limit of [K] [ESI]+[Emim] [ESI] is comparable to that of [Na] [ESI]+[Emim] [ESI] and [Li] [ESI]+[Emim] [ESI] (Fig. 9) [174]. For the system of [M][FSI]+[PmPyrr][FSI] (M = K, Na, Li), the order of electrochemical stability is similar [175].

Similarly, EW of [K][Tf₂N] (0.5 mol L-1)+[PmPyrr] [Tf₂N] (6.01 V vs. Fc/Fc⁺) is higher than that of [Na][Tf₂N] (0.5 mol L^-1)+[PmPyrr][Tf₂N] (5.66 V vs. Fc/Fc⁺) and [Li] [Tf₂N] (0.5 mol L^-1)+[PmPyrr][Tf₂N] (5.65 V vs. Fc/Fc⁺) (cut-off current density is ±0.1 mA cm^-2i [176]. Anodic limit of [K][Tf₂N]+[PmPyrr][Tf₂N] (6.01 V vs. Fc/Fc⁺) is also Fig. 9. Cyclic voltammograms of [K][FSI]+[Emim][FSI] (a), [Na] [ESI]+ [Emim][FSI] (b), and [Li] [ESI]+[Emim] [ESI] (c) at the mole fraction of xILs = 0.8. Copper disk and glassy carbon disk are working electrodes for the negative and positive region, respectively. Scan rate is 5 mV s^-1. Cut-off current density is ±0.1 mA cm^-2 [174]. Reprinted with permission from Ref. [174]. Copyright (2021) American Chemical Society.

higher than that of [Na] [Tf₂N]+[PmPyrr] [Tf₂N] (2.55 V vs. Fc/Fc⁺) and [Li] [Tf₂N]+[PmPyrr] [Tf₂N] (2.45 V vs. Fc/Fc⁺) (Pt as working electrode) [176]. Cathodic limit of [K][Tf₂N]+ [PmPyrr] [Tf₂N] (-3.48 V vs. Fc/Fc+) is more negative than that of [Na] [Tf₂N]+[PmPyrr] [Tf₂N] (-3.11 V vs. Fc/Fc⁺) and [Li] [Tf₂N]+[PmPyrr] [Tf₂N] (-3.20 V vs. Fc/Fc⁺) (Ni as working electrode) [176]. All the results suggest that [K] [Tf₂N]+[PmPyrr] [Tf₂N] owns the highest electrochemically stability, which is very helpful for their application in potassium-ion batteries [176].

Cathodic limit of [K][FSI]+[Emim] [FTA] with the mole fraction of x_ILs = 0.45 is 0 V vs. K/K⁺, which could be ascribed to the potassium metal reduction [177]. Anodic limit of [K][FSI]+[Emim] [FTA] with the same concentration is 5.6 V vs. K/K+, which could be ascribed to oxidation reaction of FSA or ETA (Fig. 10a) [177]. By using [K] [ESI]+[Emim] [ETA] as electrolytes, 1st and 50th charge/discharge capacity of potassium-ion batteries is 640/273 mAh g^-1 and 254/230 mAh g^-1, respectively (Fig. 10b) [177]. The coulombic efficiency of potassium-ion batteries is 90%, indicating the electrochemical decomposition of FSA or FTA [177].

[K][Tf₂N] (0.5 mol L^-1)+[BmPyrr][Tf₂N] with aprotic ILs is more electrochemically stable than that of [K][Tf₂N] (0.5 mol L-1)+[BPyrr][Tf₂N] with protic ILs [178]. Specifically, anionic limit of the above two electrolytes is comparable, while cathodic limit of [K][Tf₂N] (0.5 mol L-1)+ [BmPyrr][Tf₂N] is ca. -3 V vs. Ag wire, much lower than that (-1 V vs. Ag wire) of [K][Tf₂N] (0.5 mol L"')+[BPyrr] [Tf₂N] [178]. Thus [K][Tf₂N] (0.5 mol L ')+[BPyrr][Tf₂N] is not suitable for potassium-ion batteries due to the low electrochemical stability, while the insertion/deinsertion of potassium ion could be reversible when [K][Tf₂N] (0.5 mol L^-1)+[BmPyrr][Tf₂N] is used as the electrolytes [178].

