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. 2024 Nov 19;146(48):33169–33178. doi: 10.1021/jacs.4c12616

Poly(Ionic Liquid) Electrolytes at an Extreme Salt Concentration for Solid-State Batteries

Shinji Kondou 1,2,3,4,*, Mohanad Abdullah 5, Ivan Popov 6, Murillo L Martins 5, Luke A O’Dell 1,2, Hiroyuki Ueda 1,2, Faezeh Makhlooghiazad 1,2, Azusa Nakanishi 1, Taku Sudoh 4, Kazuhide Ueno 4,7, Masayoshi Watanabe 7, Patrick Howlett 1,2, Heng Zhang 8, Michel Armand 9, Alexei P Sokolov 5,10, Maria Forsyth 1,2,*, Fangfang Chen 1,*
PMCID: PMC11636621  PMID: 39558642

Abstract

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Polymer-in-salt electrolytes were introduced three decades ago as an innovative solution to the challenge of low Li-ion conductivity in solvent-free solid polymer electrolytes. Despite significant progress, the approach still faces considerable challenges, ranging from a fundamental understanding to the development of suitable polymers and salts. A critical issue is maintaining both the stability and high conductivity of molten salts within a polymer matrix, which has constrained their further exploration. This research offers a promising solution by integrating cationic poly(ionic liquids) (polyIL) with a crystallization-resistive salt consisting of asymmetric anions. A stable polymer-in-salt electrolyte with an exceptionally high Li-salt content of up to 90 mol % was achieved, providing a valuable opportunity for the in-depth understanding of these electrolytes at an extremely high salt concentration. This work explicates how increased salt concentration affects coordination structures, glass transitions, ionic conductivity, and the decoupling and coupling of ion transport from structural dynamics in a polymer electrolyte, ultimately enhancing electrolyte performance. These findings provide significant knowledge advancement in the field, guiding the future design of polymer-in-salt electrolytes.

Introduction

Solvent-free solid polymer electrolytes (SPEs) are emerging as solid-state ionic conductors for electrochemical devices, offering enhanced safety and cycle performance in lithium (Li) batteries, by replacing volatile organic solvents with polymers.1 While this concept has been acknowledged for several decades,2,3 its widespread implementation remains hampered by the low cation conductivity. Traditional SPEs such as poly(ethylene oxide) (PEO) exemplify a coupled ion transport system, where the ion species, particularly Li ions, migrate through the polymer structural (segmental) dynamics. This coupled transport, due to strong ion-dipole interactions between the Li ion and polymer, results in a lower Li-ion transference number (<0.5) and brings significant internal polarization and premature cell failure for the corresponding Li batteries.4

Polymer-in-salt electrolytes (PISEs) with a high Li-salt concentration (>50 mol %) are a promising approach to overcome this challenge.5−7 Angell et al. first proposed the concept that a low-mole-percentage polymer can be added into a highly conducting salt solution to form a rubbery and highly conductive glassy electrolyte.8−11 The salt is the dominant component in this system, enabling the partial decoupling of ion conduction from polymer dynamics. However, the Li salts and polymer initially chosen by Angell are not suitable for batteries. Subsequent research by him and others has been devoted to exploring other polymers and salts.

Numerous neutral polymer materials (such as polyacrylonitrile (PAN), poly(ethylene carbonate) (PEC), and poly(ethylene oxide) (PEO)) have been applied to PISEs, demonstrating both relatively high ionic conductivity (>10–6 S cm–1 at 30 °C) and Li-ion transference number (>0.5).12−15 In parallel, the combination of ionic liquids and polymers as PISEs was also demonstrated to realize the decoupled ionic motion.16−19 However, the correlation between the complex coordination structures and coupling–decoupling of ion transport and the effect of salt concentration in PISEs remains unclear. Recently, cationic poly(ionic liquid)s (polyIL) have been utilized in the field of PISEs, termed polyIL-in-salt electrolytes.20−23 In such systems, the strong ion–ion interaction between the anion and polycation does not directly restrict the Li-ion motion. Our previous work suggests that a dominant coordination structure among the metal ion, anion, and polycation formed at high salt concentrations can facilitate metal-ion transport in these electrolytes.21 Wang et al. demonstrated that the polydiallyldimethylammonium bis(fluorosulfonyl) imide (PDADMAFSI) system containing lithium bis(fluorosulfonyl) imide (LiFSI) exhibits an ionic conductivity of 7.0 × 10–5 S cm–1 and a Li-ion transference number of 0.56 at 80 °C with a moderately high salt content (i.e., [polycation]:[Li+] = 1:1.5, by mole).20 A higher Li-salt ratio exceeding 1:1.5 results in the formation of an anion–Li+ aggregation regime where some anions coordinate only with Li ions, potentially leading to further enhanced Li-ion diffusion, as predicted by molecular dynamics (MD) simulations. However, this regime, being thermodynamically metastable, undergoes crystallization within a few days, impeding experimental validation. Nevertheless, this work points us in a new direction to study the ion decoupling motion in polymer-in-salt systems, and it also motivates us to further push the boundaries of Li-salt content beyond previous thresholds and delve deeper into polyILs with stable molten-salt states.

