An ionic-liquid functionalized metal–organic framework and its high performance as a solid electrolyte for lithium-ion conduction

Abstract

Crystalline porous metal–organic frameworks (MOFs) have attracted great interest, including in the field of solid-state electrolytes. Herein, we report a new type of solid-state electrolyte based on an MOF matrix and a Li⁺ ionic liquid. By covalently bonding the Li⁺ ionic liquid (MIMS·LiTFSI) on the stable UiO-67 framework, the obtained crystalline IL_Li-MOF material exhibited high ion conductivities within a wide temperature range (30 °C 1.62 × 10⁻³ S cm⁻¹, 110 °C 1.26 × 10⁻² S cm⁻¹) and efficient Li⁺ transport (t_Li⁺ = 0.88) [MIMS: 1-(1-mthyl-3-imidazolio) propane-3-sulfonate, LiTFSI: lithium bis(trifluoromethane sulfonyl)imide]. Characterization and control experiments demonstrated the ordered structure of the ionic-liquid moiety (MIMS·LiTFSI) arranged along the infinite channels, with the ultramicropores (<1 nm) in the MOF well accounting for the high and efficient targeted Li⁺ transfer. Additionally, this two-in-one strategy endows the crystalline electrolyte with desirable advantages, such as inflammable properties, stability and no leakage. The structure, electrochemical properties and ion conduction mechanism of the IL_Li-MOF were investigated and discussed. We hope that this work will provide a new strategy for the design and synthesis of high-performance solid-state electrolytes for lithium-ion batteries.

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Introduction

Lithium-ion batteries (LIBs), as non-polluting energy sources, present an excellent combination of high energy density, fast charging, and prolonged cycle life.¹ Traditionally, some organic liquids such as propylene carbonate, methyl carbonate, and dimethyl carbonate are used as electrolytes for LIBs.² Unfortunately, organic liquid electrolytes often suffer from side reactions with high-activity lithium metal to form lithium dendrites, resulting in decomposition of the electrolyte.^3–5 Moreover, the low boiling point and flash point of these organic liquids put electrolytes at risk of leakage and combustion.^6,7 To overcome these shortcomings, solid-state electrolytes (SSE) have received attention, as they are able to impede the growth of lithium dendrites and solve the leakage and combustion issues.^8–11 The most studied SSEs are inorganic ceramics and organic polymers. Nevertheless, inorganic ceramics are commonly limited by poor interfacial contacts and high grain boundary resistance, while organic polymers are considerably blocked by low ionic conductivity.^12–15 Therefore, developing new types of SSEs is crucial but also very challenging.

During the past few decades, metal–organic frameworks (MOFs) as crystalline organic–inorganic hybrid porous materials, have attracted significant scientific and technological attention due to their potential applications as separators, sensors, catalysts, and elegant ionic conductors.^16–18 Due to the long-range ordered structure and directional channels, MOFs offer the potential for directional and fast transport of ions with low activation energy. Some MOFs have been used as solid-state Li⁺ conducting electrolytes.^19,20 However, some challenges remain. Li⁺ salts do not diffuse well in the cavity of MOFs, and they tend to stay as tightly bound ion pairs, resulting in low conductivity. This phenomenon also occurs in ionic liquids (ILs) incorporated within porous MOFs (ILs@MOF) or flexible organic polymer blenders (ILs@polymer), as people use ionic liquids as safe electrolytes due to their high thermal stability, non-volatility, non-flammability, and low corrosivity.^21–23 Covalently grafting ILs into the directional channels of MOFs to create a novel soft-media-in-hard-matrix material not only improves the ion conductivity through efficient dispersion over the large internal surface of the MOFs but also overcomes the viscosity drawback of bulk ILs and ion cluster formation even in the subzero region.^24–26 For instance, our group recently reported a zwitterion-modified MIMS-MOF, namely UiO-67-MIMS, which was impregnated with methylsulfonic acid (MSA) to engender the binary IL moieties MIMS·MSA arranged along the MOF channels with ultramicropores (<1 nm), demonstrating a 2–4 orders of magnitude higher H⁺ conductivity than its counterpart bulk IL (MIMS = 1-(1-mthyl-3-imidazolio)propane-3-sulfonate) from −40 °C to 80 °C.²⁷ Furthermore, through the same method, we recently reported a crystalline Na⁺ ionic-liquid EN-1@UiO-67-MIMS by incorporating the ionic liquid EIMS-NaTFSI (abbreviated as EN-1) within UiO-67-MIMS (EIMS = 1-(1-ethyl-3-imidazolio)propane-3-sulfonate, NaTFSI = sodium bis (trifluoromethanesulfonyl) imide). These crystalline Na⁺ ionic-liquid derivatives also exhibited 1–2 orders of magnitude higher conductivities and related low activities compared to their counterpart bulk ILs (EN-1) within a wide temperature range.²⁸ Such structural design enables a new type of solid electrolyte (crystalline ILs) to overcome the typical limitations of solid ILs, having lower ion conductivity than their counterpart bulk ionic liquids, and related new solid electrolytes for lithium-ion batteries with wide applications are desired, specifically those with high performances in the low temperature region.²⁹