As to the ternary ILs-based electrolytes [K][B(CN)4]+ [Emim][B(CN)₄]+poly (ethylene oxide) for potassium-ion batteries, lower concentration of [K][B(CN)4] would lead to a higher ionic conductivity and a lower viscosity; medium poly (ethylene oxide) could improve the transport properties while much more poly (ethylene oxide) would have negligible on the transport properties [179]. The selection of [Emim][B(CN)₄] is because EW of [Emim][B(CN)₄] with B(CN)₄ is large; further, they are less viscous [179] [PmPyrr] [FSI]+[K][FSI] (1 mol kg^-1) is selected as the electrolyte for potassium-ion batteries because of the high anodic stability of ESI anion; similarly [PmPyrr] cation has high cathodic stability and shows liquid state near room temperature [180].

3.3.3. Magne sium-ion batteries

The current density is not high below 3.0 V vs. Mg/Mg²⁺ by utilizing [Mg] [(bis(diisopropyl) amide)₂]+[Emim] [A1C1₄] (mole ratio is 1:2) as electrolytes in different electrodes (Fig. Ila) [181]. However, the oxidation current for different electrodes is ordered as Pt ~ graphite > stainless steel ~ Mo ~ Mo all phenyl complex when the potential ranges from 3 Mg/Mg²⁺ to 4.0 V Mg/Mg²⁺. More importantly, the oxidation current is the lowest even the potential is as high as 5 V vs. Mg/Mg²⁺ (Fig. Ila), suggesting that [Mg][(DIPA)₂]+ [Emim] [A1C1₄] shows the highest electrochemical stability by Mo electrode [181]. It could explain the oxidation current is much lower by Mo electrode than by stainless steel when chronoamperometry is conducted for ca. 50 h (Fig. 11b) [181]. EW of poly (vinylidenefluoride-co-hexafluoropropylene) [Mg] [TEO]+[Emim] [TEO] gel electrolyte is ca. 4.8 V, which shows promising potential in magnesium-ion batteries (TEO means trifluoromethanesulfonate) [182].

For the electrolytes [Mg][Tf₂N]+ILs + acetonitrile with different kinds of ILs, the anodic limit at 0.1 mA cm-2 is ordered as [EmPyrr][Tf₂N] > [PmPyrr] [Tf₂N] [PmPy] [Tf₂N] > [EmPy][Tf₂N] [Pmim][Tf₂N] > [Emim] [Tf₂N] ; piperidinium > pyrrolidinium > pyridinium > imidazolium (by substituting methyl and propyl) [183,184]. It suggests that ethyl could be less or more electrochemical stable than propyl depending on the cation type. For the electrolytes [Mg] [Tf₂N]+ILs + acetonitrile di (propylene glycol) dimethyl ether with different kinds of ILs, the anodic limit of all the electrolytes is comparable (4.40-4.44 V Mg/Mg2+) [183,184]. However, the charge/discharge capacity by different electrolytes is different. For example, the discharge capacity of [PmPip][Tf₂N] [PmPyrr][Tf₂N] and [Pmim][Tf₂N] is 7.8, 7.2 and 4.6 mAh g-1, respectively [183,184]. It means that ILsbased electrolytes with higher electrochemical stability do not necessarily lead to higher charge/discharge capacity of magnesium-ion batteries.

Although magnesium-ion batteries are highly safe and cheap, there are still some obstacles for the wide application of magnesium-ion batteries [185]: (1) exploring appropriate electrolytes for steady charge/discharge; (2) developing stable electrode; (3) achieving high voltage; (4) reducing side reactions; (5) slower shuttle rate of Mg²⁺ than that of Li⁺; (6) showing low charge/discharge efficiency due to the breakdown of cathode material.

33.4. Aluminum-ion batteries

EW of AlCl₃/adelaide could reach as high as 2.5 V vs. A1/A1³⁺, which is comparable to that of commercial ILs [186]. By using AlCl₃/adelaide as electrolyte, the cathodic capacity of aluminium-ion batteries could reach as high -66.8 mAh g^-1 at 500 mA g^-1; in addition, this aluminiumion batteries are also safe, cheap, highly conductive and sustainable [186].

