Ionic liquids and their derivatives for lithium batteries: role, design strategy, and perspectives

IL and Li-salt system

The replacement of OLEs with new electrolytes based on ILs aims to realize safer electrolytes with enhanced thermal and electrochemical stabilities while maintaining an acceptable ionic conductivity and t_Li⁺.

To design suitable IL-based mixtures for use as electrolytes, it is paramount that the employed ILs have a low melting point (or preferably a total absence of a crystallization behavior even at sub-zero temperatures) and a low viscosity, in addition to the ability to dissolve lithium salts and a wide ESW. Given these conditions, a combination of a bulky and asymmetric cation (for a lower T_m) without unsaturated bonds and acidic protons (for better cathodic stability) coupled with a fluorinated anion (for a lower T_g, better ionic conductivity^[57] and anodic stability^[58]) is usually employed.

The cation is usually an imidazolium or an aliphatic quaternary ammonium. Imidazolium is the most common due to its association with high conductivity and low viscosity due to the planarity of the imidazolium ring^[31]; yet, the presence of double bonds and the acidity of the proton at the C2 position results in ILs that are unstable at low potential. The issue of deprotonation can be mitigated by substituting this proton with a methyl group, increasing the cathodic stability of the IL, as reported by Bonhôte et al.^[59]. On the other hand, aliphatic quaternary ammonium cations, such as tetraalkylammonium, pyrrolidinium, and piperidinium, are slightly less conductive compared to imidazolium ILs but are stable at lower potentials^[60]. In addition, the use of phosphonium and sulphonium ILs is currently under investigation as they display a wide ESW and high thermal stability^[61,62]. From a comparative study, the cathodic stability of these cations is found to increase in the following order: sulfonium < ammonium < phosphonium^[63]. The superior stability of phosphonium cations arises from the more delocalized positive charge on the P atom center. However, phosphonium ILs are less common, which may be due to the air/moisture-sensitive and spontaneously flammable nature of trialkylphosphines and their successive toxic gas formation^[64]. It is also possible to enhance the cathodic stability of ILs with alkyl side chains through a shielding effect of positive charge, which can be maximized by alteration of the alkyl chains with a moderate length such as a butyl group^[65], because a shorter chain has a weak shielding effect whereas a longer chain is a good leaving group and not tolerant against reduction^[66]. A common approach to reduce T_m is the introduction of an ether group in the side chain of the cation. This causes an increment of the degrees of freedom of the side chain, reducing the T_m or even inhibiting the crystallization behavior of ILs^[31,38,67]. In particular, an ethoxyethyl group has been reported to contribute to the reduction of the T_m than the shorter methoxyethyl group. Jin et al. observed that 1-ethoxyethyl-2-methyl-3-propylimidazolium has T_m 80 °C lower than that of 1-methoxyethyl-2-methyl-3-propylimidazolium^[68]. This is probably due to the larger free space formed by the terminal ethyl group.

With respect to the anions, BF₄^- and PF₆^- are among the first choices as counter ions (especially the latter corresponding to LiPF₆, the most common lithium salt found in commercial electrolytes^[69]). However, due to their perfect symmetry causing an increase in the T_m (e.g., T_m are 15 °C and 60 °C for [EMI]BF₄ and [EMI]PF₆, respectively^[70,71]) and their tendency to be hydrolyzed in the presence of even a small amount of water resulting in the formation of HF, the use of these anions has declined, and imide-based anions have become the preferred choice for IL-based electrolytes. Bis(trifluoromethanesulfonyl)imide ([TFSI]) and bis(fluorosulfonyl)imide ([FSI]) are the main representatives of this family. ILs with [TFSI] anions exhibit a notable ESW, thermal stability, and good ionic conductivity (8.8 mS cm^-1 for [EMI][TFSI]^[59]) and tend to have a reduced melting point (-3 °C for [EMI][TFSI]^[59]). ILs with [FSI] anions can exhibit enhanced ionic conductivity (15.6 mS cm^-1 for [EMI][FSI]^[72]) due to the reduced steric hindrance of the anion in comparison to TFSI^[31], while they suffer from a reduced electrochemical and thermal stability because of the low stability of S-F bonds compared to S-CF₃ bonds^[31]. Other (less common) imide-based anions are bis(pentafluoroethanesulfonyl)imide ([BETI]) and the asymmetrical ones ([IM₁₄] and [FTFSI], see Table 1)^[61,73]. The use of these asymmetric anions allows the formation of RTIL, even with symmetric cations, such as tetrabutylphosphonium [P_4,4,4,4], which is a promising cation due to its remarkable cathodic stability^[61]. In recent years, oxalatoborate-based anions, such as difluoro(oxalate)borate [DFOB] and the bis(oxalate)borate [BOB], have also been studied^[38] because of their ability to form a protective film on cathodes, i.e., CEI, containing borate-based species, which is beneficial for high-voltage cathode materials^[38].

The current choice of lithium salt is Li[FSI] or Li[TFSI] because the same anions are used for the ILs, which act as solvents. IL electrolytes (ILEs) can also utilize different anions. For example, the use of a TFSI salt in an FSI-based IL has been reported to exhibit positive synergistic effects, such as higher anodic stability, a wider liquid range^[74], an efficient formation of favorable EEIs on the surface of electrodes^[75,76], an improved wettability^[77], and a higher ionic conductivity/reduced viscosity^[78]. Another important point regarding the additive salt is its concentration. Usually, with the aim of maintaining low viscosity of the mixtures, the molar ratio of IL:lithium salts varies between 9:1 and 8:2. However, addition of a large amount of salts in the electrolyte (more than 2 M) is known to reduce the gradient of Li⁺ concentration in the electrolyte, favoring a more uniform lithium deposition on lithium metal and the formation of a more robust SEI^{[7,8,34,77,79,80]}.