As a continuation of our work, herein we hypothesize that anchoring the zwitterionic group MIMS into the channels of an MOF could form an ideal Li⁺ conduction route. The following incorporation of lithium bis(trifluoromethanesulfonyltrifluoromethane sulfonyl)imide (LiTFSI) generates a binary Li⁺ ionic liquid (MIMS·LiTFSI) within the MOF. Thus, the covalently bonded MIMS groups dictate the arrangement of the ILs along the ordered channels, engendering a new type of solid electrolyte for fast Li⁺ transport. Accordingly, a newly synthesized IL_Li-2-MOF incorporating LiTFSI with the total van der Waals’ volume equaling that of UiO-67-MIMS was obtained (Fig. 1). IL_Li-2-MOF with the Li⁺ IL of MIMS·LiTFSI within the channel is nonflammable and stable in high temperature cycling tests, and exhibits the highest conductivity of 1.62 × 10⁻³ S cm⁻¹ at room temperature and 1.35 × 10^–2 S cm⁻¹ at 110 °C. Meanwhile, the control IL_Li-4-MOF, with an excess amount of LiTFSI and the counterpart ILs@UiO-67, containing the same component but a random IL (EIMS-LiTFSI), displayed conductivities that were 1 and 3 orders of magnitude, respectively, lower than those of IL_Li-2-MOF, respectively. Importantly, the conductivities of IL_Li-2-MOF are even 2–4 orders of magnitude higher than that of IL_Li-4, a bulk ionic liquid bearing Li⁺ salts and linkers the same as those of IL_Li-MOF (Fig. 1), from 30 °C to 110 °C. At 30 °C, the conductivity of IL_Li-2-MOF reached 1.62 × 10⁻³ S cm⁻¹, which is one order of magnitude higher than the reported Na⁺ conduction of EN-1@UiO-67-MIMS.²⁸ A high Li⁺ transport number was recorded, and the conduction mechanism was discussed. Li⁺ ionic liquids with ordered structures dictated by the covalently bonded MIMS in the long-range ordered channels of the MOF account for the high performance of crystalline IL_Li-2-MOF.

Fig. 1 Diagram of the design and synthesis of solid Li⁺ electrolyte ILs@MOF with a random ionic liquid (EIMS-LiTFSI) and IL_Li-MOF with a covalently bonded ionic liquid (top), and binary Li⁺ ionic liquid (bottom) bearing the same H₂BPDC-MIMS linker and LiTFSI composition as that of IL_Li-MOF in comparison.

Experimental section

General methods and materials

All chemicals were of analytical grade and obtained from commercially available sources, and were used without further purification. Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D8 Advanced diffractometer using Cu Kα radiation (λ = 1.540598 Å) at 40 kV and 40 mA in the range of 5° ≤ 2θ ≤ 30°. Thermogravimetric analysis (TGA) experiments were carried out on an STA 449 F5 Jupiter, using a heating rate of 10 °C min⁻¹ from room temperature to 800 °C under a N₂ atmosphere, with the sample heated in an Al₂O₃ crucible. FT-IR spectra were obtained on a Bruker Equinox 55 FT-IR analyzer using KBr pellets in the 4000–400 cm⁻¹ range. N₂ adsorption isotherms were measured on a Micromeritics ASAP 2020 surface area analyzer for the guest-free samples at 77 K with liquid nitrogen used. Before the nitrogen adsorption tests, all samples were washed with hot CH₃OH, soaked in CH₂Cl₂ for 3 days, and further dried under vacuum at 150 °C for 12 h of activation. AC impedance analysis was carried out on a CHI 760E electro-chemical workstation over a frequency range of 1 Hz–10⁶ Hz with an input voltage amplitude of 15 mV.