In this study, by incorporating a glass-forming salt, lithium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (LiFTFSI), into PDADMAFSI (Figure 1a), we successfully created a stable room temperature PISE with an extremely high salt concentration (i.e., [polycation]: [Li+] = 1:8, by mole; 90 mol % of Li salt). This offers an opportunity to investigate a polyIL-in-salt electrolyte in an extreme salt concentration range, as well as the effect of the use of mixed anions in this case. LiFTFSI is shown to be a good ionic conductor (>10–4 S cm–1 at 100 °C24,25) with a crystallization-resistant feature, helping maintain a glassy state in this polyIL-in-salt system. As the salt concentration increases, ion motion shifts from being decoupled (at a 1:2 ratio) to coupled (at a 1:8 ratio) with the structural dynamics, therefore affecting the glass transition temperature (Tg). Promisingly, the overall ionic conductivity remains high at such a superhigh salt concentration, with an exceptionally high Li-ion transference number of 0.8. Furthermore, we show that the conductivity can be further optimized by changing the ratio of two anions while maintaining 90 mol % Li, due to the reduced Tg, further improving ionic conductivity to 9.0 × 10–5 S cm–1 and Li transference number to 0.81 at 80 °C. The extreme polyIL-in-salt system shows the benefit of enhancing the electrolyte performance, including high oxidation stability, remarkable stability in Li deposition/dissolution cycling at a current density of up to 0.5 mA cm–2, and highly reversible charge–discharge cycling in prototype solid-state Li cells. Our finding provides critical insight into the interplay between salt concentration, coordination environments, and ion transport mechanisms in polyIL-in-salt systems across an unexplored middle-high to extremely high salt concentration range, with demonstrated enhanced electrolyte performance. This new understanding fills an important knowledge gap in the polymer-in-salt field, contributing to the design of highly metal-ion-conductive solid-state polymer electrolytes.

Figure 1.

Figure 1

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Thermal properties of the polyIL-in-salt systems. (a) Chemical structures of poly(diallyldimethylammonium) bis(fluorosulfonyl)imide (PDADMAFSI) and lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (LiFTFSI). (b) Photographs of polymer electrolytes at the polyIL unit/LiFTFSI cation molar ratios of 2:1, 1:2, and 1:8. (c) DSC thermograms for polymer electrolytes with different LiFTFSI concentrations. (d) Relationship between Tg and the mole fraction of Li salt in both mixed anion system and single anion systems with either a FSI (up to 1:2) or a FTFSI anion (up to 1:8). (e) DSC thermograms of mixed anions and single FTFSI anion systems at a molar ratio of 1:8 after 5 days and 2 months.

Results and Discussion

Thermal Properties

The mixtures of PDADMAFSI and LiFTFSI can form self-standing membranes at the polyIL cation/Li-ion molar ratio of 2:1, while the mechanical strength diminishes upon the increase of the salt content, ultimately transforming into a rubbery state at a 1:8 ratio (Figure 1b), as expected for polymer-in-salt systems. It is noted that PDADMAFSI is a rigid polymer, with its glass transition temperature (Tg) around 120 °C.26 LiFTFSI is a low crystallinity salt, with a melting point (Tm) of 106 °C and a glass transition temperature (Tg) of −8.0 °C, as seen in differential scanning calorimetry (DSC) thermograms (Figure S1).27 Cleary, the asymmetric FTFSI shows an advantage in suppressing crystallinity compared to FSI. Only Tg was observed for LiFTFSI salt after the second heating, whereas crystallization (Tc) and Tm were observed for LiFSI salt even after the second cycles.

DSC analysis in Figure 1c shows that all PDADMAFSI/LiFTFSI mixtures can remain completely amorphous, with a single Tg value. The Tg decreases monotonically with increasing Li-salt ratios from 2:1 to 1:2, reaching its lowest value of −30.2 °C due to the plasticizing effect of Li salt, i.e., the incorporation of Li salt weakens polycation–anion interactions via Li-ion-mediated cocoordination (polycation–anion–Li+), enhancing polymer chain dynamics.21 In addition, the low crystallinity of LiFTFSI can allow for higher Li-salt incorporation without inducing crystallization. As Li-salt content exceeds 1:2, the Tg of the system starts to increase gradually to approach that of pure LiFTFSI at −8.0 °C. Remarkably, the superhigh Li-salt composition, with a PDADMAFSI/LiFTFSI mole ratio of 1:8 (mLi= 89 mol %), exhibits no crystalline phase and maintains its molten-salt state even after 2 months at 25 °C (Figure 1e and Figure S1c). This is in stark contrast to the single FTFSI anion system, signifying the role of mixed anions in further enhancing the mixing entropy and, thus, improving the stability of molten salts. The overall relationship between Tg and the mole fraction of Li salt with mixed anions exhibits parabolic behavior (Figure 1d). In contrast, the single anion systems exhibit a more gradual reduction in Tg, either with the FSI anion (up to 1:2) or with the FTFSI anion (up to 1:8), and in both cases, Tg is higher than that in the mixed anion system. The plasticizing effect on the polymer is insufficient to explain the parabolic behavior of Tg in the mixed anion system, making it essential to consider the ion–ion coordination structure at extreme salt ratios.

It should be noted that these polymer-in-salt electrolytes are considered to be in a thermodynamically metastable state. Therefore, sample preparation conditions and measurement intervals in subsequent experiments were kept consistent to ensure the reproducibility of each result.

Ion Coordination Environment with MD Simulations

We analyzed the ion coordination changes in PDADMAFSI/LiFTFSI electrolytes at 1:2 and 1:8 ratios using molecular dynamics (MD) simulation at 80 °C. Figure 2a shows the distribution of Li ions in the polymer matrix. At a 1:2 ratio, PDADMA chains and Li ions are homogeneously distributed throughout the simulation box, whereas at a 1:8 ratio, the lower polymer proportion suggests that salt-rich domains predominantly govern the structure. Three distinct anion coordination states in Figure 2b, denoted in previous research as Type I (polycation–anion–polycation), Type II (polycation–anion–Li+ cocoordination), and Type III (anion–Li+ aggregates),20,21,28 are quantified to analyze the ion–ion coordination environment (see Supplementary Note 1). Only Type II and Type III were identified in 1:2 and 1:8 systems, as Type I is normally associated with the neat polyIL or occurs at low salt concentrations. At a 1:2 ratio, around 95% of anions are Type II, nearing the maximum anionic cocoordination state, and only 5% are Type III. Increasing Li-salt content increases Type III coordination. At a 1:8 ratio, Type II decreases to 59.4%, and Type III increases to 40.6%, signifying a prominent anion–Li+ aggregation within the system. In this case, the Tg of the system begins to be influenced by the growth of the anion–Li+-aggregated component, and it gradually increases and approaches that of pure LiFTFSI (Figure 1d).