A mixture that has shown excellent properties in recent years is 0.8[PYR_1,4][FSI]-0.2Li[TFSI]. This dual anion mixture possesses all the previously mentioned synergistic effects, especially good conductivity^[13,78] coupled with a wide ESW^[13,76]. Furthermore, the combination of these two anions permits the formation of suitable inorganic EEIs based on LiF, Li₃N, LiNSO, and Li_xSO_y, both on the surfaces of a lithium-metal anode and cathode, demonstrating a promising feature for the realization of high-energy LMBs. Employing this ILE in a high-voltage Li||LRNM system, Wu et al. obtained a discharge capacity of ~155 mAh g^-1 and a capacity retention rate of 56.0% after 2,000 cycles at 1 C^[13], which is an outstanding result among different ILEs from the viewpoints of C-rates and the number of cycles [Supplementary Table 1]^[31,61,73]. The enhanced cycle life can be explained by the formation of a protective CEI on the surface of the cathode, which suppressed the structural transformation of the LRNM to a rock-salt phase^[13]. The same ILE was also used in a Li||NM88 battery, which achieved a discharge capacity of ~200 mAh g^-1 with 88% capacity retention over 1,000 cycles at 0.3 C, while a similar system where an OLE (instead of ILE) was used, attained only a capacity retention rate of 36.9%^[76]. Figure 2 shows the morphology of the cathode surface after cycling. The crack formation was evident when OLE, specifically 1M LiPF₆ in EC/DMC 1:1 v/v (LP30), was used. XPS measurement on the same cathodes showed that the CEI present on the surface was rich with LiF and S-based species when the ILE was used, which is beneficial for a high mechanical strength^[21,24,25] and electrochemical stability coupled with high Li⁺-conducting and electron insulating properties^[21,24,25]. In addition, these CEI products are considered favorable to suppress electrolyte decomposition when in contact with highly charged and highly reactive metal species, i.e., Ni⁴⁺. Furthermore, due to the absence of LiPF₆, HF was not produced upon cycling, avoiding the formation of rock-salt phases^[76].

Figure 2. Top-view and cross-sectional SEM micrographs of pristine (fresh) NCM88 electrodes (A, D, G, and J) and NCM88 electrodes after 200 cycles at 0.3 C-rate in LP30 (B, E, H, and K) and ILE (C, F, I, and L). The electrodes shown in (A-I) were pressed at 8 t cm², while those shown in (J-L) are unpressed electrodes. This figure is quoted with permission from Wu et al.^[76]. Copyright (2021) The Author(s). Published by Elsevier Inc.

For super-concentrated ILEs, Heist and Lee showed that 4.2 M Li[FSI] in [PYR_1,3][FSI] allowed the cycle of a Li||NMC811 battery for 1,000 times at 1 C maintaining ~77% of the initial capacity retention, in contrast to the cell with an OLE resulted in far lower capacity retention (~17%)^[79]. With the increase in the salt concentration, the ionic conductivity decreased as expected. However, t_Li⁺of the electrolyte increased, and SEI resistance decreased^[79]. Sun et al. further developed this approach using a high-concentration ILE composed of 5M Li[FSI] in [EMI][FSI] with and without 0.16M Na[TFSI]^[8]. Using this electrolyte, a Li||LCO battery was able to retain a capacity retention of 87% at 1 C after 900 cycles and 81% at 0.7 C over 1,200 cycles. These values are far superior to those obtained in similar systems but without Na[TFSI] and those with a reduced amount of lithium salts (the latter is shown in Figure 3A)^[8]. Furthermore, a Li||NMC811 system achieved an initial discharge capacity of 181 mAh g^-1 and retained 94% after 200 cycles, as shown in Figure 3B^[8]. These improvements can be ascribed to the presence of both Na[TFSI] as an additive and the high-concentration lithium salt. The latter contributes to forming a LiF-rich SEI on the lithium surface, while the former provides a “positive shielding effect” on an initial growth tip of dendrites, which limits dendrite growth^[8]. In addition, this ILE was able to protect the NMC811 cathode by covering it with a stable CEI. This CEI was highly Li⁺-conductive due to the presence of a large amount of F-based species and capable of protecting the NMC811 from oxygen release due to the presence of S-based species^[8].

Figure 3. (A) Cyclic stability of Li||LCO batteries using [EMI][FSI] + 1M Li[FSI] (orange) and [EMI][FSI] + 5M Li[FSI] + 0.16M Na[TFSI] (blue) electrolytes. The batteries were first charged/discharged at 0.25 C for two cycles and then cycled at 0.7 C (≈1.2 mA cm^-2). (B) Cycling stability of Li||NCM 811 batteries using EMIFSI + 1M Li[FSI] with (blue) and without (black) 0.16 M Na[TFSI] at 0.5. This figure is reprinted (adapted) with permission from Sun et al.^[8]. Copyright (2020) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

ILs as co-solvent/additive

ILEs possess strong Coulombic interactions, which, in some cases, hinder their use as electrolytes, especially when a high concentration of lithium salts is added, the viscosity of ILEs increases, causing a reduced ionic conductivity and sluggish Li⁺ transport^[77,81,82]. A possible approach to overcome this challenge is the use of the conventional organic solvents employed in the traditional OLEs as a co-solvent^[77,82].

The combination of ILs and organic solvents achieves a medium with high ionic conductivity, which is mainly governed by the ions solubilized in the organic solvent. The role of the IL is to provide two functions in this type of system: first, it ensures enhanced safety, maintaining the thermal stability at the same level as ILEs. Secondly, a well-designed IL allows the formation of stable and protective EEIs on both electrodes. Since the former has been discussed extensively in the literature, the focus of this section is the formation of robust EEIs.

When ILs are added in OLEs, inorganic species in EEIs are present, and CF₃- derived from [TFSI], N- due to ammonium cations^[13], and B- from borate anions^[38] are particularly important for CEI. In addition, P- species from phosphonium cations are reported to enhance the thermal stability of the CEI^[61], and S-based ones are known to prevent lattice oxygen exposure^[8].

On the anodic side, it is well known that the addition of an IL tends to produce a stable and inorganic SEI on the surface of lithium metal^{[7-9,75,77,83-85]}. The most important component of this inorganic SEI is LiF, and it is usually coupled with other species produced by the decomposition of both the anion and the cation, depending on the structure of the employed IL. Recently, it has been reported that the cations play another significant role in the improvement of the performance of lithium metal because of the formation of a lithiophobic protective layer on Li protuberant tips^[75,83]. Jang et al. reported that the addition of 1M [PYR_6,6][FSI] to a solution of 1M Li[TFSI] in DOL/DME (1:1 v/v) achieved capacity retention of 80% and a coulombic efficiency of 99.8% over 600 cycles in a Li||NMC622 battery, while the same electrolyte without ILs displayed only 55.7% capacity retention over 200 cycles [Figure 4A-C]^[75]. The main reason for this improvement is related to the efficient deposition of [PYR_6,6]⁺ on lithium protruding tips.