Synthesis of the H₂BPDC-MIMS ligand

The ligand 3-(1-((4,4′-dicarboxy-[1,1′-biphenyl]-2-yl)methyl)-1H-imidazol-3-ium-3-yl)propane-sulfonate (abbreviated as H₂BPDC-MIMS) was synthesized following a previously reported method.²⁷ Dimethylm2-((3-propylsulfonate)methylimidazole)biphenyl-4,4′-dicarboxylate (4.73 g, 10.0 mmol) was dissolved in 90 mL of CH₃OH : H₂O (CH₃OH : H₂O = 3 : 1), and then LiOH (478.8 mg, 20.0 mmol) was added slowly. The solution was stirred and refluxed for 12 h. After cooling to room temperature, the solution of the reaction was adjusted to pH = 1 with concentrated hydrochloric acid, resulting in a white precipitate. This precipitate was filtered, washed with water, and dried under a high temperature vacuum, obtaining the H₂BPDC-MIMS ligand (4.22 g, yield 95%), the ¹H-NMR spectrum of which was measured and shown in Fig. S1.†

Synthesis of UiO-67 and UiO-67-MIMS

UiO-67 and UiO-67-MIMS were synthesized through a hydrothermal method at 120 °C for 24 h as in a previous report, using biphenyl-4,4′-dicarboxylate (abbreviated as H₂BPDC) and H₂BPDC-MIMS, respectively.²⁷ The sediment was collected by filtering, washed with DMF (3 × 10 mL) and then with methanol (3 × 10 mL) to remove unreacted salts and ligand, and dried at 120 °C under vacuum for complete removal of the residual solvents. UiO-67-MIMS bears the stoichiometric formula Zr₆O₄(OH)₄(BPDC-MIMS)_5.3(CH₃CO₂)_1.4 with CH₃CO₂⁻ as an auxiliary ligand, which was characterized and determined via the ¹H-NMR spectra by digesting the powdered MOF in DCl (Fig. S2†).

Synthesis of IL_Li-2-MOF and IL_Li-4-MOF

All IL_Li-MOFs were synthesized through an impregnation-evaporation method, by incorporating a certain amount of Li⁺ salts into the MOF. For IL_Li-2-MOF, 50 mg of activated UiO-67-MIMS (0.016 mmol, based on the formula Zr₆O₄(OH)₄(BPDC-MIMS)_5.3(CH₃CO₂)_1.4) and 56.8 mg of LiTFSI (0.40 mmol) with the total van der Waals’ volume equal to that of the pores of UiO-67-MIMS (0.423 cm³ g⁻¹) were mixed in a vial (Table S2†). Then, 50 μL of anhydrous methanol was added with stirring for 24 h at 50 °C and left to slowly volatilize. After two days, the remaining powder was dried in an oven at 80 °C for 24 h to completely diffuse the ionic liquid into the pores of UiO-67-MIMS. The obtained powder (IL_Li-2-MOF) was collected, dried at 100 °C under vacuum to remove possible absorbed moisture from the air, and stored under a N₂ atmosphere before use without further purification. The stoichiometric formula based on UiO-67-MIMS is Zr₆O₄(OH)₄(BPDC-MIMS)_5.3(CH₃CO₂)_1.4(LiTFSI)_12.5 with the BPDC-MIMS : LiTFSI ratio of 1 : 2.4. IL_Li-4-MOF was synthesized similarly to IL_Li-2-MOF, except for the H₂BPDC-MIMS : LiTFSI ratio equaling 1 : 4.8 (Table S1†). Herein, the H₂BPDC-MIMS : LiTFSI ratio equals that of IL_Li-4.

Synthesis of room-temperature Li⁺ ionic liquids EIMS-LiTFSI, IL_Li-4

EIMS-LiTFSI was synthesized according to the reported procedure for common binary ionic liquids.³⁰ Lithium bis(tri-fluoromethanesulfonyl)-imide (abbreviated as LiTFSI), 1.909 g (4.8 mmol), and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (abbreviated as EIMS), 0.107 g (1 mmol), were added to the methanol solution. The mixture was stirred at 60 °C for 24 h and then heated in a rotary evaporator to remove the methanol. The remaining liquid was placed in a vacuum oven and dried at 100 °C for 24 h to obtain a slightly light-yellow Li⁺ ionic liquid EIMS-LiTFSI at room temperature, which was then stored in N₂ environment for further use. The colorless liquid IL_Li-4 at room temperature was synthesized following the same procedure as that for EIMS-LiTFSI, with the molar ratio of LiTFSI to H₂BPDC-MIMS of 4.8 : 1, which is the same as that in IL_Li-4-MOF and of EIMS to LiTFSI in EIMS-LiTFSI (Scheme S1, Table S1†).