Mole ratio of A1C1₃/XC1 (X = organic cation) and XC1 category determines the overall electrochemical stability of electrolytes in aluminium-ion batteries. On the one hand, anodic limit of ILs decreases and cathodic limit increases when mole ratio of A1C1₃/XC1 increases [187]. On the other hand, both anodic limit and EW affected by XC1 is ordered as AlCl₃/[N_m][Cl] > AICl₃/[Bmim] [CI] > A1C1₃/[N₂₂₂] [Cl] > AlCl₃/[Emim][Cl] [187]. Electrolytes AlCl₃/[PmPy] [Cl] with the mole ratio from 1.4 to 1.7 could be used as favorable electrolytes for rechargeable aluminum-graphite batteries [188]. Compared to common AlCl₃/[Emim][Cl], AlCl₃/[PmPy] [Cl] shows higher voltage window. However, AlCl₃/[PmPy] [Cl] own high overpotential because of higher viscosity and lower conductivity [188], which could be improved to be more energy-efficient.

3.3.5. Zinc-ion batteries

EW of ZnCl₂/ethylene glycol is ~2 V vs. Ag wire, where Zn²⁺ reduction and ethylene glycol oxidation occur at -12 V and 1.05 V vs. Ag wire, respectively [189]. Among all the kinds of electrolytes, ILs-based electrolytes own very high EW when compared to aqueous electrolytes, conventional organic electrolytes, all-solid-state electrolytes and quasi-solid-state electrolytes [157]. When stainless sheet and zinc sheet are used as electrode, EW of triazolium-based ILs without hydroxyl group is the widest (i.e., 4.76 V), which is higher than that of triazolium-based ILs with one hydroxyl group (i.e., 4.11 V) and two hydroxyl groups (i.e., 3.52 V) [190]. The presence of [Zn][TFO]₂ and [Li][Tf₂N] in triazolium-based ILs without hydroxyl group could further broaden the EW into 6.36 V, which shows 81 mAh g^-1 initial discharge capacity at 30 °C in zinc-ion batteries [190]. EW of [Zn][TEO]₂+[Emim] [TEO] is 2.8 vs. Zn/Zn²⁺ by using stainless steel and zinc plate as working electrode and counter/reference electrode, respectively, which is higher than that of the aqueous system [191]. Fan et al. find that the mobility of Zn²⁺ cation in ILs is favorable; moreover, the deposition and strip of zinc in ILs could be easily [191]. The suppression of dendrite formation on zinc surface in ILs is also noticeable. It should be noted that the addition of [Zn][TFO]₂ in [Emim][TF0] would increase the viscosity and hence decrease the conductivity. When the concentration of [Zn][TFO]₂ is higher, the conductivity of ILsbased electrolytes is lower [191].

3.3.6. Calcium-ion batteries

Calcium is more abundant than sodium, potassium, magnesium and lithium. Electrochemical stability of ILs-based gel electrolytes ([Emim] [TEO]+[Ca][Tf₂N]2+poly (ethylene glycol)diacrylate) is investigated by using calcium metal and stainless steel as reference/counter electrode and working electrode, respectively [192]. Below 0 V vs. Ca/Ca²⁺, there is negligible cathodic current, indicating hard deposition of calcium. Anodic current could not be ignored when the potential is higher than 3 V vs. Ca/Ca²⁺, which could be ascribed to the oxidation of ILs-based gel electrolytes [192]. ILs-based gel electrolytes suffer from significant oxidative decomposition at ca. 4.5 V vs. Ca/Ca²⁺ and could completely breakdown at ca. 6 V vs. Ca/Ca²⁺. The electrochemical stability of ILsbased electrolytes is higher than that of ether- or carbonatebased electrolytes. The high anodic stability of ILs-based gel electrolytes could be ascribed to the polymer host and the ILs. However, the charge/discharge capacity are fading with the time because of the hard reinsertion of Ca²⁺ into Ca₃Co₄O₉ [192]. Shiga et al. constructs rechargeable calcium-oxygen batteries by using ILs-based electrolytes; instead, the capacity is easy to fade due to the deactivaition of calcium anode and there are many gases produced (e.g., oxygen, hydrogen, methane) during the charge/discharge process [193].

4. Conclusions and outlooks

In summary, electrochemical stability of ILs/DESs is very important for their application as electrolytes in lithium-ion, lithium-metal, and post-lithium-ion batteries. The summary of electrochemical stability of ILs/DESs could also provide a guide for the application of ILs/DESs as electrolytes in electroconversion of carbon dioxide, supercapacitor, oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, electrochemical deposition, electrochemical separation and electroconversion of biomass. Nevertheless, electrochemical stability of ILs/DESs during the application process in batteries is seldom paid much attention, which is instead the main focus in this review.