Figure 2.

Figure 2

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(a) MD snapshots showing the structures of two polyIL-in-salt systems at two salt concentrations and the coordination environment of a selected polymer segment at 1:2 and 1:8 systems. The simulation box snapshots show the distribution of Li ions (purple balls) and PDADMA chains (green sticks). The polycation chain segment snapshots show the surrounded cocoordinated anions and Li. FSI is in yellow bold sticks; FTFSI is in blue sticks. (b) Schematic diagram showing the three distinct anion coordination states with increasing salt concentrations.

The detailed mixed anion coordination environments, distinguishing between FSI and FTFSI anions, were analyzed from their coordination numbers (CNs). At a 1:2 ratio, the CNs for FSI and FTFSI with PDADMA are 2.25 and 6.35 (around 1:3), respectively (Figure S3c and Table S2). They are 2.96 for FTFSI and 1.84 for FSI with Li. The ratio of the two anions in the first solvation shell of the cation is not fixed at 1:2. Clearly, there is more FTFSI in the PDADMA’s solvation shell, whereas FSI shows an advantage in competing with FTFSI for Li coordination. The mixed FTFSI and FSI coordination mitigates crystallization and further lowers the Tg at a 1:2 ratio compared to the systems with either single FSI20 or FTFSI anions (Figure 1d). At a 1:8 ratio, CNs are 9.3 for FTFSI and 0.46 for FSI with PDADMA and 4.46 for FTFSI and 0.6 for FSI with Li (Figure S3a and Table S2), showing that the FTFSI becomes predominant in both cation’s first solvation shell, due to its high number. The change in the coordination environment is shown in the snapshots of a selected polycation segment and its coordination environment in Figure 2a.

RDF analysis between Li+ and the anions reveals the three distinct peaks between 2 and 6 Å.29,30 The nearest peak at ∼2.5 Å is assigned to coordination between Li+ and negatively charged nitrogen atoms of anions. Larger peaks at 3.9 and 4.5 Å correspond to bidentate and monodentate coordination with oxygen atoms, respectively (Figure S3a). Monodentate coordination increases due to multiple Li coordinating with each anion at both 1:2 and 1:8 ratios, akin to behavior in highly concentrated ionic liquid electrolytes.31 It has been reported that the FTFSI anion plays a crucial role in inhibiting crystallization in highly concentrated liquid electrolytes.32,33 Reber et al.34 explained supercooling behavior in water-in-salt electrolytes with the FTFSI anion in MD simulation, noting that the monodentate coordination restricts N–SO2F bond rotation due to the preferential coordination of the Li ion with its S=O site, while the rotation of the N–SO2CF3 bond is relatively unrestricted. This leads to a greater rotational mobility of the N–SO2CF3 bond, resulting in a lower likelihood of crystallization. Similar behavior could occur in the polyIL-in-salt system, which helps hinder crystallization. We observe that the monodentate LiFTFSI coordination is more prevalent through the SO2F side than through the SO2CF3 side, as indicated by the Li–S RDFs calculated with the two S sites in Figure S3b,f. Further density function theory (DFT) calculations of CHelpG charges show that the F on the SO2F side is twice as negative as that on the SO2CF3 side (Figure S2), suggesting more delocalized negative charges on the SO2CF3 side, causing its weaker interaction with Li compared to the SO2F side. The conformational energy of different monodentate-coordinated LiFTFSI ion pairs also indicates more stable coordination geometries through the SO2F side.

Ion Transport Properties

We explored the impact of elevated Li-salt concentrations on the ion transport for battery applications. Figure 3a presents the ionic conductivity of PDADMAFSI/LiFTFSI electrolytes at various polyIL unit/LiFTFSI cation molar ratios at 30, 50, and 80 °C. At 80 °C, ionic conductivity increases with Li-salt content from 2:1 to 1:2, peaking at 1.1 × 10–4 S cm–1 for the 1:2 ratio, which is higher than the highest conductivity obtained for this polyIL with the LiFSI salt (7.0 × 10–5 S cm–1). This increase corresponds to a decrease in Tg values (Figure 1d). Further increases in Li-salt content decrease the ionic conductivity to 4.7 × 10–5 S cm–1 at a 1:8 ratio, with a slight increase in Tg. It is noteworthy that the decrease in the conductivity from 1:4 to 1:8 is minimal. At 30 and 50 °C, conductivity even slightly increases from 1:4 to 1:6, indicating that changes in ionic conductivity and Tg behavior are not fully aligned. This implies that a shift in dominant structural dynamics from polymer segmental dynamics to structural dynamics of the anion–Li+ aggregation domain affects the ionic conductivity, which will be discussed below.

Figure 3.

Figure 3

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Ion transport properties of the polyIL-in-salt systems. (a) Ionic conductivities for different polyIL unit/LiFTFSI ratios at 30, 50, and 80 °C. (b) Current–time curve at 10 mV polarization and estimated Li-ion transference numbers for different polyIL unit/LiFTFSI ratios of 1:2, 1:4, and 1:8.

Furthermore, the Li-ion transference number (tLi+), a crucial transport parameter for fast charge–discharge performance, was measured under anion blocking condition using a potentiostatic polarization method in a Li°||Li° symmetrical cell (see Supplementary Note 2).4,35,36Figure 3b shows the current–time curve at 10 mV polarization for different ratios, with impedance spectra and an equivalent circuit for estimating tLi+ in Figure S4. At a 1:2 ratio, tLi+ is 0.57, which is comparable to polyIL-in-salt systems using LiFSI.20 As the Li-salt content increases, the polarization current decay diminishes, mitigating concentration gradient formation in the cell. Notably, tLi+ increases continuously from a 1:2 to 1:8 ratio, achieving a maximum of 0.8 at a 1:8 ratio. Therefore, the benefit of suppressing crystallization at high salt concentrations is not only to prevent the conductivity decrease associated with crystallization but also to enhance tLi+. This indicates superior ion transport properties compared to those reported in recent studies on polymer-in-salt electrolytes (Table S3 and Figure S5).