Synthesis of EIMS-LiTFSI@UiO-67

EIMS-LiTFSI@UiO-67, the counterpart of IL_Li-4-MOF, was synthesized via the incipient wetness technique. Desolvated UiO-67 (50 mg) was added to a mortar under N₂ atmosphere protection in a dry box. Based on the pore volume of 1.067 cm³ g⁻¹ of UiO-67 (Table S2†), the total pore volume of which is 53 μL, the corresponding liquid EIMS-LiTFSI (53 μL) equal to the total pore volume of UiO-67 was added dropwise with continuous grinding until the EIMS-LiTFSI was completely absorbed. The mixture was sealed in a glass vial and heated in a drying oven at 80 °C for 24 h to uniformly distribute the EIMS-LiTFSI into the MOF channels, and then stored under a N₂ atmosphere before use without further purification.

Alternating current (AC) impedance spectroscopy analysis

At each temperature point, the measurements were conducted repeatedly with an interval of half an hour until the equilibrium was reached. The ionic conductivity was calculated using the following equation:
where l and S represent the thickness (cm) and the area (cm²) of the cylindrical sample, respectively, and R, which was extracted directly from the impedance plots, is the bulk resistance of the sample (Ω). The activation energy (E_a) for the material conductivity was estimated from the following equation:
where σ is the Li⁺ conductivity, σ₀ is the pre-exponential factor, k_B is the Boltzmann constant, and T is the temperature.

Conductivity of IL_Li-2-MOF, IL_Li-4-MOF, IL_Li-4 and EIMS-LiTFSI@UiO-67

Electrochemical impedance spectroscopy was conducted for all conductors of the synthesized composites. The EIS was recorded within a frequency of 1 Hz to 10⁶ Hz and amplitude 0.01 V at different selected frequencies called decades. All temperature-dependent conductivities of the samples were measured in an oven (in anhydrous conditions). To avoid the possible moisture adsorption from the air, all samples were heated to 100 °C for 8 h, and then the measurements were conducted from a high temperature to a low temperature. At each temperature point, the measurements were conducted repeatedly with an interval of half an hour until the equilibrium was reached. For IL_Li-x-MOF, the sample was placed into a quartz tube whose inner diameter was 2 mm and compacted into a cylindrical shape. A gold wire was rolled into a cylindrical shape to make intimate contact with the material, and the other end was extended out of the quartz tube. Finally, the two ends of the tube were sealed with epoxy resin and cured by heating in an oven. For IL_Li-4 liquid, a certain amount of ionic liquid (IL_Li-4) was placed in a 1 cm × 1 cm × 3 cm colorimetric dish, in which two sheets of carbon paper opposite to each other were attached as electrodes. The conductivity of UiO-67 (5.71 × 10⁻⁸ S cm⁻¹) and UiO-67-MIMS (4.60 × 10⁻⁸ S cm⁻¹) is negligible even at 150 °C, likely due to the lack of mobile ions as charge carriers.

Li⁺ transference number measurement

The Li⁺ transference number was evaluated in a Li| IL_Li-2-MOF |Li system using a potentiostatic polarization method. A polarization with the potential of 10 mV was applied until the current reached a steady state. The Li⁺ transference number could be calculated from Bruce–Vincent–Evans (BEV) equation:
where I_O and I_S are the initial current and steady-state current, respectively, ΔV is the polarization voltage, and R₀ and R_S are the impedance values before and after polarization, respectively.