Figure 4. Cycling performance of Li-metal batteries and Li-metal morphology after three initial cycles at a different C rate (E/C: electrolyte/cathode ratio and N/P: negative/positive ratio). (A) Li||LiFePO₄ cell performance at 1 C (Li thickness: 300 μm and LiFePO₄ loading: 3-4 mg cm^-2). (B) Li||NCM622 cell performance at 1 C (Li thickness: 300 μm and NCM622 loading: 4-5 mg cm^-2). (C) Li||NCM622 cell performance at 0.5 C (Li thickness: 40 μm and NCM622 loading: ≈12 mg cm^-2). Cross-section SEM images of Li-metal anodes retrieved from the 40 μm Li||NCM622 cells with (D) [Pyr_6,6][FSI, (E) [Pyr_1,12][FSI], and (F) [Pyr_3,3][FSI] containing electrolytes after 100 cycles at 0.5 C. (G) Schematic illustration of the Li-metal stabilization by lithiophobic protective layers on Li tips formed by three different IL cations [Pyr_1,12]⁺, [Pyr_6,6]⁺, and [Pyr_3,3]⁺ (from left to right). This figure is reprinted (adapted) with permission from Jang et al.^[75]. Copyright (2022) Wiley-VCH GmbH.

This layer produces a positive shielding effect that repels Li⁺ and hinders inhomogeneous lithium deposits, maintaining a smooth lithium surface, as shown in Figure 4G^[75]. The effect was less pronounced when ILs with an asymmetric structure or with shorter alkyl chains were added. From SEM analysis of NMC622 recovered from the cycled cells [Figure 4D-F], the thickness of the SEI formed in the presence of [PYR_6,6]⁺ was thinner and more compact than with other ILs.

Wang et al. assembled Li||LFP batteries by adding 20 wt% [BzMI][TFSI]/[EMI][TFSI] (9:1 in mol) in a 1 M Li[FSI] in DOL/DME (1:1 v/v) electrolytes, which exhibited capacity retention of 76% after 500 cycles^[85]. This suggests that the synergistic effects between both the [FSI] and [TFSI] anions and the [BzMI] and [EMI] cations enabled outstanding cycling stability. The former two quickly produced a stable, thin, and LiF-rich SEI on the anode surface, while [BzMI] provided a fast Li⁺ conduction path through the SEI and [EMI] enhanced the fast diffusion of Li⁺ through the liquid electrolyte^[85].

On the cathodic side, the addition of ILs leads to the formation of a CEI based on inorganic species similar to the ones found on the SEI. The CEI is a nanoscale layer that has demonstrated numerous beneficial features^{[13,38,61,69,76,86]}, including protection from corrosion, reduction of oxygen evolution, and prevention of the dissolution of transition metals. Typical anions that form CEI are oxalatoborate-based ones, such as [BOB] and [DFOB] (see Table 1)^[87]. By adding a 0.3 M [Pip_1,2O1][BOB] or [Pip_1,2O1][DFOB] to 1 M LiPF₆ in EC/DMC/DEC (1:1:1 v/v/v), so-called LP71, capacity retentions of 99.2% and 98.1% after 100 cycles at 1 C in Li||LNMO cells were obtained. This excellent result has been attributed to the formation of a protective CEI on the cathode surface, which included the favorable presence of B-based species^[38]. The CEI formed from BOB is known to contain B_xO_y, which possesses robustness against electrochemical abuse, preventing transition-metal crossover and also attack of HF. Nair et al. dissolved 0.3 M Li[TFSI] in a mixture of EC/DMC+[Pyr_1,3][TFSI]+FEC (45:45:10), obtaining an electrolyte, allowing a discharge capacity of 153 mAh g^-1 with stable performances for 1,200 cycles in a Li||LRNMC cell^[88], which was related to a sufficiently reduced degradation of the electrode morphology.

Although an effective reduction of the viscosity of the electrolyte is possible by using OLEs in addition to ILs, as a tradeoff, this approach potentially leads to an increase of the flammability of the electrolyte^[77]. Furthermore, due to strong coordination of Li⁺ by alkyl-carbonates, these organic solvents change the Li⁺-solvation structure and affect the formation of SEI on the lithium metal surface, increasing its organic content and reducing its stability^[77,82]. To solve this problem, another approach emerged in recent years using hydrofluoroethers (HFEs) instead of alkyl-carbonates as a co-solvent in ILEs, realizing the so-called “locally highly concentrated electrolytes, LHCEs”^[9,77,80,84]. HFEs possess a low viscosity, are non-flammable, and due to their dielectric constant, they are not able to strongly coordinate Li^+[9,77,84]. Because of these properties, their presence does not alter the coordination environment of Li⁺, allowing the formation of the inorganic-based SEI with faster Li⁺ transport [Figure 5A-C]^[84]. Using TTE as a HFE, Wang et al. realized an LHCE with a composition of Li[FSI]/[Pip_1,3][FSI]/TTE (1:2:4 in mol) that allowed a Li||LFP battery to achieve a capacity retention of 87% after 1,000 cycles at a rate as high as 5 C. This cycling performance is superior to the same electrolyte without the addition of the HFE [Figure 5D]^[84].

Figure 5. Schematic diagrams of Li plating on Cu current collectors in (A) commercial electrolyte, (B) ILE, and (C) LHCE. (D) Cycling performance of Li||LFP cells at room temperature with different electrolytes at 5 C. This figure is reprinted (adapted) with permission from Wang et al.^[84]. Copyright (2021) Wiley-VCH GmbH.