Coupling/Decoupling Ion Transport from Structural Dynamics

This section delves into the ion transport mechanisms within the polyIL-in-salt system, examining how to achieve a superior ionic conductivity. Pulsed-field gradient (PFG) NMR was used to measure the self-diffusion coefficients of Li+ ions (DLi+) and FTFSI anions (DFTFSI) at 80 °C as shown in Figure 4a. Unfortunately, the self-diffusion of the FSI anion could not be measured due to its short 19F transverse relaxation time (T2). DLi and DFTFSI increase as the Li-salt content shifts from a 1:1 to 1:2 ratio, with DFTFSI at a 1:1 ratio being below the measurement limit. The increase in D is consistent with the increase in ion conductivity in Figure 3a. Increasing the Li salt to a 1:4 ratio lowers the diffusion, aligning with a decreased ionic conductivity and increased Tg. Interestingly, from a 1:4 to 1:8 ratio, diffusion does not decrease, and the ratio of DLi to DFTFSI increases, despite an increase in Tg and a decrease in conductivity. This could suggest an increase in ion correlation at a 1:8 ratio, which will be discussed next.

Figure 4.

Figure 4

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Analysis of ion transport behavior in the polyIL-in-salt systems. (a) Self-diffusion coefficients of different polyIL unit/LiFTFSI systems at 80 °C. (b) Temperature dependencies of structural relaxation time and conductivity relaxation time of different polyIL unit/LiFTFSI systems.

MD simulations at 1:2 and 1:8 ratios and 80 °C provide further ion dynamics information (Figure S6). It is important to note that this analysis is not intended to accurately reproduce NMR diffusivity results, which are overestimated in this work, due to limitations in classical MD, including the use of a nonpolarizable force field, a simplified charge scaling method for polarization, and the significantly shorter polymer chains in the simulation compared to those in experiments. Nonetheless, this analysis still provides supportive and complementary information about relative ion diffusion measured by experiments. For example, mean square displacement (MSD) shows that Li+ ions move faster than other species at both ratios. As the ratio goes from 1:2 to 1:8, the displacement of all species decreases, but the relative motion ratio of the Li ion to the FTFSI anion increases, which supports the NMR results. This enhanced relative motion of Li ions, with an extremely high Li-salt concentration, boosts the Li-ion transference number (Figure 3b). Furthermore, MD simulations also suggest that the diffusion of FSI is higher than that of FTFSI, suggesting the possibility of enhancing ion conductivity by increasing the FSI component, which will be discussed in the next section.

We analyzed the conductivity relaxation time (τσ) and the structural relaxation time (τs) to further understand how ion transport couples or decouples from structural dynamics within the system across Li-salt concentrations. The temperature dependencies of τσ and τs were evaluated using broadband dielectric spectroscopy (Figure S7) and rheology (small amplitude oscillatory shear (SAOS) experiments, Figure S8), respectively. A detailed analysis is presented in Supplementary Note 3. These relaxation times follow a typical Vogel–Fulcher–Tamman (VFT)-type temperature dependence (Figure 4b), as observed in glass-forming materials.37,38 It is evident that τσ is consistently shorter than τs at 1:1 and 1:2 ratios in the measured temperature range, signifying a decoupling of ion transport from structural dynamics. Conversely, at a 1:8 ratio, τσ and τs are comparable, indicating a coupling of ion motion to structural dynamics. Based on these results, the structure at a 1:2 ratio facilitates the decoupling of Li-ion transport from structural relaxation essentially controlled by polymer segmental dynamics. In contrast, at a 1:8 ratio, the structural relaxation is dominated by the salt dynamics leading to the Li-ion transport coupling to the structural dynamics.

Enhanced Electrochemical Performance via Mixed Anion Ratio Modulation

Since MD simulation suggests a faster FSI diffusion compared to FTFSI, it is reasonable to speculate that the Li+ diffusion could be enhanced through increasing the FSI component. To prove this, an MD simulation was conducted on a 1:4:4 PDADMAFSI/LiFSI/LiFTFSI system at 80 °C, which still maintains a high 1:8 salt concentration but changes the ratio of FSI to FTFSI from 1:8 to 5:4. MSD results in Figure 5a clearly show the enhancement of ion diffusion for all species in the 5FSI:4FTFSI compared to the 1FSI:8FTFSI. A systematic experimental study was followed by adjusting the FTFSI/FSI anion ratio while keeping the polyIL/Li+ ratio constant at 1:8. Figure 5b and Figure S9 illustrate the Tg, ionic conductivity, and DSC thermograms of mixed LiFTFSI and LiFSI systems, combined with or without PDADMAFSI. The Tg of mixed Li salts without polycation correlates with that of the polyIL-in-salt system. In particular, the Tg of mixed salts gradually decreases as the LiFSI fraction increases, bottoming out at a LiFTFSI/LiFSI ratio of 1:1. An MD snapshot of the 1:4:4 system suggests that the FTFSI and FSI anions are coordinated with the polycation and Li in a mixed manner and are homogeneously distributed (Figure S10), mitigating crystallization and further lowering the Tg. The experimental ionic conductivity of the polyIL-in-salt system increases as the Tg decreases, reaching a maximum of 9.0 × 10–5 S cm–1 at 80 °C (Figure 5b). This result is consistent with the increased diffusion coefficients of ionic species measured by MSD and PFG-NMR (Figure 5a,c). The tLi remains high at 0.81 in the mixed system, which also exhibits significantly reduced interfacial resistance in the Li symmetrical cell (Figure 5d), likely due to a less resistive solid–electrolyte interface (SEI) originating from FSI decomposition.39,40

Figure 5.