Results and discussion

Structure and components

As shown in Fig. 1, UiO-67-MIMS was firstly synthesized via the hydrothermal method by reacting Zr⁴⁺, CH₃COOH and H₂BPDC-MIMS as reported in previous work.²⁷ The X-ray diffraction patterns of the as-made UiO-67-MIMS fit well with that of the parent UiO-67 and its simulation from the Cambridge Crystallographic Data Center (CCDC) (Fig. 2a).³¹ To obtain a dense electrolyte with a fully impregnated MOF, LiTFSI with the total van der Waals volume equaling that of the pores of the activated UiO-67-MIMS was used and engendered the IL_Li-2-MOFvia an impregnation-evaporation method (Experimental section).³² Reference IL_Li-4-MOF was also synthesized following the same procedure as that for IL_Li-2-MOF, except that double the amount of LiTFSI was used to investigate the ion concentration effect on the conductivity. As shown in Fig. 2a, the patterns of IL_Li-2-MOF and IL_Li-4-MOF after loading LiTFSI all match well with that of the pristine UiO-67-MIMS, indicating that LiTFSI was uniformly diffused into the MOF and the structure of UiO-67-MIMS remained unchanged. Notably, no diffraction of neat LiTFSI was observed, which explicitly signifies the efficiently diffused lithium salts within the micropores of UiO-67-MIMS, and no large ion-cluster aggregation occurred as was done in salt LiTFSI.

Fig. 2 (a) PXRD patterns of IL_Li-4-MOF, IL_Li-2-MOF, UiO-67-MIMS and UiO-67 in comparison, and (b) SEM images of UiO-67-MIMS (upper left), IL_Li-2-MOF (upper right), and IL_Li-4-MOF (bottom) with different zoomed-in views. (c) Comparison of the N₂ adsorption–desorption isotherms and (d) pore-size distributions of UiO-67-MIMS, IL_Li-2-MOF and IL_Li-4-MOF.

N₂ adsorption/desorption isotherms were measured at 77 K. As shown in Fig. 2 and Table S2,† after grafting -MIMS groups into the framework, UiO-67-MIMS shows decreased Brunauer–Emmett–Teller (BET) surface area (320.22 m² g⁻¹) and pore volume values (0.423 cm³ g⁻¹) as compared with that of the parent UiO-67 (1100 m² g⁻¹, 0.98 cm³ g⁻¹), and depressed micropores with a size distribution of ∼0.6 nm are observed. Moreover, after incorporating LiTFSI, the pore volumes of IL_Li-2-MOF and IL_Li-4-MOF are negligible (Table S1†), indicating that the pores of the MOF were fully occupied by the efficiently diffused LiTFSI.

Additionally, evidence for the efficient diffusion of LiTFSI within the micropores of UiO-67-MIMS was provided using Fourier transform infrared spectroscopy of IL_Li-2-MOF, and IL_Li-4-MOF (FT-IR, Fig. S2†). The ν_s(CF₃) stretching vibration peak of TFSI at 1198 cm⁻¹, δ_s(CF₃) at 740 cm⁻¹, and the symmetric vibration of S–N–S at 597–575 cm⁻¹ are observed upon LiTFSI incorporation. In comparison with the 1180 cm⁻¹ of the pristine UiO-67-MIMS, new strong ν_a(SO₃⁻) bands toward lower wavenumbers at 1142 and 1055 cm⁻¹ are found in IL_Li-2-MOF and IL_Li-4-MOF, with a separation of 87 cm⁻¹. This separation between the two ν_a(SO₃⁻) bands in IL_Li-2-MOF and IL_Li-4-MOF is due to the (–SO₃⁻)O⋯Li interactions.³³ Furthermore, the unchanged at 777 cm⁻¹ and 660 cm⁻¹ peaks of Zr–O vibration and 1657 cm⁻¹ and 1414 cm⁻¹ bands of η²:μ₂-bridging COO⁻ signify the remaining Zr₆ clusters and stable framework of UiO-67-MIMS after LiTFSI incorporation.

The morphologies of UiO-67-MIMS, IL_Li-2-MOF and IL_Li-4-MOF before and after the incorporation of salts were further characterized using scanning electron microscopy (SEM). As shown in Fig. 2b, the UiO-67-MIMS particles present a regular octahedron shape with a uniform size of ∼50 nm. Comparatively, IL_Li-2-MOF and IL_Li-4-MOF exhibit a similar morphology to that of UiO-67-MIMS, but an aggregating phenomenon of the MOF particles was observed. As the content of LiTFSI salts in IL_Li-4-MOF is twice that of IL_Li-2-MOF, the agglomeration is more pronounced in the former. This difference may be ascribed to the excess salts which engage in the ionic liquid MIMS·LiTFSI outside of the pore, resulting in some infiltrated interfaces of the MOF particles.