Other fluorinated additives, namely fluorinated aromatic compounds (FACs), are also of interest. Liu et al. demonstrated that dFBn added to Li[FSI]/[EMI][FSI] do not strongly interact with Li⁺ but instead preferentially interact with [EMI]⁺ through π-π interactions^[82]. Using a mixture of Li[FSI]/[EMI][FSI]/dFBn (in a molar ratio of 1:2:2), they obtained a highly stable cycling of Li||NMC811 cells at C/3 charge and 1 C discharge for 500 cycles with a discharge capacity of 192 mAh g^-1 and a capacity retention of 93% [Figure 6A]^[82]. This result is ascribed to two factors: the formation of thick SEI on the lithium metal anode suppressing the formation of dendrites and the formation of thin CEI on the surface of the cathode^[82]. Through an XPS analysis of the two interphases, the decomposition products of [EMI] and [FSI] were identified as an important component of the SEI, which can be confirmed by the high intensity of the S-F, N_cation and N_anion signals, while the CEI is mainly based on the decomposition products of dFBn, as indicated by the high intensity of the signals associated to fluorine-based species and the low intensity of the N_cation and N_anion signals [Figure 6B-E]^[82].

Figure 6. (A) The evolution of the discharge specific capacity and Coulombic efficiency (CE) during long-term cycling of the cells at C/3 charge and 1C discharge after two formation cycles at C/10. XPS detail spectra in the C 1s, F 1s, and N 1s regions for the lithium cycled in (B) Li[FSI]/[EMI][FSI] (FE) and in (C) Li[FSI]/[EMI][FSI]/dFBn (FEdF) and for cathodes in (D) FE and (E) FEdF. This figure is reprinted (adapted) with permission from Liu et al.^[82]. Copyright (2022) The Authors. Advanced Energy Materials published by Wiley-VCH GmbH.

The use of co-solvents has other roles besides the reduction in viscosity. For example, the addition of co-solvents forming a protective SEI on the Li-metal anodes enables the use of ILs with an acidic proton (so-called “protic ILs”), which generally reacts with lithium. Protic ILs are beneficial in terms of an easier and cheaper preparation method in comparison to common ILs because they can be synthesized merely by mixing a Brønsted base and acid^[89-91]. In addition, they possess a high Li⁺ mobility when they are mixed with lithium salts due to the less shielded cation in comparison to aprotic ILs, leading to a “competition” between Li⁺ and the cation to coordinate the anion that results in a loosely coordinated Li^+[89-91]. Lingua et al. recently showed that the addition of vinylene carbonate (VC) to 0.8[PYR_H4][FSI]-0.2Li[FSI] allows the use of this electrolyte in a Li||NMC622 battery, obtaining a discharge capacity of 155 mAh g^-1 and maintaining a capacity retention of 95% after 60 cycles^[89]. This excellent result is ascribed to the formation of a protective layer on the lithium surface due to the decomposition of VC during the first cycle, which protects the electrolyte from further degradation upon cycling^[89]. This preliminary result demonstrates that with further optimization, protic ILs can play an important role in the future development of high-voltage LMBs.

Besides the use of the co-solvent, ILs have been used as an additive with OLEs to improve their safety and to optimize the formation of both the SEI and CEI. Zhang et al. reported the addition of only a 1 wt% of [Pip_1,3][FSI] to a Li[FSI]/PC electrolyte that was able to significantly reduce the formation of dead lithium and lithium dendrites due to the lithiophobic effect of the cation and the formation of a stable SEI^[83]. The use of ILs as additives in conjunction with OLEs has another advantage: it is possible to use a greater variety of ILs since low viscosity or a low melting point is not a prerequisite for such use cases. ILs possessing a T_m above room temperature, such as [Pyr_1,4]PF₆, improve the safety and the stability of LP30, allowing Li||LNMO cells to achieve a discharge capacity of 110 mAh g^-1 after 200 cycles^[69].

Polymerized ionic liquids

The first polymerized ionic liquids (PIL) was reported in 1998 by Ohno and Ito^[99]. In the past decades, many different PILs, such as (i) linear polycationic and polyanionic types; (ii) copolymer type (co-PIL); and (iii) cross-linked types (xl-PIL), including star-shaped types, have been reported^[43]. Most PILs for LMB applications have polycationic structures, in which single or multiple cations are included either in the main chain, side chains, or bridges between cross-links, and ion species are mainly limited to imidazolium, quaternary ammonium (including pyrrolidinium structure), and rarely pyridinium.

Polymerization of [Vmim]Y, in which Y is the type of anion structure, with water-soluble azo-initiator yields linear PILs having imidazolium structure in the side chain. By selecting diallyldimethylammonium Y ([DADMA]Y) or imidazolium cation with two allyl groups, it is possible to include pyrrolidinium and imidazolium structures in the polymer backbone, respectively. There are numerous reports of poly([DADMA]Y)-derivatives containing different anion structures Y^[100] and modified alkyl side chains^[101]. Among them, the most conventional one is poly([DADMA][TFSI]), and its nanofiber prepared by electrospinning was found to show good wettability with ILEs, with an uptake almost five times more than that of polypropylene separators^[102]. It was also demonstrated that the replacement of [TFSI] with [FSI] enables higher ionic conductivity of Li⁺, specifically poly([DADMA][FSI]-[Pyr_1,3][FSI]-Li[FSI] (1:1:1, by molar ratio) possessed σ_Li of 0.22 mS cm^-1 at 50 °C because of a lower amount of ion aggregates^[103]. By employing poly[DADMA][FSI] mixed with 1M Li[FSI] in [Pyr_1,3][FSI] (6:4 by weight), Fu et al. developed high-voltage LMBs^[46]. The cell with NMC811 exhibited an initial capacity of 162 mAh g^-1 (cut-off voltage of 4.4 V), and the one with LNMO displayed an initial capacity of 132 mAh g^-1[46]. Both were cycled at 0.1 mA cm^-2 at 25 °C^[46]. In the latter case, a capacity retention of 76% after 300 cycles was achieved, while in the former, although the capacity retention was good, the formation of soft shorts was observed. To address this problem, PIL-based electrolytes were combined with a glass fiber separator, and a capacity retention of 72% after 600 was achieved^[46]. Dicationic PILs are also potential materials highlighting the many possibilities of rational structure design of ILs^[104,105].

In a similar manner, there is also a growing trend to develop polyanions, which are generally classified as SPEs rather than PILs because the cation for polyanions is Li⁺ and not organic cations. Since anion structures are fixed in the polymer main chain, the t_Li⁺ of unity can be achieved. Recently, fixation of sulfonylimide anion moieties on SPEs has been dominating material development because their delocalized negative charge is favorable for ion dissociation^[106]. By enlarging the conjugation structure adjacent to the anion structure, the ionic interactions can be further reduced, and several poly-styrene derivatives with sulfonylimide structures have been designed as Li⁺ conductive SPEs^[106].

