Ionic Liquid Hybrid Nanoparticles for Solid-State Lithium Ion

Article pubs.acs.org/cm

SiO2/Ionic Liquid Hybrid Nanoparticles for Solid-State Lithium Ion Conduction Sébastien Delacroix,†,‡,§,⊥ Frédéric Sauvage,‡,§,⊥ Marine Reynaud,‡,§,⊥ Michael Deschamps,∥,⊥ Stéphanie Bruyère,‡,§,⊥ Matthieu Becuwe,‡,§,⊥ Denis Postel,†,§,⊥ Jean-Marie Tarascon,‡,§,⊥ and Albert Nguyen Van Nhien*,†,§,⊥ †

Laboratoire de Glycochimie, des Antimicrobiens et des Agroressources, CNRS FRE 3517, ‡Laboratoire de Réactivité et de Chimie des Solides, CNRS UMR 7314, and §Institut de Chimie de Picardie, FR 3085 CNRS, UFR des Sciences, Université de Picardie Jules Verne, 33 rue Saint Leu, Amiens 80039 Cedex 1, France ∥ CEMHTI-CNRS UPR3079, 1D Avenue de la Recherche Scientifique, Orléans 45071 Cedex 2, France ⊥ Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 33 rue Saint Leu, Amiens 80039 Cedex, France ABSTRACT: We report the physical and electrical characterization of a series of substituted imidazolium-based ionic liquids grafted on Stöber-type SiO2. This hybrid architecture affords an increase of the lithium transference to 0.56 by hindering TFSI− (bis(trifluoromethane)sulfonimide) mobility to the total ionic conductivity. When doped with 16 wt % wt LiTFSI, the resulting hybrid organic/inorganic solid material exhibits a lithium diffusion coefficient of 2 × 10−12 m2/s at 87 °C and a conductivity of ca. 10−6 S/cm at room temperature and 10−4 S/cm at 65 °C with an activation energy barrier of 0.89 eV.

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INTRODUCTION In recent years, increased attention has been paid to ionic liquids (ILs), which can be defined as a salt having the property to melt at temperatures below 100 °C. The gain of interest for such a class of compounds stems from the combination of unique characteristics among low vapor pressure, nonflammability, and excellent chemical, thermal, and electrochemical stability. They were even quoted as “greener” solvents than the most reputed organic counterparts because of their potentialities to get reused and even recycled. Because of these features, research on ILs has expanded to almost all fields from (bio)organic, (bio)catalysis, and inorganic synthesis1 to electrochemistry. For the latter, IL electrolytes demonstrated superior durability in dye-sensitized solar cells, lithium ion batteries, and supercapacitors and are now investigated as a chemical bath for electrodeposition2−9 or for ionothermal synthesis to design new electrode materials.10−15 These appealing properties stem from their unique structures, which classically associate a bulky delocalized cation with a weakly coupled large anion. This allows minimization of the overall lattice energy, therefore lowering their melting point temperature. Their high electrochemical stability has been taken advantage of in their use as an electrolyte for lithium ion batteries to enhance the energy density thanks to the possible integration of more oxidative electrode materials (i.e., >4.5 V vs Li+/Li) and to hamper the growth of lithium dendrite at the lithium metal negative electrode during battery recharge.16 © XXXX American Chemical Society

Nevertheless, important challenges for their development still remain to supplant sooner or later the conventional carbonatebased electrolytes, e.g., lithium ion conductivity penalized by their higher viscosity, low lithium ion transference number (±0.3), and low tensile and compressive strengths, which constrain the shape adopted by the electrodes. Different approaches have been followed to enhance both the conductivities and lithium transference numbers by taking advantage of interfacial fast ionic transport in “soggy sand” electrolytes,17−19 tailoring single-ion ionomers,20−22 or optimizing nanocomposite polymer electrolytes based on PEG400 and anionic nanoparticles which reached a lithium transference number of close to 1.23,24 One other promising approach to address these drawbacks is the design of efficient hybrid gel nanocomposite electrolytes constituted of an intimate blend of ILs supported on insulating inorganic nanoparticles such as ZrO2 or SiO2.25−27 This new class of hybrid organic/inorganic materials, still modestly developed, offer mechanical strength comparable to that of solid polymer electrolytes and have been reported by Archer et al. to display lithium ion conductivity approaching the standards of battery liquid electrolytes when mixed with an optimized amount of lithium bis(trifluoromethane)sulfonimide (LiTFSI) (>77%).25−31 In this Received: July 30, 2015 Revised: November 6, 2015