Figure 5

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Mixed anion effect on ion transport and electrochemical properties. (a) MSD profiles obtained from MD simulations for Li+, FSI–, FTFSI–, and PDADMA+ at a PDADMAFSI/LiFTFSI ratio of 1:8 (red lines) and a PDADMAFSI/LiFTFSI/LiFSI ratio of 1:4:4 (blue lines) at 80 °C. (b) Tg and ionic conductivity measured as a function of the FSI mole fraction in mixed LiFTFSI/LiFSI systems with PDADMAFSI at polyIL unit:Li+ of a 1:8 ratio. (c) Experimental self-diffusion coefficients of FTFSI– and Li+ and ionic conductivity at 80 °C. (d) Nyquist plots after the polarization for PDADMAFSI/LiFTFSI at a 1:8 ratio and for PDADMAFSI/LiFTFSI/LiFSI at a 1:4:4 ratio at 80 °C.

To explore the impact of extremely high Li-salt concentrations in polyIL-in-salt systems on electrochemical properties for Li-metal batteries, we measured the galvanostatic cycling of a symmetrical Li°||Li° cell. This is a useful method to evaluate not only the electrochemical stability on the Li metal but also the bulk ion transport properties, unaffected by composite electrode fabrication. Cycling performance at current densities from 0.1 to 0.5 mA cm–2 for every 10 cycles at 80 °C, with a fixed Li deposition/dissolution of 1.0 mAh cm–2, revealed that PDADMAFSI/LiFTFSI at a 1:2 ratio exhibits an arcing behavior during Li deposition/dissolution at low current densities (Figure 6a). This behavior became more pronounced at a higher current density, which is attributed to the Li-ion mass transport limitation within the cell.41−43 At a 1:8 ratio, the slope of the voltage profile is more gradual with less polarization, although the conductivity decreases compared to the 1:2 system. This can be attributed to the extremely high Li-salt concentration and a high tLi of 0.80 at a 1:8 ratio, mitigating the concentration gradient in the cell. The optimized mixed PDADMAFSI/LiFTFSI/LiFSI at a 1:4:4 ratio, with a higher conductivity and lower interfacial resistance, shows a lower polarization and stable Li deposition/dissolution profile up to 0.5 mA cm–2 without a short-circuit. The CV measurement shows enhanced oxidative stability up to ∼4.7 V with an increased Li-salt content (Figure 6b). This leads to highly reversible charge–discharge cycling in the LiFePO4 solid-state cell over 200 cycles with an average coulombic efficiency of >99.9% (Figure 6c). Thus, the overall improved electrolyte performance was demonstrated by pushing up the salt limit and using the mixed anions in a dry cationic polyIL/salt electrolyte.

Figure 6.

Figure 6

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Electrochemical performance of the polyIL-in-salt systems at 80 °C. (a) Rate performance of Li deposition/dissolution cycling in the symmetrical Li cell with PDADMAFSI/LiFTFSI at ratios of 1:2 and 1:8, and PDADMAFSI/LiFTFSI/LiFSI at a 1:4:4 ratio. The enlarged view is presented in Figure S11. (b) Linear sweep voltammograms on a Pt electrode in PDADMAFSI/LiFTFSI at ratios of 1:2 and 1:8. (c) Discharge capacities and coulombic efficiencies of the Li/LiFePO4 cell using PDADMAFSI/LiFTFSI at a ratio of 1:8. Inset: charge–discharge curves of the cell.

Outlook

Although this study has demonstrated the enhanced electrolyte performance by increasing the salt concentration limit, further improvement of the electrolyte performance for practical performance is still necessary. Here, we open a discussion on factors that should be considered for future design and optimization of polymer-in-salt electrolytes (PISEs). Using Li salts with low salt crystallinity is one key factor since high salt concentrations tend to form crystalline phases that reduce electrolyte conductivity. To suppress salt crystallization in polymer/salts melt electrolytes, increasing anion asymmetry is an effective way together with the use of mixed salts, which further lower the Tg of the electrolytes and enhance ionic conductivity.

Interestingly, in the polyIL-in-salt electrolyte, we found that the decoupled ion transport from structural relaxation in the medium-high salt concentration range (e.g., 1:2 here) changes to coupled ion transport at ultrahigh salt concentrations. Nevertheless, the electrolyte performance has improved. Therefore, it is not always necessary to pursue highly decoupled ion transport during electrolyte design. In addition, at extreme salt concentrations, the properties of Li salts become dominant factors in the overall electrolyte properties. We can see that both the Tg and conductivity approach those of the Li salts. Therefore, the excellent electrolyte properties of Li salts are crucial, such that the high conductivity of the Li salts should be the basic prerequisite.

On the other hand, the polymer should not significantly reduce the ionic conductivity of the PISEs. We believe that it is necessary to delve in-depth into the role of the polymer. According to our preliminary comparison of two ionic polymer matrices (polycation vs polyanion) at high salt concentrations, the nature of the polymer does show different effects on the physicochemical properties of electrolytes, and the polycationic electrolytes have obvious advantage in achieving high conductivity, which should be associated with interactions between polymers and salts. Future comprehensive studies and comparisons of various polymer electrolyte systems will certainly help provide more insights into the role of the polymer.