Electrochemical stability of ILs/DESs not in batteries is investigated by many researchers. Many factors are comprehensively investigated, including chemical structures (cation type, alkyl chain length, C2 methylation, functional group, substitution position, anion) and non-structural factors (e.g., working electrode, reference electrode, conductivity, viscosity/ melting point, cut-off current density, scan rate, AC/DC, AC frequency, temperature, water, oxygen, other compounds, pH, ultrasound). The importance of these structural factors could be approximately ordered as anion > cation > functional group > alkyl chain length > C2 methylation > substitution position. In this case, many ILs/DESs are claimed to be electrochemically stable. ILs with the anion of FAP or Tf₂N tend to roughly own high electrochemical stability. Azepanium, piperidinium, pyrrolidinium and phosphonium generally show higher electrochemical stability than other cations, such as ammonium, imidazolium and pyridinium. Shorter alkyl chain length, functionalization and C2 methylation are not necessarily lead to a higher electrochemical stability, which depends on the types of cation, anion and interaction between cation and anion. On the other hand, effect of non-structural factors on electrochemical stability of ILs/DESs seems to be more complicated and more irregularly. The presence of ubiquitous oxygen and moisture in the air would generally narrow the EW, while adding different concentration of water in different ILs/DESs also increase the EW in some cases. Conductivity, viscosity, melting point and pH could be used to roughly estimate the electrochemical stability. During the measurement, values of temperature, scan rate, cut-off current density and working electrode must be provided; instead, many references do not give these details. AC, DC and AC frequency are very important in specific conditions. Reference electrode should not have any influence on the electrochemical stability of ILs/DESs; however, the presence of junction potential, wettability and possible solvents leak would affect the electrochemical stability.

The reports on the electrochemical stability of ILs are enormous; however, very few literatures have reported the electrochemical stability of DESs.

However, electrochemical stability of ILs/DESs not in batteries is only the simulated stability rather than the real evaluation. In the real environment of batteries, ILs/DESs are always mixed with salts, polymers or organic solvents; electrode are also specific. These special requirements in lithiumion, lithium-metal, and post-lithium-ion batteries would lead to the different electrochemical stability of ILs/DESs when compared to condition not in batteries. Electrochemical decomposition mechanism of ILs/DESs in batteries are hardly investigated. The decomposed produced after many cycles in batteries are scarcely reported. It means that long-term electrochemical stability of ILs/DESs in these batteries should be paid much attention. ILs/DESs as electrolytes in lithium-ion, lithium-metal, and post-lithium-ion batteries are mainly these containing anion of Tf₂N/FSI or cation of pyrrolidinium/ piperidinium/imidazolium. As indicated above, adding the organic solvents into ILs-/DESs-based electrolytes generally improve their efficiency; instead, it would contradict with the concept of green chemistry and sustainable development. Pure ILs-/DESs-based electrolytes are encouraged for the future application in batteries, which is still demanding the developments of novel ILs/DESs with high electrochemical stability.

Different from the electrochemical stability of ILs and DESs not in batteries, how to more accurately judge the EW of ILs-/DESs-based electrolytes in batteries is very important. We believe that in addition to EW, cathodic limit and anodic limit, mechanism of electrochemical decomposition could be used to judge the electrochemical stability.

Moreover, for the better of applying ILs/DESs as electrolytes in lithium-ion, lithium-metal, and post-lithium-ion batteries to fulfil the green and stable batteries. Several strategies (Fig. 12) are proposed and summarized as below for the improvement of electrochemical stability of ILs-/DESs-based based on the above discussion [63,65,66,72].

Strategy 1. Select reactants with high electrochemical stability.