Ionic conduction

The temperature-dependent conductivities of IL_Li-MOF were measured through alternating current (AC) impedance analysis. To exclude the possible moisture from the air, all samples were treated in a glove box, and each sample was measured from 110 °C (high temperature) to room temperature (Experimental section). At each temperature point, the measurements were conducted repeatedly with an interval of half an hour until equilibrium was reached. The Nyquist plots of IL_Li-2-MOF and IL_Li-4-MOF show a semicircle at high frequency and a linear tail at low frequency (Fig. 3a and b). The semicircular region is the dielectric nature of the electrolyte, and the different regions occurred due to the electrode–electrolyte interactions. As shown in Fig. 3a and b, the diameter of the semicircle decreases with increasing temperature, indicating the increase in conductivity and fast ion transfer upon heating. As shown in Fig. 3c, the σ values of IL_Li-2-MOF and IL_Li-4-MOF increase with elevated temperature. IL_Li-2-MOF and IL_Li-4-MOF exhibited a high σ_Li⁺ of 1.62 × 10⁻³ S cm⁻¹ and 2.39 × 10⁻⁴ S cm⁻¹ at room temperature, respectively. Due to the negligible conduction of UiO-67-MIMS and UiO-67 (∼10⁻⁸ S cm⁻¹) even at 150 °C, here the conductions of the IL_Li-MOF mainly originate from ion transfer. Notably, the σ value of IL_Li-2-MOF reaches 1.62 × 10⁻³ S cm⁻¹, which is one order of magnitude higher than that of the IL_Li-4-MOF at 30 °C. They have the same framework and comparable PXRD patterns, but IL_Li-4-MOF bears a ratio of LiTFSI : H₂BPDC-MIMS (4.8 : 1) twice that in IL_Li-2-MOF (2.4 : 1). The larger ion concentration of the former does not lead to a higher conductivity. In comparison, the uniform structure with LiTFSI pairing with the MIMS along the channel of the MOF engenders fast ion transfer, overcoming the cut off effect of lower ion concentration in IL_Li-2-MOF.

Fig. 3 (a) Nyquist plots of IL_Li-2-MOF from −40 °C to 110 °C. (b) Nyquist plots of IL_Li-4-MOF from −30 °C to 110 °C. (c) Temperature-dependent conductivities of IL_Li-2-MOF, IL_Li-4-MOF, EIMS-LiTFSI@UiO-67 and ionic liquid IL_Li-4 in comparison. (d) I–t polarization curves for IL_Li-2-MOF with the inset electrochemical impedance spectra (EIS) before and after polarization.

To elucidate the structure effect within IL_Li-MOF, a counterpart EIMS-LiTFSI@UiO-67 was synthesized by blending ILs (EIMS-LiTFSI) with UiO-67 via an incipient wetness technique (Experiment section). EIMS-LiTFSI has the molar ratio (4.8 : 1) of LiTFSI to zwitterion EIMS the same as that of LiTFSI : H₂BPDC-MIMS in IL_Li-4-MOF (Fig. 1, Table S1†). The difference is that the former is liquid while the latter is in the solid state according X-ray diffraction at room temperature, although they bear the same composition. As shown in Fig. 3c, the σ value of IL_Li-4-MOF (2.39 × 10⁻⁴ S cm⁻¹) is 24 times than that of EIMS-LiTFSI@UiO-67 (9.80 × 10⁻⁶ S cm⁻¹) at 30 °C. In the −30 °C to 110 °C range, over 1 order of magnitude higher ionic conductivities for IL_Li-4-MOF than those of EIMS-LiTFSI@UiO-67 were observed. Considering that the ion concentration of IL_Li-4-MOF is almost the same as that of EIMS-LiTFSI@UiO-67, the markedly improved conductivities are ascribed to the relatively ordered structure of LiTFSI pairing with the zwitterion MIMS groups (via H-bonding and electrostatic interaction)²⁷ and covalent-bonding to the framework of the former. In contrast, the ILs incorporated within the latter are random, although they bear the same framework. This conclusion is further confirmed by the much depressed ion conductivities of IL_Li-4, an ionic liquid containing the same composition of LiTFSI and H₂BPDC-MIMS (Fig. 1, Table S1†). Their conductivities follow the order of IL_Li-4-MOF > EIMS-LiTFSI@UiO-67 > IL_Li-4 within a wide temperature range. Therefore, we speculated here that the very high ion conductivity of IL_Li-2-MOF as compared to IL_Li-4-MOF is mainly ascribed to its uniform crystalline structure. The excess LiTFSI salts outside of the MOF of IL_Li-4-MOF may result in a low ion conduction.³⁴