Conclusions

In conclusion, we have developed poly(ionic liquid)s-in-salt systems at exceptionally high Li-salt concentrations by incorporating crystallization-resistant LiFTFSI into PDADMAFSI. A comprehensive investigation has been conducted, combining molecular dynamics simulations with experimental validation, including DSC, PFG-NMR, BDS, and rheology measurements. A specific focus was to clarifying the electrolyte behavior and different mechanisms involved in such a system. A notable transition from polycation–anion–Li cocoordination to an anion–Li+ aggregation-dominated coordination environment was unveiled as the polycations-to-Li ratio increases from 1:2 to 1:8. This transition significantly impacts the electrolyte Tg, ion transport, and Li transfer number of the electrolytes. Our findings illustrate how the ion transport behavior shifts between decoupling and coupling with the structural dynamics. At a 1:2 ratio, a decoupling from structural dynamics dominated by the polymer is observed, transitioning to coupling with structural dynamics at a 1:8 ratio, due to the formation of anion–Li+ aggregates alongside the polycation–anion–Li+ cocoordination structure at increased salt concentrations. The Li-ion transference number significantly increases above 0.8. Nevertheless, ion motion is intricately affected by the structural dynamics and the Tg. The Tg changes nonmonotonically with an increased salt concentration in the mixed anion system and can be further lowered at the highest salt concentration by modifying the ratio of two anions. This strategy facilitates structural motion and enhances ionic conductivity at the 1:8 salt system, thus maintaining a high Li-ion transference number. The presence of anion–Li+ aggregation domain was also shown to have a positive impact on improving battery performance compared to the previously reported 1:1.5 PDADMAFSI/LiFSI system, with enhanced Li conductivity, lowered interfacial resistance and polarization, and a long stable Li deposition/dissolution. Although further improvement in electrolyte performance is still required to achieve practical applications, an important contribution of this study is to demonstrate the feasibility of stabilizing molten salt by using asymmetric anions and mixed anions, which sheds light on strategies for designing a broad range of polymer-in-salt systems. More importantly, for the first time, ion coordination and coupling/decoupling transport in an unprecedented salt domain of cationic polymer electrolytes were clearly elucidated, greatly advancing knowledge in the field.

Acknowledgments

S.K., L.A.O., H.U., F.M., P.H., M.F., and F.C. acknowledge the Australian Research Council (ARC) for funding through the Industry Transformation Training Centre for Future Energy Technologies (storEnergy) (IC180100049). S.K. acknowledges Japan Society for the Promotion of Science (JSPS) Overseas Research Fellowship, and Yamada Science Foundation for funding support. M.W., K.U., and S.K. acknowledge JSPS KAKENHI (23KK0102) for funding support. F.C. and M.F. acknowledge the Australian Research Council for funding support through the discovery projects (DP210101172, DP240101661). The simulation work was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University through the National Computational Merit Allocation Scheme supported by the Australian Government. This research was undertaken, in part, at the Deakin University Battery Research and Innovation Hub (BattRI-Hub), Australia.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c12616.

  • Experimental procedures, DSC thermograms, details of computational methods, DFT calculations, details of Li-ion transference number, MSD profiles, details of conductivity relaxation time and structural relaxation time, and 1H NMR spectrum (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja4c12616_si_001.pdf (1.5MB, pdf)