Electrochemical stability of reactants is strongly related to the corresponding electrochemical stability of ILs/DESs. Generally, stable reactants would lead to the ILs/DESs with high electrochemical stability; instead, reactants tending to decompose electrochemically would results in a narrow EW of ILs/DESs. For example, Tf₂N or FAP salts as the reactants of ILs would make ILs highly electrochemically stable in most cases, while electrochemical stability of ILs with the iodide anion synthesized from iodide salts tend to be very low. Similarly, electrochemical stability of DESs is determined by that of their individuals; for example, EW order affected by HBD is methyl urea > xylitol > urea > malonic acid > butanediol > EG > GL > oxalic acid when ChCl is used as HBA [63]. It means that the components methyl urea, xylitol and urea tend to contribute to a higher electrochemical stability of DESs. ILs could also be used as the components of DESs [194-196], implying that stable ILs would lead to the DESs with high electrochemical stability when ILs are the raw materials of DESs.

Strategy 2. Consider all the chemical structures comprehensively.

ILs/DESs are reported to own high designability and high tunability; however, every part of structure in ILs/DESs (e.g., cation type, anion, C2 methylation, chain length, functional group, substitution position) would affect the electrochemical stability of ILs/DESs. All the structural factors should be considered comprehensively for obtaining ILs/DESs with high electrochemical stability. For example, although phosphonium cation [Pi4666İ are highly electrochemically stable when compared to other cations (e.g., [Brnim]), electrochemical stability of [P444i6]-based ILs is not necessarily higher when being bonded with a vulnerable anion (e.g., DCA anion) if [Bmim] cation is bonded with Tf₂N anion. Furthermore, the interaction strength between anion and cation is also very important for the electrochemical stability of ILs; the electrochemical stability of DESs is also affected by the interaction strength between HBA and HBD.

Strategy 3. Treat non-structural factors seriously.

Much attention has been paid to the structural factors; instead, non-structural factors (e.g., temperature, pressure, time, impurities, mixtures, contactant, cut-off current density, pH) are easy to be ignored or concealed. In order to enhance electrochemical stability, all the non-structural factors should be treated carefully, based on which the electrochemical stability of ILs/DESs could be precisely evaluated. For example, ILs/DESs are generally easier to electrochemically decompose when they are contaminated by oxygen or water in the air, which would affect the electrochemical stability of ILs/DESs. It is suggested to conduct all the experiments in glovebox and quantify all the impurities contents when the electrochemical test is conducted, because ILs/DESs would be contaminated by the air or other substances during the transferring and preparation process. Moreover, some reports claim that ILs/ DESs are highly electrochemically stable while the cut-off current density is not available, which makes the conclusion from the comparison of stability is not accurate. Cut-off current density should be fixed at a specific value (e.g., 1 mA cm-2) for the better comparison.

Strategy 4. Discriminate different kinds of stability.

Actually, there are many other kinds of stability (thermal, chemical, radiolytic and biological stability) in addition to electrochemical stability. Although this review concentrates on the electrochemical stability of ILs/DESs in or not in lithium-ion, lithium-metal and post-lithium-ion batteries, there is high possibility that ILs/DESs could also decompose thermally, chemically or biologically during the process of electrochemical decomposition. For example, ILs/DESs could chemically react with lithium salts, water, organic solvents or polymers. Temperature would increase during the process of operating batteries, which could also intrigue the thermal decomposition and evaporation of ILs/DESs. These decomposed products from thermal, chemical, radiolytic and biological decomposition of ILs/DESs would affect evaluating the electrochemical stability of ILs/DESs. Thus, for the purpose of obtaining reliable data for the electrochemical stability of ILs/DESs, several guides are proposed below. (1) other kinds of decomposition should be avoided, which could be used as the basis for the simple evaluation of electrochemical stability of ILs/DESs; (2) products are discriminated and assigned to thermal, chemical, electrochemical, radiolytic and biological, respectively; (3) contrast experiment without electric power should be conducted to verify whether there are other kinds of decomposition or to quantify degree of theses decompositions.

Strategy 5. Utilize relationship between electrochemical stability and other physical properties.

There are some brief relationships between stability and other physical properties (e.g., viscosity, melting point, electrical conductivity), which would be very helpful for the design of ILs/DESs with high electrochemical stability. For example, ILs/DESs with high viscosity, high melting point and low electrical conductivity are generally difficult to be volatilized and electrochemically oxidized/reduced. However, it should be noted that the design of ILs/DESs with high viscosity, high melting point and low electrical conductivity could result in their unfavourable application in lithium-ion, lithium-metal and post-lithium-ion batteries. Particularly, high electrical conductivity of ILs-/DESs-based electrolytes is one of the most important prerequisites for achieving high-performance batteries. However, high melting point and high viscosity could be used to construct (quasi) solid-state ILs-/DESs-based electrolytes for (quasi) solid-state batteries. In this sense, electrochemical stability could be enhanced according to other physical properties although these properties should be balanced. It is anticipated to summarize the relationship between electrochemical stability and other physical properties, before which large quantity of data should be determined.