A

DOI: 10.1021/acs.chemmater.5b02944 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

at 70 °C before dropwise addition of TEOS (15 mL, 67.25 mmol). After the resulting solution was stirred for 12 h, the solvent was eliminated under vacuum and the silica placed in an oven at 500 °C for 6 h. After being cooled to room temperature, the silica nanoparticles were hydrated in a concentrated solution of HCl (37%) at 100 °C for 12 h. Then the suspension was centrifuged, and the silica was washed with water until pH 6−7 and finally dried under vacuum at 100 °C before use. General Procedure for the Synthesis of a Disubstituted Imidazolium Ring (Method A). (3-Chloropropyl)triethoxysilane or (11-chloroundecyl)triethoxysilane (1 equiv) was added to Nbutylimidazole (1 equiv) under an argon atmosphere. After the resulting solution was stirred at 120 °C for 36 h, the crude product was washed several times with Et2O and then dried, yielding the desired compound. General Procedure for the Synthesis of a Disubstituted Imidazolium Ring Using Microwave Irradiation (Method A′). (3Chloropropyl)triethoxysilane or (11-chloroundecyl)triethoxysilane (1 equiv) was added to N-butylimidazole (1 equiv), and the reaction mixture was irradiated under microwaves (200 W) at 150 °C for 1−2 h. After completion of the reaction, Et2O was added to the mixture, and the resulting mixture was poured onto a silica pad and consecutively washed with Et2O until no (chloroalkyl)triethoxysilane remained (as detected on thin-layer chromatography, TLC). Finally, a mixture of MeOH/EtOAc (70/30, v/v) was used for filtration. After elimination of the solvents under vacuum, the residue was adsorbed on Celite and washed with cyclohexane to eliminate unreacted Nalkylimidazole. Finally, the Celite was flushed with dichloromethane to afford the desired product. General Procedure for Anion Metathesis Exchange (Method B). The triethoxysilane imidazolium chloride compound was dissolved in CH3CN. A 1.1 equiv sample of LiTFSI was added. After the resulting solution was stirred at 50 °C for at least 1 h, the solvent was eliminated under vacuum. Water and CH2Cl2 were then added, and the resulting product was extracted. The organic phase was washed 4−5 times until no chlorides remained (by adding AgNO3). After the organic phase was dried over Na2SO4 and filtrated, the solvent was eliminated under vacuum, yielding the desired substituted imidazolium bis(trifluoromethane)sulfonimide compound. General Procedure for Grafting on Synthesized Silica Nanoparticules (Method C). To 470, 500, or 1000 mg of a silica nanoparticle suspension in 20−30 mL of dried toluene was added the appropriate 1-alkyl-3-[(triethoxysilyl)alkyl]imidazolium bis(trifluoromethane)sulfonimide (0.20−0.90 mmol), and triethylamine (0.38 mmol) if needed. After the resulting solution was stirred at 90 or 110 °C for at least 13 h and up to 72 h under an Ar atmosphere, the silica nanoparticles were extracted through centrifugation (20 min at 8000 rpm at 10 °C) with toluene (3 × 35 mL), yielding the hybrid grafted silica nanoparticles by the desired IL. Preparation of SiO2−IL−TFSI/LiTFSI Mixtures. The desired amount of LiTFSI was dissolved in water and added to SiO2−IL−TFSI. The mixtures were sonicated to form a uniform phase, and the samples were lyophilized to remove water before use. Synthesis of 1-Butyl-3-[3-(triethoxysilyl)propyl]imidazolium Chloride (4). Following general method A, reaction of 4.21 mL of (3-chloropropyl)triethoxysilane and 2.22 mL of N-butylimidazole for 72 h yields 1-butyl-3-[3-(triethoxysilyl)propyl]imidazolium chloride (5.50 g, 91%) as a dark orange syrup, which was used afterward without further purification. Alternatively, following general method A′, reaction of 5.00 mL of (3-chloropropyl)triethoxysilane and 2.73 mL of N-methylimidazole for 2 h leads to 1-butyl-3-[3-(triethoxysilyl)propyl]imidazolium chloride (7.05 g, 92%) as a dark orange syrup, which was used afterward without further purification. Synthesis of 1-Butyl-3-[11-(triethoxysilyl)undecyl]imidazolium Chloride (5). Following general method A, reaction of 1.39 g of (11-chloroundecyl)triethoxysilane and 0.53 mL of N-butylimidazole for 36 h leads to 1-butyl-3-[11-(triethoxysilyl)undecyl]imidazolium chloride (1.54 g, 82%) as a slight orange syrup, which was used afterward without further purification. Alternatively, following general method A′, reaction of 1.52 g of (11-chloroundecyl)triethoxysilane