References

  1. Mindemark J.; Lacey M. J.; Bowden T.; Brandell D. Beyond PEO—Alternative host materials for Li+-conducting solid polymer electrolytes. Prog. Polym. Sci. 2018, 81, 114–143. 10.1016/j.progpolymsci.2017.12.004. [DOI] [Google Scholar]
  2. Wright P. V. Electrical conductivity in ionic complexes of poly (ethylene oxide). Br. Polym. J. 1975, 7 (5), 319–327. 10.1002/pi.4980070505. [DOI] [Google Scholar]
  3. Armand M. The history of polymer electrolytes. Solid State Ionics 1994, 69 (3), 309–319. 10.1016/0167-2738(94)90419-7. [DOI] [Google Scholar]
  4. Galluzzo M. D.; Maslyn J. A.; Shah D. B.; Balsara N. P. Ohm’s law for ion conduction in lithium and beyond-lithium battery electrolytes. J. Chem. Phys. 2019, 151 (2), 020901 10.1063/1.5109684. [DOI] [PubMed] [Google Scholar]
  5. Wang Y.; Sokolov A. P. Design of superionic polymer electrolytes. Current Opinion in Chemical Engineering 2015, 7, 113–119. 10.1016/j.coche.2014.09.002. [DOI] [Google Scholar]
  6. Bresser D.; Lyonnard S.; Iojoiu C.; Picard L.; Passerini S. Decoupling segmental relaxation and ionic conductivity for lithium-ion polymer electrolytes. Molecular Systems Design & Engineering 2019, 4 (4), 779–792. 10.1039/C9ME00038K. [DOI] [Google Scholar]
  7. Bocharova V.; Sokolov A. P. Perspectives for Polymer Electrolytes: A View from Fundamentals of Ionic Conductivity. Macromolecules 2020, 53 (11), 4141–4157. 10.1021/acs.macromol.9b02742. [DOI] [Google Scholar]
  8. Angell C. A.; Liu C.; Sanchez E. Rubbery solid electrolytes with dominant cationic transport and high ambient conductivity. Nature 1993, 362 (6416), 137–139. 10.1038/362137a0. [DOI] [Google Scholar]
  9. Fan J.; Marzke R.; Sanchez E.; Angell C. Conductivity and nuclear spin relaxation in superionic glasses, polymer electrolytes and the new polymer-in-salt electrolyte. Journal of non-crystalline solids 1994, 172, 1178–1189. 10.1016/0022-3093(94)90641-6. [DOI] [Google Scholar]
  10. Angell C. A. Polymer electrolytes—Some principles, cautions, and new practices. Electrochim. Acta 2017, 250, 368–375. 10.1016/j.electacta.2017.07.118. [DOI] [Google Scholar]
  11. Austen Angell C.; Fan J.; Liu C.; Lu Q.; Sanchez E.; Xu K. Li-conducting ionic rubbers for lithium battery and other applications. Solid State Ionics 1994, 69 (3), 343–353. 10.1016/0167-2738(94)90422-7. [DOI] [Google Scholar]
  12. Commarieu B.; Paolella A.; Collin-Martin S.; Gagnon C.; Vijh A.; Guerfi A.; Zaghib K. Solid-to-liquid transition of polycarbonate solid electrolytes in Li-metal batteries. J. Power Sources 2019, 436, 226852 10.1016/j.jpowsour.2019.226852. [DOI] [Google Scholar]
  13. Forsyth M.; Sun J.; Macfarlane D. R.; Hill A. J. Compositional dependence of free volume in PAN/LiCF3SO3 polymer-in-salt electrolytes and the effect on ionic conductivity. J. Polym. Sci., Part B: Polym. Phys. 2000, 38 (2), 341–350. . [DOI] [Google Scholar]
  14. Pożyczka K.; Marzantowicz M.; Dygas J. R.; Krok F. IONIC CONDUCTIVITY AND LITHIUM TRANSFERENCE NUMBER OF POLY(ETHYLENE OXIDE):LiTFSI SYSTEM. Electrochim. Acta 2017, 227, 127–135. 10.1016/j.electacta.2016.12.172. [DOI] [Google Scholar]
  15. Matsuoka R.; Shibata M.; Matsuo K.; Sai R.; Tsutsumi H.; Fujii K.; Katayama Y. Importance of Lithium Coordination Structure to Lithium-Ion Transport in Polyether Electrolytes with Cyanoethoxy Side Chains: An Experimental and Theoretical Approach. Macromolecules 2020, 53 (21), 9480–9490. 10.1021/acs.macromol.0c01634. [DOI] [Google Scholar]
  16. Watanabe M.; Yamada S.-I.; Sanui K.; Ogata N. High ionic conductivity of new polymer electrolytes consisting of polypyridinium, pyridinium and aluminium chloride. J. Chem. Soc., Chem. Commun. 1993, 11, 929–931. 10.1039/c39930000929. [DOI] [Google Scholar]
  17. Noda A.; Watanabe M. Highly conductive polymer electrolytes prepared by in situ polymerization of vinyl monomers in room temperature molten salts. Electrochim. Acta 2000, 45 (8), 1265–1270. 10.1016/S0013-4686(99)00330-8. [DOI] [Google Scholar]
  18. Susan M. A. B. H.; Kaneko T.; Noda A.; Watanabe M. Ion Gels Prepared by in Situ Radical Polymerization of Vinyl Monomers in an Ionic Liquid and Their Characterization as Polymer Electrolytes. J. Am. Chem. Soc. 2005, 127 (13), 4976–4983. 10.1021/ja045155b. [DOI] [PubMed] [Google Scholar]
  19. Seki S.; Susan M. A. B. H.; Kaneko T.; Tokuda H.; Noda A.; Watanabe M. Distinct Difference in Ionic Transport Behavior in Polymer Electrolytes Depending on the Matrix Polymers and Incorporated Salts. J. Phys. Chem. B 2005, 109 (9), 3886–3892. 10.1021/jp045328j. [DOI] [PubMed] [Google Scholar]
  20. Wang X.; Chen F.; Girard G. M.; Zhu H.; MacFarlane D. R.; Mecerreyes D.; Armand M.; Howlett P. C.; Forsyth M. Poly (ionic liquid) s-in-salt electrolytes with co-coordination-assisted lithium-ion transport for safe batteries. Joule 2019, 3 (11), 2687–2702. 10.1016/j.joule.2019.07.008. [DOI] [Google Scholar]
  21. Chen F.; Wang X.; Armand M.; Forsyth M. Cationic polymer-in-salt electrolytes for fast metal ion conduction and solid-state battery applications. Nat. Mater. 2022, 21 (10), 1175–1182. 10.1038/s41563-022-01319-w. [DOI] [PubMed] [Google Scholar]
  22. Malunavar S.; Gallastegui A.; Wang X.; Makhlooghiazad F.; Mecerreyes D.; Armand M.; Galceran M.; Howlett P. C.; Forsyth M. Formulation and Characterization of PS-Poly (ionic liquid) Triblock Electrolytes for Sodium Batteries. ACS Applied Polymer Materials 2022, 4 (12), 8977–8986. 10.1021/acsapm.2c01326. [DOI] [Google Scholar]
  23. Makhlooghiazad F.; Miguel Guerrero Mejía L.; Rollo-Walker G.; Kourati D.; Galceran M.; Chen F.; Deschamps M.; Howlett P.; O’Dell L. A.; Forsyth M. Understanding Polymerized Ionic Liquids as Solid Polymer Electrolytes for Sodium Batteries. J. Am. Chem. Soc. 2024, 146 (3), 1992–2004. 10.1021/jacs.3c10510. [DOI] [PubMed] [Google Scholar]