There are also reports of polymer blends or copolymers of PIL to compensate for the weakness of each component or to improve the properties by their synergic effect. Hu et al. reported tensile strength and toughness enhanced through a composite formation based on poly(vinylidene fluoride) (PVdF) and PIL owing to ion-dipole interactions among the components^[107]. For the design of suitable composite materials, compatibility between polymers and ILs should be a useful parameter to control microphase separation and properties of resulting couples^[108,109]. In contrast, by forming a copolymer of PIL and other polymers, it is possible to control their phase separation. Zhu et al. prepared a copolymer based on PILs and poly(ethylene oxide) (PEO) that exhibited reduced crystallinity in PEO and microphase separation with enhanced t_Li^+[110]. Other examples of copolymer-type PILs are through integration with task-specific moieties, such as a flexible ether chain structure, to reduce glass transition temperature, or boron as an anion receptor^[44,111].

Cross-linking has also been attempted, which is advantageous in terms of improved mechanical stability and ionic conductivity. The latter is because structural flexibility of the polymer chain allows faster ion conduction; thus, PILs with cross-links connecting short molecular chains are favorable for the ion conduction^[112]. As summarized in Supplementary Table 2, most PILs contain a considerable amount of ILs as plasticizers that allow them to function as the electrolyte, while some copolymer and cross-linked PILs can operate as the electrolyte by merely adding lithium salts due to their intrinsically low T_g and presence of coordinating sites that allow the dissociation of the salt. Although it is difficult to systematically summarize all series of PILs because of their complicated structures, including unidentified molecular weight between cross-linking points and cross-link density, materials can be classified into two groups, one comprising an IL structure in the main polymer (in backbone or side chain structures) and one consisting of an IL structure as the bridge between polymer backbones. In the first case, cross-linkers such as Bisphenol A ethoxylate dimethacrylate^[88], trimethylolpropane triacrylate^[113], epoxy polyhedral oligomeric silsesquioxanes (POSS)^[114], poly(ethylene glycol) diacrylate^[115,116], and isocyanoethyl ethacrylate^[117] have been used. In contrast, in the second case, cross-links are formed between ILs modified in two or more places with epoxy, vinyl, or allyl, and cross-link acceptors such as sulfur^[118], pentaerythritol tetrakis(3-mercaptopropionate)^[119], azobis(isobutyronitrile)^[120], and poly(ethyleneimine)^[121].

In the polymer-based electrolytes, their ion conduction property is governed by the segmental motions of polymers, which strongly correlate to T_g. The molecular weight is one of the most critical factors in determining the T_g, together with cross-linking density and the presence of plasticizer and functional groups. Therefore, it greatly affects their performance in batteries; thus, it is necessary to assess these factors by reliable methods. Yin et al. suggested that the polymerization degree is affected by the synthesis procedure, but determination of accurate molecular weight by means of chromatography is difficult because of polymer aggregation and column interaction^[122]. To address this problem, high-performance IL chromatography (HPILC) is a promising technique in which ILs are used as an eluent^[123].

As summarized in this section, there are different classes of PILs. However, their performance as electrolytes for LMBs is strongly dependent on the additives, such as liquid electrolytes and ILs, especially in the case of linear or copolymer type PILs. Therefore, the development of PILs should focus on increasing the ionic conductivity while incorporating other additional functionalities, such as improved mechanical stability, anti-folding, and self-healing properties. As a new type of PIL, toward greener materials, PIL based on biomass has also been reported^[124]. Since the material can be synthesized through the reaction between chitosan and glycidyl trimethylammonium chloride, complicated polymerization steps can be avoided^[125].

Ionic liquids in solid-polymer electrolytes

Regarding solid-polymer electrolytes (SPEs), as suggested by Armand at the Second Meeting on Solid Electrolytes in 1978, the first studied system is PEO with the addition of a lithium salt, namely Li[TFSI], which is still a relevant system even today^{[94,97,126-128]}. This system works as electrolytes because of the Lewis acid-base interactions between Li⁺ (due to its high positive charge density) and the oxygen atoms in PEO chains (due to their lone pairs of electrons) that permit the dissociation of the lithium salt when Li⁺ and the anion are not strongly associated, as in the case of Li[TFSI]^{[94,97,126-128]}. This system is particularly appealing due to the absence of liquid electrolytes, avoiding any risk of leakage and thus improving the safety profile of devices^[129]. Furthermore, the polymer can partly suppress the growth of dendrites on the surface of the lithium metal anode due to its high elastic modulus^[129,130]. However, the main disadvantage of this system is related to its low ionic conductivity at room temperature^{[94,97,126-128]}. This is primarily owing to Li⁺-conduction occurring along the chains of PEO, where it breaks the coordination with one oxygen atom and establishes coordination with the next one. This process is promoted by the segmental motion of PEO in the amorphous phase^[94,97,127]. Due to the high T_g of PEO, the crystalline phase is predominant at room temperature, reducing the amount of amorphous phase available for Li⁺-conduction^[94,97,127]. To overcome this limitation, the use of ILs as additives with PEO, resulting in GPE, is a desirable approach^[126-128]. ILs in PEO act as plasticizing agents, reducing the crystallinity of the polymer and its T_g while simultaneously supporting the solvation of the lithium salt^[128] and increasing the ionic conductivity. Atik et al. recently reported a promising PEO-based electrolyte with ionic conductivity of 0.66 mS cm^-1, achieved by adding Li[TFSI]:[Pyr_1,(2O)7][TFSI] ([Pyr_1,(2O)7][TFSI] is an oligoether-functionalized IL with seven ether groups)^[126] to the polymer matrix. This improvement is related to a “molecular-shuttle” effect: the oligoether chains of the cation solvate Li⁺ and unlock a new Li⁺ transport path, decoupling the movement from the usual chain-related transport characteristic of PEO-based electrolytes [Figure 7]^[126]. This efficient and faster Li⁺ transport reduces the formation of lithium dendrites, attaining a 99.3% capacity retention after 200 cycles in a Li||LFP cell^[126], with an average discharge capacity of ~150 mAh g^-1.