work, we report the synthesis and electrical determination of a diversely substituted imidazolium cation anchored onto SiO2 nanoparticles, yielding a solid-state electrolyte having a lithium cation conductivity of ca. 10−6 S/cm at room temperature and even 10−4 S/cm at 65 °C. Solid-state NMR investigations clarify the covalent interaction between the ILs and the support as well as validate the current approach of immobilizing TFSI− anions by imidazolium to significantly augment the lithium transference number to 0.56.

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EXPERIMENTAL SECTION

Reagents. All chemicals and solvents were purchased from Aldrich as reagent grade or better. Toluene was predried over sodium and distilled prior to use. Measurements. Solution NMR spectra were recorded on a Bruker Advance 300 (1H at 300.13 MHz). Solid-state NMR: The correlation experiments were recorded on a Bruker 850 MHz wide-bore spectrometer equipped with a Bruker 3.2 mm triple-resonance probe tuned to 1H, 13C, and 29Si. The magic angle spinning (MAS) frequency was set to 22 kHz for the 1H spin-diffusion experiment, with a spin diffusion delay of 50 ms and with the usual nuclear Overhauser effect spectroscopy (NOESY) pulse sequence. The sweep width in the indirect dimension was 11 kHz, and 1048 points were recorded with 16 transients for each with a recovery delay of 1 s. The 1H−13C and 1 H−29Si heteronuclear correlation (HETCOR) spectra were obtained with νR = 14286 kHz in the same probe, using 1.4 and 4 ms contact pulses, with ramped spin-lock pulses on 1H and 70 kHz sweptfrequency two-pulse phase modulation (SW(f)-TPPM) decoupling during 13C and 29Si signal acquisitions.32,33 The self-diffusion coefficients were measured on a Bruker 750 MHz spectrometer equipped with a Micro 2.5 imaging system providing a pulsed field gradient (PFG) along the MAS axis up to 150 G/cm, using a 3.2 mm double-resonance Bruker probe tuned to 19F and 7Li and a stimulated echo sequence, using bipolar pulses with gradient pulse durations of δ = 10 ms for 7Li and 3 ms for 19F, and diffusion delays Δ of 200 ms for 7 Li and 600 ms for 19F. The temperature was calibrated with the 207Pb chemical shift of lead nitrate, and the MAS rate was set to 10 kHz. Transmission electron microscopy (TEM) was realized on an FEI Tecnai F20 S-TWIN high-resolution transmission electron microscope operating at 200 kV. The hybrid material was dispersed using 2propanol, and a drop was deposited on the copper grid. Thermogravimetric analysis (TGA) was realized on a Netzsch STA 449C Jupiter thermal analyzer equipped with a differential analysis microbalance coupled with a QMS 403 Aëolos mass spectrometer with a stainless steel capillary and a Secondary Electron Multiplier (SEV) detector (Channeltron). The counting time for the mass spectrometer was 20 ms per m/z value (scanning width: m/z = 10−150 amu) with a resting time of 1 s. The samples (10−15 mg) were heated in an alumina crucible until 1000 °C with a 10 °C·min−1 scan rate under static air. FT-IR spectra in diffuse reflectance (DRIFT) mode were collected at room temperature under air over a range of 4000−400 cm−1 with a resolution of 2 cm−1 using a Nicolet AVATAR 370 DTGS spectrometer from Thermo Electron Corp. The ionic conductivity was measured on about 70% dense pellet using impedance spectroscopy. Prior to analysis, the powders were dried under vacuum at 60 °C overnight and kept under an Ar-filled glovebox to remove residual adsorbed water, which could influence the conductivity values. The airtight cell for determining conductivity was assembled in an argon-filled glovebox to avoid water uptake by IL− TFSI/LiTFSI. A platinum thin layer was sputtered on each polished side of the pellet to improve the electrical electrode/pellet contacts. The equivalent electrical circuit used to extract the kinetic parameters was Rohm‑drop in series with Rct//CPEct in series with a Warburg element. Zview software was used for adjusting the spectra. Synthesis. Synthesis of Silica Nanoparticles. In a 1 L roundbottom flask, 38 mL (0.29 mol) of ammonia solution (30 wt %) was added to 500 mL of absolute ethanol. The solution was stirred for 1 h B