The calculated activation energies (E_a) of IL_Li-2-MOF, IL_Li-4-MOF, IL_Li-4 and EIMS-LiTFSI@UiO-67 through the Arrhenius equation are compared in Fig. 3c. IL_Li-2-MOF exhibits a linear Arrhenius behavior with the lowest activation energy (E_a = 0.37 eV). However, IL_Li-4-MOF exhibits a nonlinear Arrhenius behavior, but two activation energies over 0.37 eV in the two temperature ranges of above and below 10 °C are observed. Moreover, LiTFSI salt (twice the pore volume) is composited in IL_Li-4-MOF as compared with IL_Li-2-MOF. Therefore, the nonlinear Arrhenius behavior of IL_Li-4-MOF may be ascribed to a possible phase transition as the temperature increases because it's structure is different from that of the stable and uniform crystalline IL_Li-2-MOF. This fact is consistent with their slightly different morphologies (Fig. 2b). However, the activation energies of IL_Li-2-MOF and IL_Li-4-MOF are much lower than those of bulk ionic liquid IL_Li-4 (0.41–0.59 eV) and EIMS-LiTFSI@UiO-67 (0.65–0.80 eV). The lowest E_a of IL_Li-2-MOF is consistent with its highest σ value, further indicating that fast Li⁺ transport occurred in the crystalline electrolyte IL_Li-MOF. Herein, the anions and cations of the LiTFSI pair with MIMS groups to construct an ionic liquid subsystem (MIMS-LiTFSI), leading to the delocalization of charge and reduced bond ability of Li⁺ that facilitate the dissociation and fast ion transfer. The ordered and ultra-micropore (0.6/1.2 nm) structure of UiO-67-MIMS not only dictates the MIMS-LiTFSI arranging along the channel to provide an infinite high-speed channel for fast ion transport but also well disperses the MIMS-LiTFSI within the MOF to avoid aggregation of crystallization. Therefore, no phase transformation occurred in IL_Li-2-MOF from −40 °C to 110 °C. In sharp contrast, non-linear Arrhenius behaviors that may relate to the phase transformations are commonly investigated for IL_Li-4, EIMS-LiTFSI@UiO-67 and IL_Li-4-MOF (Fig. 3c). The almost identical temperature of transformation around 30 °C signifies the same nature of EIMS-LiTFSI within EIMS-LiTFSI@UiO-67 as that of bulk ionic liquid IL_Li-4.

The average Li ion transference number (t_Li⁺) is an important parameter for evaluating ion transport in a solid electrolyte. To investigate the transport ability of the target Li⁺ in IL_Li-MOF, 20 wt% of propylene carbonate (PC) was added to make the solid IL_Li-MOF feasibly fabricated into the symmetric battery Li| IL_Li-2-MOF |Li. Here, the ionic transference number is the overall contribution of non-faradaic current due to the ions and neutral nature of the MOF matrix. The t_Li⁺ of IL_Li-2-MOF calculated from the Bruce–Vincent–Evans (BEV) equation is 0.88 (Fig. 3d, Experimental section). It is comparable to that of UiO-66-LiSS-50%EC/PC (0.88),³⁵ being the highest value with respect to many Li⁺ electrolytes including hybrid salt derivatives (Fig. 4a). However, the ion conductivity of IL_Li-2-MOF (1.62 × 10⁻³ S cm⁻¹) is 2.3 times that of the UiO-66-LiSS-50%EC/PC analogue (7.04 × 10⁻⁴ S cm⁻¹) at room temperature. The high conductivity is also one order of magnitude higher than that of UN-LiM-EMIM (4.75 × 10⁻⁴ S cm⁻¹)³⁶ and Li-MOF@NWF/PEO (1.0 × 10⁻⁴ S cm⁻¹),³⁷ and many analogues derived from MOFs (Fig. 4a). The large target ion transfer number along with the high conductivity even at −40 °C (1.25 × 10⁻⁵ S cm⁻¹) would endow IL_Li-2-MOF with high performance for practical applications.