Figure 7. Illustration of the accelerated Li⁺ conduction modes in PEO-based electrolytes by state-of-the-art oligoether-functionalized IL plasticizer [Pyr_1,(2O)7][TFSI], which enables new conduction modes. This figure is quoted with permission from Atik et al.^[126]. Copyright (2021) The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.

Other polymeric materials frequently combined with ILs are fluorinated polymers such as PVdF and PVdF-HFP. These polymers have higher T_m (165 °C for the former, 142 °C for the latter) than PEO, which is advantageous for high temperature applications. PVdF-HFP has ionic conductivities between 10^-6 and 10^-3 mS cm^-1[131]; thus, it acts only as a matrix similar to other polymers.

Poiana et al. obtained a non-flammable GPE with ionic conduction of 1.46 mS cm^-1 and a t_Li⁺of 0.4 at 20 °C by soaking a PVdF-HFP membrane in a mixture of LP30/[Pyr₁₄]PF₆ (70:30 w/w)^[129]. The GPE was found to be suitable for high-voltage applications, and in a Li||LNMO system, it sustained a specific capacity of 110.4 mAh g^-1 after 150 cycles at 0.2 C. Tseng et al. mixed PVdF-HFP/[EMI][TFSI]/Li[TFSI]/EC in a weight ratio of 4:11:3.94:1, obtaining a similar thermally stable GPE with ionic conductivity of 1.95 mS cm^-1 at 25 °C^[132]. In a Li||LFP battery, the GPE achieved a discharge capacity of 146.5 mAh g^-1 and a capacity retention of 95% after 300 cycles at 0.2 C^[132].Besides PEO, PVdF, and PVdF-HFP, other polymers are also being investigated as matrices for GPEs, such as polyacrylonitrile (PAN)^[133] and poly(methyl methacrylate) (PMMA)^[134].

To further improve the ionic conductivity of GPEs, they have also been combined with inorganic solid electrolytes due to their many complementary features^{[92-98,131,135-139]}.Several GPEs, such as PEO with [Pyr₁₄][TFSI]+Li[TFSI]^[93] and PVdF-HFP with [EMIM][TFSI] + Li[TFSI]^[131], have been combined with ceramic-type solid electrolytes [e.g., Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) and Li₇La₃Zr₂O₁₂ (LLZO)]. The presence of the IL introduces several favorable effects: the IL enhances the wetting of the polymer-ceramic interphase, reducing interfacial resistances, decreases the crystallinity of PEO, and participates in the formation of a stable SEI that avoids the formation of lithium dendrites and the side-reaction between Ti⁴⁺ and lithium^[93]. Using these composite electrolytes, extremely long battery cycling was achieved. Specifically, a Li||LFP cell could sustain a specific capacity of 119 mAh g^-1 after 1,200 cycles at 0.5 C^[93], and a Li||LTO cell showed capacity retention of 91% after 1,000 cycles at 0.5 C, with an initial discharge capacity of 164 mAh g^-1[131].

Solvated ionic liquids and zwitterionic ionic liquids

Beyond traditional ionic liquids (ILs), other ionic materials have also been developed for the realization of task-specific liquid electrolytes, such as solvated ionic liquids (SILs) and zwitterionic ionic liquids (ZILs). SILs, extensively studied by Prof. Watanabe and coll^[48,140-143], are composed of a lithium salt and an oligomeric ether such as triglyme or tetraglyme, predominantly solvating the cation^[48,140]. Due to the strong Lewis basicity given by the lone pairs on the oxygen atoms, the ethers coordinate the lithium ion, forming the complex cations [Li(Glyme)]⁺, and dissociate the salt^[48,143,144]. This phenomenon reduces the T_m of the lithium salt, producing a molten salt with properties similar to traditional ILs. In an equimolar mixture of Li[TFSI] and triglyme or tetraglyme, only a negligible amount of free-glyme molecules are left^{[140,144,145]}, as shown in Figure 8^[145]. Thus, high thermal stabilities, negligible volatility, and non-flammability can be attained^[48,143,144]. Furthermore, they can possess a high t_Li⁺ due to their high concentration of Li⁺ and the presence of a reduced number of different charged species in comparison to binary IL-lithium salt mixtures, as well as a notable stability toward oxidation because of the scarce presence of free-glyme molecules and their HOMO energy level lowered by a donation of lone pairs^[144]. Due to this improvement of the oxidative stability, Yoshida et al. realized a Li||LCO battery capable of 200 stable cycles at 0.8 C using [Li(G4)][TFSI] or [Li(G3)][TFSI] as the electrolyte, with an initial discharge capacity of 130 mAh g^-1[47,146]. In the same study, it was suggested that the Li⁺-conduction provided from the [Li(glyme)]⁺ migration through the vehicular mechanisms is coupled with a ligand exchange mechanism induced by the electrochemical reactions on the surface of the electrodes. The Li⁺ released from the anode requires prompt solvation, and it strips a glyme from the nearest [Li(glyme)]⁺ complex, while on the cathode, a free glyme produced by the intercalation of Li⁺ immediately receives a Li⁺ from the nearest [Li(glyme)]^+[47,146].

Figure 8. Estimated percentages of free glyme (c_f) over total glyme amount (c_G) in equimolar molten mixtures [Li(glyme)₁]Y at 30 °C. Reproduced from Ref.^[145] with permission from the PCCP Owner Societies. Copyright (2015) The PCCP Owner Societies.

Unlike ILs, whose properties can be altered by inserting functional groups via covalent bonds, the properties of SILs are designed by tuning the physical bonds, which are influenced by the combination and composition of lithium salts and glyme. For the selection of lithium salts, their Lewis basicity is a critical factor and determines whether the resulting material will be a SIL or simply a highly concentrated salt solution in glyme^[147]. This is because the interaction of glyme-Li⁺ and counter anion-Li⁺ is competitive. Glyme forms superior SIL, behaving as typical ILs with lithium salts with a weak Lewis basicity.

ZILs are another promising material for batteries, pioneered by Prof. Ohno et al., and have organic ion structures that can be found in ILs, but the cation and the anion are tethered together by covalent bonds^{[39-41,148-151]}. Due to the presence of positive and negative charges in one molecule enhancing intermolecular electrostatic interaction, ZILs usually have a high T_m and are solid at room temperature^[41]. Similar to ILs, by choosing ion structures with charge delocalization, such as imidazolium cations and sulfonamide anions, as well as flexible alkyl side chains, it is possible to realize ZILs that are liquid at room temperature^[41].