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Scheme 1. Synthesis of N-Alkyl[(triethoxysilyl)propyl(undecyl)]imidazolium Chlorhydrates 4 and 5 and Their Corresponding N-Alkyl[(triethoxysilyl)propyl(undecyl)]imidazolium Bis(trifluoromethane)sulfonimides IL1 and IL2a

a

Reagents and conditions: (i) Ar, 120 °C for 36 h or 200 W at 150 °C under microwave irradiation; (ii) LiTFSI, CH3CN, 50 °C. Grafting of 1-Butyl-3-[3-(triethoxysilyl)propyl]imidazolium Bis(trifluoromethane)sulfonimide on Stö ber Silica Nanoparticles (SiO2−IL1A). Following general method C, reaction of 500 mg of Stöber silica nanoparticles (⌀ = 52 nm, ∼52.5 m2/g) and compound IL1 (497 mg, 0.69 mmol) for 48 h leads after centrifugation to grafted silica nanoparticles (IL loading 0.17 mmol/g). Grafting of 1-Butyl-3-[11-(triethoxysilyl)undecyl]imidazolium Bis(trifluoromethane)sulfonimide on Stö ber Silica Nanoparticles (SiO2−IL2A). Following general method C, reaction 500 mg of Stöber silica nanoparticles (⌀ = 52 nm, ∼52.5 m2/g) and compound IL2 (217 mg, 0.30 mmol) for 70 h leads after centrifugation to grafted silica nanoparticles (IL loading 0.10 mmol/g). Grafting of 1-Butyl-3-[3-(triethoxysilyl)propyl]imidazolium Bis(trifluoromethane)sulfonimide on Stö ber Silica Nanoparticles (SiO2−IL1B). Following general method C, reaction of 500 mg of Stöber silica nanoparticles (⌀ = 52 nm, ∼52.5 m2/g), compound IL1 (183 mg, 0.30 mmol), and triethylamine (34 μL, 0.50 mmol) for 64 h leads after centrifugation to grafted silica nanoparticles (IL loading 0.27 mmol/g). Grafting of 1-Butyl-3-[11-(triethoxysilyl)undecyl]imidazolium Bis(trifluoromethane)sulfonimide on Stö ber Silica Nanoparticles (SiO2−IL2B). Following general method C, reaction of 500 mg of Stöber silica nanoparticles (⌀ = 52 nm, ∼52.5 m2/g), compound IL2 (217 mg, 0.30 mmol), and triethylamine (34 μL, 0.50 mmol) for 64 h leads after centrifugation to grafted silica nanoparticles (IL loading 0.16 mmol/g).