Strategy 6. Strengthen intermolecular interaction.

Design of ILs/DESs with high electrochemical stability should consider both the ions/components and the interactions. Electrochemically stable cation, anion and component would enhance the electrochemical stability of ILs/DESs; however, ILs/DESs tend to be deteriorated if the interaction is weak. Improving the interaction strength between cation and anion or between HBA and HBD could be helpful for the construction of ILs/DESs with high electrochemical stability. The strong intermolecular interaction related to ILs/DESs would protect ILs/DESs from being disrupted (i.e., oxidized or reduced) by electricity. For example, polymerization [197] of ILs and DESs is a favourable strategy for strengthening the interaction between cation and anion or between HBA and HBD, which could generally enhance the electrochemical stability of ILs/DESs used for lithium-ion, lithium-metal and post-lithium-ion batteries. In addition to polymerization, eutectogel [198,199], hydrogel [200], ionicgel [201] could also be very helpful for strengthening the intermolecular interaction.

Strategy 7. Ensure the same evaluation standards and conditions.

We doubt that some conclusions about the electrochemical stability of ILs/DESs drawn from the references are misleading because these data are based on different standards or conditions. For the purpose of improving electrochemical stability by selecting appropriate strategies, it should be ensured that the evaluation standards and measuring conditions, the analytic methods and the corresponding measuring parameters should be the same. Comparison in electrochemical stability and the rules used to design ILs/DESs with high electrochemical stability would be invalid if the evaluation standards, the analytic methods or the corresponding measuring parameters is different. We strongly suggest a uniform standard to measure the electrochemical stability of ILs/DESs; at least, all the related details about the measuring process should be provided in the reports. For example, these non-structural factors are fixed as 1 mA cm-2 cut-off current density, scan rate 100 mV s-1, 25 °C, no water, no oxygen, no ultrasound, and DC. The values of EW, cathodic and anodic limit of ILs/DESs could be converted into potential vs. normal hydrogen electrode for a better comparison.

Strategy 8. Apply computer-aided methods to predict and design.

Theoretical methods are commonly used to predict the physical properties and the phase behaviours of ILs and DESs for separation, purification and catalysis [202-207]. Similarly, electrochemical stability of ILs and DESs could also be theoretically predicted and estimated by many methods.

This review would be particularly important for achieving green, sustainable and safe energy. The main compositions of batteries include cathode, anode, separator, current collector and electrolyte. ILs-/DESs-based electrolytes with high electrochemical stability could reduce the cost, increase the recyclability and enhance the safety, which is the necessity for green energy; meanwhile, these ILs-/DESs-based electrolytes are favourable to construct batteries with high flexibility, high wearability, high specific energy, high conductivity, low cost, dendrite suppression, high safety, non-flammability, and high performance. However, some reported ILs-/DESs-based electrolytes used in batteries are not so electrochemically stable in the above discussion; fortunately, the rules drawn from the effect of chemical structure and non-structural factors on the electrochemical stability of ILs/DESs in and not in batteries could be utilized for the design of more stable, greener and safer ILs-/DESs-based electrolytes.

In conclusion, the factors affecting the electrochemical stability of ILs/DESs in or not in lithium-ion, lithium-metal and post-lithium-ion batteries are summarized. Further, the corresponding strategies to improve the stability are proposed. Although ILs/DESs are considered as green solvents, their instability is unfavourable for sustainable economy particularly in the field of lithium-ion, lithium-metal and postlithium-ion batteries. In common condition, the electrochemical stability of ILs/DESs could keep still high and the electrochemical decomposition ratio would be low. These proposed strategies would be the potential routes to obtain electrochemically stable ILs/DESs and applying ILs/DESs with high sustainability for lithium-ion, lithium-metal and post-lithium-ion batteries.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (22103030, 22073112) and Youth Topnotch Talent Program of Hebei Institution of Higher Learning (BJ2021057) for financial support.

Footnote

* authors.

E-mail addresses: [email protected] (Y. Chen), [email protected] (T. Mu).

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