Another interesting approach for liquefaction, which is specific to ZIL, is the addition of Li[TFSI]. Although Li[TFSI] is solid having T_m at 234-238 °C, specific combinations with ZIL, such as 1-(1-ethyl-3-imidazolio)butane-4-sulfonate and Li[TFSI] equimolar mixture, do not show crystallization behavior^[152]. This is because of the formation of an IL-like phase between imidazolium cation in ZIL and added TFSI anions. However, even carefully designed room temperature ZILs are more viscous compared to RTILs^[41]. As a consequence, it is difficult to realize ZIL-based electrolytes with satisfying ionic conductivity^[41], and it is common to couple them with OLEs. The main advantage of ZILs is that the molecules do not move according to the electric field because of their neutrality, increasing the ionic conductivity of Li^{+[39,40,148-151]}. Furthermore, due to their large dipole moment, they can easily dissolve lithium salts^[148], enhancing the Li⁺-transport^{[148,150,153]}. In addition, it is also reported that their presence is beneficial in terms of anti-freeze properties^[153], the formation of favorable SEI morphologies with reduced resistances^[151,154], and the ability to hinder the degradation of organic electrolytes^[148,153]. Guan et al. reported on the use of a liquid ZIL as the electrolyte, which is based on quaternary ammonium structures modified with ether oxygen and sulfonate anions, containing Li⁺ and ClO₄^-, with the addition of only 7 wt% of EC^[149]. The realized electrolyte achieved conductivity of 1 mS cm^-1 at 20 °C and allowed a Li||LFP cell to operate stably for 100 cycles, achieving a discharge capacity of 120 mAh g^-1. This was possible by forming a thin and stable SEI on lithium metal^[149].

Deep eutectic solvents

For large-scale applications, complicated synthesis and purification of ILs must be simplified^[30]. Furthermore, for these syntheses, a notable amount of organic solvents, such as acetonitrile, dichloromethane, and acetone, are consumed^[30]. Thus, the implementation of ILs on the industrial scale is hindered by these aspects. Recently, deep eutectic solvents (DESs), which possess similar properties to ILs, and a facile synthesis method have been proposed as alternatives^[35,49].

In general, DESs are defined as a mixture of an HBD and an HBA that, due to the charge delocalization caused by the hydrogen bonds formed between them, results in a T_m far below that of the employed components^[35,49]. Some of the HBDs used for the realization of DESs are amides, such as urea, N-methylacetamide (NMA), 2,2,2-trifluoroacetamide (TFA), methanesulfonamide (MSA), and N-dimethyl MSA (DMMSA), while the most famous HBA is choline chloride ([Ch]Cl), which is not RTIL but can be regarded as ILs in a broader sense. The mixture between urea and [Ch]Cl in a molar ratio 1:2 (called “reline”) is the most famous DES, which has been extensively studied over the years for applications such as electrodeposition of metals^[35,155-157], CO₂ capture^[35,158], redox-flow battery testing^[35,159], biocatalysis^[35,160], biomass pretreatment^[35,161], and synthesis of nanoparticles^[35,162-166]. However, this type of DES is not adequate for LMB applications because of its high viscosity^[35,167] and its low cathodic stability due to the presence of N-H groups in the HBDs. As in the case of ILs, the structure of DESs can also be modulated^[49] (the estimated number of possible structures of DESs composed from two materials is between 10⁶ and 10^8[35]). This enables the formation of DESs displaying high ionic conductivity and exerting control on the EEIs.

A type of DESs studied for applications in lithium batteries as the electrolyte is the mixture of an amide and Li[TFSI] or LiPF₆^[50,168-170]. In this case, the driving forces for lowering the T_m are both the interaction between the N-H group and the anion and the interaction between the Li⁺ and the lone pair on the C=O group^[50]. These two interactions weaken the hydrogen bonding among the amide molecules, lowering its melting point and producing a DES with (usually) a high t_Li⁺ due to the high content of lithium. While the first HBD used for the realization of DES-based electrolytes is acetamide^[170], its use has quickly become obsolete due to the instability of N-H bonds with lithium metal anodes. As a result, NMA is the main choice as the HBD to realize DES-based electrolytes for LMBs^[171-176].

Ogawa and Mori recently suggested that the interaction between Li⁺ and C=O is more intense and influential than that between the N-H group and the anion since the mixture between tetramethylurea and Li[TFSI] can also produce a DES, even though the N-H group is absent in tetramethylurea^[50]. This result is critical for future DES development because the presence of the N-H group generally causes low cathodic stability. Recent results on the use of DESs in electrolytes are reported in Supplementary Table 3.

Wu et al. produced a mixture of Li[TFSI]:Li[DFOB]:succinonitrile (SN) (0.8:0.2:10 in mol), in which the CN groups of SN are the HBD^[177]. In this case, SN has a high polarity and can dissociate the lithium salts^[177]. The mixture was found to have a t_Li⁺ of 0.75, higher than the typical range for common OLEs, which is usually 0.3-0.4^[177,178]. The use of Li[DFOB] is advantageous in forming a stable and efficient SEI on the lithium surface, preventing the reduction of SN, and creating a protective CEI on the cathode surface. A Li||LCO cell with the DES showed a specific discharge capacity of 110 mAh g^-1 after 700 cycles using this electrolyte^[177]. A similar DES but with a reduced amount of SN [Li[TFSI]:Li[DFOB]:SN (0.17:0.03:0.8 in mol)] was studied by Hu et al.^[179]. Their electrolyte had a t_Li⁺ of 0.44 and did not show any crystallization behavior, even at sub-zero temperatures^[179]. Furthermore, a Li||LCO cell employing this electrolyte achieved capacity retention of more than 70% after 500 cycles at 1 C^[179]. This result is related (besides the formation of optimized EEIs) to the reduced amount of free SN, improving its stability against lithium metal^[179].