and 0.58 mL of N-butylimidazole for 2 h leads to 1-butyl-3-[11(triethoxysilyl)undecyl]imidazolium chloride (1.51 g, 74%) as a slight orange syrup, which was used afterward without further purification. Synthesis of 1-Butyl-3-[3-(triethoxysilyl)propyl]imidazolium Bis(trifluoromethane)sulfonimide (IL1). Following general method B, reaction of 5.50 g (15.10 mmol) of compound 4 and 4.76 g (16.57 mmol) of LiTFSI for 1.5 h leads to 1-methyl-3-[3-(triethoxysilyl)propyl]imidazolium bis(trifluoromethane)sulfonimide (8.58 g, 93%) as a slight brown syrup. IR (ATR): ν (cm−1) 3149, 3109, 3081, 2974, 2942, 2884, 1565, 1351, 1188, 1135, 1056, 960, 789, 740. T°g: −77.8 °C. T°c: not determined. T°f: not determined. 1H NMR (CD3OD, 300 MHz): δ (ppm) 8.94 (s, 1 H, NCHN), 7.63 (d, JCHHC = 0.5 Hz, 1 H, CHN), 7.62 (d, JCHHC = 0.5 Hz, 1 H, CHN), 4.14−4.30 (m, 4 H, NCH2CH2CH2Si(OEt)3, CH3(CH2)2CH2N), 3.82 (q, JOCH2−CH3 = 7.0 Hz, 6 H, Si(OCH2CH3)3), 1.79−2.10 (m, 4 H, NCH2CH2CH2Si(OEt)3, CH3(CH2)2CH2N), 1.31−1.48 (m, 2H, CH3(CH2)2CH2N), 1.21 (t, JCH3−CH2O = 7.0 Hz, 9 H, Si(OCH2CH3)3), 0.98 (t, JCH3−CH2 = 7.3 Hz, 3H, CH3(CH2)3N), 0.54−0.67 (m, 2 H, NCH2CH2CH2Si(OEt)3). 13C NMR (CD3OD, 75 MHz): δ (ppm) 137.2 (NCHN), 124.0 (CHN), 123.9 (CHN), 121.3 (q, J1C,F = 321 Hz, 2 × CF3), 59.8 (Si(OCH2CH3)3), 53.1 (NCH2CH2CH2Si(OEt)3), 50.8 (CH3(CH2)3N), 33.2 (CH3(CH2)3N), 25.3 (NCH2CH2CH2Si(OEt)3), 20.6 (CH3(CH2)3N), 18.8 (Si(OCH2CH3)3), 13.8 (CH3CH2CH2CH2N), 8.1 (NCH2CH2CH2Si(OEt)3). MS (ESI): [M+] = 329.0 m/z, [M−] = 279.8 m/z. HRMS: calcd for C16H33O3N2Si, 329.22550; found, 329.22684; contains nearly 0.22% water. Synthesis of 1-Butyl-3-[11-(triethoxysilyl)undecyl]imidazolium Bis(trifluoromethane)sulfonimide (IL2). Following general method B, reaction of 1.51 g (3.16 mmol) of compound 5 and 1.00 g (3.48 mmol) of LiTFSI for 2.5 h leads to 1-butyl-3-[11-(triethoxysilyl)undecyl]imidazolium bis(trifluoromethane)sulfonimide (2.28 g, 99%) as a slight brown syrup. IR (ATR): ν (cm−1) 3149, 3113, 3093, 2971, 2927, 2852, 2359, 2325, 1351, 1189, 1135, 1056, 789, 740. T°g: −66.0 °C. T°c: not determined. T°f: not determined. 1H NMR (CD3OD, 300 MHz): δ (ppm) 8.95 (s, 1 H, NCHN), 7.63 (d, JCHHC = 0.5 Hz, 1 H, CHN), 7.62 (d, JCHHC = 0.5 Hz, 1 H, CHN), 4.22 (t, JNCH2−CH2 = 7.3 Hz, 2 H, NCH2(CH2)8CH2CH2Si(OEt)3), 4.21 (t, JCH2−CH2N = 7.3 Hz, 2 H, CH3(CH2)2CH2N), 3.81 (q, JOCH2−CH3 = 7.0 Hz, 6 H, Si(OCH2CH3)3), 1.81−1.96 (m, 2H, 2H, NCH2(CH2)8CH2CH2Si(OEt)3, CH3(CH2)2CH2N), 1.25−1.50 (m, 16H, 2H, NCH2(CH2)8CH2CH2Si(OEt)3, CH3(CH2)2CH2), 1.21 (t, JCH3−CH2O = 7.0 Hz, 9 H, Si(OCH2CH3)3), 0.98 (t, JCH3−CH2 = 7.3 Hz, 3H, CH3(CH2)3N), 0.57−0.67 (m, 2 H, NCH2(CH2)8CH2CH2Si(OEt)3). 13C NMR (CD3OD, 75 MHz): δ (ppm) 137.1 (NCH N), 123.9 (CHN), 121.3 (q, J1C,F = 321 Hz, 2 × CF3), 59.5 (Si(OCH2CH3)3), 51.0 (NCH2(CH2)9CH2Si(OEt)3), 50.8 (CH3(CH2)2CH2N), 34.2 (NCH2(CH2)9CH2Si(OEt)3), 33.2 (CH3(CH2)2CH2N), 31.2, 30.7, 30.6, 30.5, 30.2, 27.4, 24.0 (8 × NCH2(CH2)9CH2Si(OEt)3), 20.6 (CH3(CH2)2CH2N), 18.8 (Si(OCH2CH3)3), 13.8 (CH3CH2CH2CH2N), 11.3 (NCH2(CH2)9CH2Si(OEt)3). MS (ESI): [M +] = 441.3 m/z, [M−] = 279.7 m/z. HRMS: calcd for C24H49N2O3Si, 441.3512; found, 441.3496; contains 0.07% water.