Fig. 4 (a) Comparison of the ionic conductivities of IL_Li-2-MOF and representative Li⁺ conductors based on ILS@MOF and salt@MOF at room temperature.^35–42 (b) PXRD patterns of IL_Li-2-MOF and IL_Li-4-MOF after testing. (c and d) Cyclical (from −30 to 110 °C) and long-term stability test of IL_Li-2-MOF. The right inset shows the images of the sheet sample before (top) and after (middle) conduction measurement and a flammability test (bottom).

Stability study

Actually, excellent long-term cyclical performance under harsh conditions plays an important role in the practical applications of solid-state electrolytes. As indicated in Fig. 4c, cyclical measurement of IL_Li-2-MOF shows a good stability with almost unvaried conductivity from −40 to 110 °C. IL_Li-4-MOF also exhibits good resistance ability to varied temperatures from −30 to 110 °C. Moreover, after being heated at 110 °C for 30 days, the ionic conductivity of IL_Li-2-MOF and IL_Li-4-MOF remained unchanged (the former: 1.35 × 10⁻² S cm⁻¹ before heating and 1.26 × 10⁻² S cm⁻¹ after 30 days, the latter: 2.25 × 10⁻³ S cm⁻¹ before heating and 2.05 × 10⁻³ S cm⁻¹ after 30 days), indicating their perfect electrochemical stability at high temperatures. Such stability is also supported by the almost unchanged PXRD patterns after cyclical measurements (Fig. 4b). Additionally, the intrinsic stability of Zr-MOF and the nature of ionic liquids covalent-bonded to the MOF matrix yield the inflammable IL_Li-2-MOF (Fig. 4).

Conduction mechanism

The equivalent LiTFSI to the total pore volume of UiO-67-MIMS implies the full occupancy of the pore within the MOF. The negligible BET or pore value confirms the efficient diffusion of lithium salts within the IL_Li-2-MOFvia the wetness incipient impregnation method (Experiment section, Scheme S1†). Therefore, the MIMS groups covalently bonded on the UiO-67-MIMS framework can pair and contact with Li⁺ and TFSI⁻ as common binary ionic liquids (ILs) and the bulk IL_Li-4 of this study (Fig. 1and 5). Thus, IL_Li-2-MOF can be viewed as a solid electrolyte (or crystalline IL) according to X-ray diffraction with ionic liquid moieties within the ultramicropore region. Li⁺ may quickly jump between sulfonate –SO₃⁻ and TFSI⁻ arranged along the infinite channel, as shown in Fig. 5. Such ordered structure and the charge delocalization effect of the ILs well accounts for the high conductivity with low activation energy (E_a) of IL_Li-2-MOF within a wide temperature range (Fig. 3c).

Fig. 5 Diagram of IL_Li-2-MOF with covalently bonded ILs on the MOF framework, and possible mechanism of Li⁺ conduction within the ionic-liquid region. The balls with different colors represent anions (TFSI⁻) or cations (Li⁺).

Conclusions

A new type of solid electrolyte IL_Li-MOF was successfully designed and synthesized via covalent bonding of ILs on the MOF matrix. This crystalline ionic liquid had the ILs arranged within the ultramicropore structure of the MOF, engendering many merits, such as fast and efficient ion transfer, low activation energy, inflammable properties and no phase transformation within a wide temperature range. This two-in-one strategy for new solid ionic liquids (ILs) provides a new pathway for high performance and safe electrolytes that have potential practical applications under harsh conditions.

Author contributions

C.-Q. Wan conceived the idea and designed the experiments. X.-K. Cui, Y. Ding and L. Feng did the synthesis and main characterizations. L.-M. Chen conducted the SEM tests, conductivity tests. N₂ adsorption isotherm measurements and data analysis. Y.-M. Hu conducted the NMR spectra recording and analyses. X.-K. Cui, Y. Ding, L. Fei, H. Chen, C.-Q. Wan contributed to data analysis and graphic drawing. C.-Q Wan, H. Chen and X.-K. Cui wrote and revised the manuscript. All authors have given approval to the final version of the manuscript. These authors contributed equally.

Data availability

The data supporting this article are included as part of the ESI.† ESI is available. See https://doi.org/10.1039/d4dt02756f.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are thankful for support from the Beijing Natural Science Foundation (L223010), National Natural Science Foundation of China (No. 22071157) and Science Basic Research Program of Shaanxi (No.2024JC-YBQN-0084). We also acknowledge support from the Key Laboratory of Life Organic Phosphorus Chemistry and Chemical Biology, Ministry of Education, Tsinghua University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4dt02756f

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