Similarly, Liang et al. demonstrated that Li[TFSI]:NMA (1:4 in mol) is a prospective DES, and by adding 2 wt% LiNO₃ to the DES, it is possible to realize a uniform SEI based on LiF, Li_xN, and Li_xNO_y species on the surface of the metal anode^[171]. The presence of these high Li⁺-conducting species leads to a more uniform Li deposition with a compact morphology^[171]. Furthermore, it was demonstrated in a Li||LiNi_0.8Co_0.15Al_0.05O₂ cell, the presence of NO₃^- contributes to the formation of a thin and compact protective and passivating CEI on the surface of cathode materials, reducing their reactivity with the electrolyte^[171]. The combination of these properties achieved a Li||NMC622 battery with a capacity of 84% after 600 cycles and a Li||NMC811 cell with a capacity retention of 74% after 600 cycles [Figure 9]^[171].

Figure 9. (A) Cycling performance of NCM622||Li cells with DES-0 (without LiNO₃ addition), DES-2 (with a 2 wt% LiNO₃ addition), and commercial electrolytes at 25 °C and 0.5 C. The inserted images present the surface of separators with the electrolytes DES-0 (left) and DES-2 (right) after 100 cycles. (B) Rate performance of NCM622||Li cells with DES-2 and commercial electrolytes at 25 °C. (C) Cycling performance of NCM811||Li cells with different electrolytes at 25 °C and 0.5 C. (D) Voltage profile of NCM811||Li cells. This figure is reprinted (adapted) with permission from Liang et al.^[171]. Copyright (2022) Wiley-VCH GmbH.

Conversely, Li et al. used the DES composed of Li[TFSI]:Urea (1:3 in mol) as an additive to a solution of 1 M LiPF₆ in EC/EMC/DMC (1:1:1 v/v/v) to suppress the dendrite-growth in a Li||LFP cell^[169]. The main advantage of this approach is that decomposition products of urea at low potentials generate a stable and uniform SEI rich in Li₃N, which provides high Li⁺-conduction^[169], enabling the Li||LFP cell showing a discharge capacity of 149 mAh g^-1 and a capacity retention of 92.1% after 1,000 cycles^[169]. Recently, Mezzomo et al. reported notable stability of performances of a Li||LFP cell^[168]. They achieved this by using an electrolyte mixture of LiPF₆:TFA (1:4 in mol) with the addition of EC and FEC to form a passivating SEI to prevent the reduction of TFA and to suppress the growth of lithium dendrites. Nonetheless, they also report on the incompatibility of this DES with NMC111 due to cathode dissolution^[168].

In the use of DESs as additives for OLEs, the hydrogen bonds maintaining the structure of DES can be destroyed by the addition of other solvents. In the case of DESs based on [Ch]Cl and urea, the maximum concentration of water in which the structure of DESs is maintained is reported to be ca. 42 wt% H₂O. Above this concentration, the solution is just an aqueous solution containing [Ch]Cl and urea separately^[180].

Similarly to GPEs and ionogels, the inclusion of DESs in polymers is possible, which are known as “eutectogels”. Joos et al. added a Li[TFSI]:NMA (1:4 in mol) to a polymer based on AcMo, and EGDMA^[172]. The eutectogel allowed a Li||LFP cell to provide a specific discharge capacity of 105 mAh g^-1 after 250 cycles. Due to hydrogen bonds formed between NMA and AcMo, the amount of free-NMA is reduced, preventing its decomposition, and the contribution of TFSI to form a more stable SEI is increased. A similar beneficial interaction has been reported by Wang et al. between NML and the oxygen in the chains of PEGDA^[181]. The DES Li[TFSI]:NML (1:6 in mol) was added to a polymer based on PEGDA and UPyMA^[181]. This composition improved both the mechanical strength and the cathodic stability of the electrolyte^[181]. This electrolyte has exhibited a t_Li⁺ of 0.66 and allows a Li||LCO cell to sustain a capacity retention of 80% after 1,000 cycles, with an initial discharge capacity of 189 mAh g^-1[181].

Furthermore, Wang et al. reported the use of an eutectogel based on Li[TFSI]:NMA (1:4 in mol) in a PEO matrix, which also contains metal-organic-framework (MOF) UiO66-NH₂^[175]. The reactivity of NMA with lithium metal was reduced due to the interaction between the NH₂ group of the MOF and the C=O group of NMA. As a result, the SEI formed in a Li||LFP cell was rich in LiF and Li₃N and allowed to retain 93.3% of the initial discharge capacity (159 mAh g^-1) after 350 cycles at 0.2 C, along with achieving a stable stripping-deposition for over 3,000 h in a Li||Li symmetric battery^[175].

Jaumeaux et al. added a Li[TFSI]:NMA (1:4 in mol) + 10 wt% FEC mixture to UPyMA and PETEA, obtaining a polymer electrolyte with ionic conductivity of 1.79 mS cm^-1 and t_Li⁺ 0.76^[173]. Using this electrolyte, a Li||LMO cell was capable of maintaining a capacity retention of 86.1% after 200 cycles at 0.1 C. This excellent result is due to the addition of FEC, which provides the formation of stable, uniform, thin, and LiF-rich SEI and CEI. The former avoids the formation of lithium dendrites, while the latter avoids the dissolution of Mn²⁺ into the electrolyte [Figure 10]^[173].

Figure 10. (A) DES and (B) DES-based self-healing polymer electrolytes in Li||LMO cells. This figure is quoted with permission from Jaumeaux et al.^[173]. Copyright (2020) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Toward the development of economical sustainable materials, Zhang et al. added Li[TFSI]:Li[DFOB]:SN (0.8:0.02:1 wt/wt/wt) (a DES similar to the one used in^[177]) in cyanoethyl cellulose obtaining an electrolyte with t_Li⁺ 0.64 to attain a Li||LCO cell with a specific capacity of 165 mAh g^-1 and an 85% capacity retention after 200 cycles at 1C^[182]. These performances are related to the presence of the DFOB anion, producing a LiF and Li₃N-rich SEI on the lithium surface, which avoided the decomposition of SN and promotes a uniform plating-stripping of Li⁺, along with a stable and thin CEI on the LCO surface, which prevented the Co²⁺ dissolution and subsequent erosion of the cathode (as schematically displayed in [Figure 11]^[182]).

Figure 11. Schematic illustration of the multifunctional mechanism of eutectogel electrolytes in a 4.45 V Li||LCO battery. This figure is reprinted (adapted) with permission from Zhang et al.^[182]. Copyright (2022) The Authors. Carbon Energy published by Wenzhou University and John Wiley & Sons Australia, Ltd.