  24. Kubota K.; Siroma Z.; Sano H.; Kuwabata S.; Matsumoto H. Diffusion of Lithium Cation in Low-Melting Lithium Molten Salts. J. Phys. Chem. C 2018, 122 (8), 4144–4149. 10.1021/acs.jpcc.7b11281. [DOI] [Google Scholar]
  25. Kubota K.; Matsumoto H. Cation Mixtures of Alkali Metal (Fluorosulfonyl)(trifluoromethylsulfonyl)Amide as Electrolytes for Lithium Secondary Battery. J. Electrochem. Soc. 2014, 161 (6), A902. 10.1149/2.026406jes. [DOI] [Google Scholar]
  26. Yunis R.; Girard G. M. A.; Wang X.; Zhu H.; Bhattacharyya A. J.; Howlett P.; MacFarlane D. R.; Forsyth M. The anion effect in ternary electrolyte systems using poly(diallyldimethylammonium) and phosphonium-based ionic liquid with high lithium salt concentration. Solid State Ionics 2018, 327, 83–92. 10.1016/j.ssi.2018.10.018. [DOI] [Google Scholar]
  27. Kubota K.; Matsumoto H. Investigation of an Intermediate Temperature Molten Lithium Salt Based on Fluorosulfonyl(trifluoromethylsulfonyl)amide as a Solvent-Free Lithium Battery Electrolyte. J. Phys. Chem. C 2013, 117 (37), 18829–18836. 10.1021/jp405068q. [DOI] [Google Scholar]
  28. Zhang Z.; Nasrabadi A. T.; Aryal D.; Ganesan V. Mechanisms of ion transport in lithium salt-doped polymeric ionic liquid electrolytes. Macromolecules 2020, 53 (16), 6995–7008. 10.1021/acs.macromol.0c01444. [DOI] [Google Scholar]
  29. Chen F.; Howlett P.; Forsyth M. Na-Ion Solvation and High Transference Number in Superconcentrated Ionic Liquid Electrolytes: A Theoretical Approach. J. Phys. Chem. C 2018, 122 (1), 105–114. 10.1021/acs.jpcc.7b09322. [DOI] [Google Scholar]
  30. Li Z.; Smith G. D.; Bedrov D. Li+ Solvation and Transport Properties in Ionic Liquid/Lithium Salt Mixtures: A Molecular Dynamics Simulation Study. J. Phys. Chem. B 2012, 116 (42), 12801–12809. 10.1021/jp3052246. [DOI] [PubMed] [Google Scholar]
  31. Haskins J. B.; Bennett W. R.; Wu J. J.; Hernández D. M.; Borodin O.; Monk J. D.; Bauschlicher C. W. Jr.; Lawson J. W. Computational and Experimental Investigation of Li-Doped Ionic Liquid Electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4]. J. Phys. Chem. B 2014, 118 (38), 11295–11309. 10.1021/jp5061705. [DOI] [PubMed] [Google Scholar]
  32. Brinkkötter M.; Mariani A.; Jeong S.; Passerini S.; Schönhoff M. Ionic Liquid in Li Salt Electrolyte: Modifying the Li+ Transport Mechanism by Coordination to an Asymmetric Anion. Adv. Energy Sustainability Res. 2021, 2 (2), 2000078 10.1002/aesr.202000078. [DOI] [Google Scholar]
  33. Gao X.; Wu F.; Mariani A.; Passerini S. Concentrated Ionic-Liquid-Based Electrolytes for High-Voltage Lithium Batteries with Improved Performance at Room Temperature. ChemSusChem 2019, 12 (18), 4185–4193. 10.1002/cssc.201901739. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Reber D.; Takenaka N.; Kühnel R.-S.; Yamada A.; Battaglia C. Impact of Anion Asymmetry on Local Structure and Supercooling Behavior of Water-in-Salt Electrolytes. J. Phys. Chem. Lett. 2020, 11 (12), 4720–4725. 10.1021/acs.jpclett.0c00806. [DOI] [PubMed] [Google Scholar]
  35. Bruce P. G.; Evans J.; Vincent C. A. Conductivity and transference number measurements on polymer electrolytes. Solid State Ionics 1988, 28-30, 918–922. 10.1016/0167-2738(88)90304-9. [DOI] [Google Scholar]
  36. Watanabe M.; Nagano S.; Sanui K.; Ogata N. Estimation of Li+ transport number in polymer electrolytes by the combination of complex impedance and potentiostatic polarization measurements. Solid State Ionics 1988, 28–30, 911–917. 10.1016/0167-2738(88)90303-7. [DOI] [Google Scholar]
  37. Fan F.; Wang W.; Holt A. P.; Feng H.; Uhrig D.; Lu X.; Hong T.; Wang Y.; Kang N.-G.; Mays J.; et al. Effect of Molecular Weight on the Ion Transport Mechanism in Polymerized Ionic Liquids. Macromolecules 2016, 49 (12), 4557–4570. 10.1021/acs.macromol.6b00714. [DOI] [Google Scholar]
  38. Novikov V. N.; Sokolov A. P. Temperature dependence of structural relaxation in glass-forming liquids and polymers. Entropy 2022, 24 (8), 1101. 10.3390/e24081101. [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Paillard E.; Zhou Q.; Henderson W. A.; Appetecchi G. B.; Montanino M.; Passerini S. Electrochemical and Physicochemical Properties of PY14FSI -Based Electrolytes with LiFSI. J. Electrochem. Soc. 2009, 156 (11), A891. 10.1149/1.3208048. [DOI] [Google Scholar]
  40. Jafta C. J.; Sun X.; Lyu H.; Chen H.; Thapaliya B. P.; Heller W. T.; Cuneo M. J.; Mayes R. T.; Paranthaman M. P.; Dai S.; Bridges C. A. Insight into the solid electrolyte interphase formation in bis (fluorosulfonyl) imide based ionic liquid electrolytes. Adv. Funct. Mater. 2021, 31 (23), 2008708 10.1002/adfm.202008708. [DOI] [Google Scholar]
  41. Kondou S.; Dokko K.; Watanabe M.; Ueno K. Rate Performance of LiCoO2 Half-cells Using Highly Concentrated Lithium Bis (fluorosulfonyl) amide Electrolytes and Their Relevance to Transport Properties. Electrochemistry 2021, 89, 389–394. 10.5796/electrochemistry.21-00052. [DOI] [Google Scholar]
  42. Kondou S.; Sakashita Y.; Morinaga A.; Katayama Y.; Dokko K.; Watanabe M.; Ueno K. Concentrated Nonaqueous Polyelectrolyte Solutions: High Na-Ion Transference Number and Surface-Tethered Polyanion Layer for Sodium-Metal Batteries. ACS Appl. Mater. Interfaces 2023, 15 (9), 11741–11755. 10.1021/acsami.2c21557. [DOI] [PubMed] [Google Scholar]
  43. Chen K.-H.; Wood K. N.; Kazyak E.; LePage W. S.; Davis A. L.; Sanchez A. J.; Dasgupta N. P. Dead lithium: mass transport effects on voltage, capacity, and failure of lithium metal anodes. Journal of Materials Chemistry A 2017, 5 (23), 11671–11681. 10.1039/C7TA00371D. [DOI] [Google Scholar]

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