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RESULTS AND DISCUSSION

Synthesis and Characterization. A family of alkyl[(triethoxysilyl)alkyl]imidazolium bis(trifluoromethane)sulfonimides have been synthesized by varying the length of the spacer arm chain separating the imidazolium cation from the siloxane anchoring moiety (propyl and undecyl) (Scheme 1). The reaction between N-butylimidazole (3) and (3chloropropyl)triethoxysilane (1) or (3-chloroundecyl)triethoxysilane (2) gives 1-butyl-3-[3-(triethoxysilyl)propyl]imidazolium chloride (4) in 92% yield and 1-butyl-3-[11(triethoxysilyl)undecyl]imidazolium chloride (5) in 74% yield by microwave irradiation. The yields are respectively 91% and 82% when classical conditions are used in the absence of microwave irradiation (Table 1). Although the yields are lower for some compounds when microwave irradiation is used, the latter is beneficial to accelerating the reaction time (1−2 h vs >36 h). Anion metathesis exchange of an equimolar amount of 4 and 5 with LiTFSI in acetonitrile leads to 1-butyl-3-[3(triethoxysilyl)propyl]imidazolium bis(trifluoromethane)sulfonimide (IL1) (yield 93%) and 1-butyl-3-[11(triethoxysilyl)undecyl]imidazolium bis(trifluoromethane)sulfonimide (IL2) (yield 99%), respectively (Table 1). The coloration of the isolated silyl bis(trifluoromethane)sulfonimide lies from a colorless to a slightly brown syrup. 1H and 13C C


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Chemistry of Materials Table 1. Reaction Yields of Silyl Chlorhydrates 4 and 5 and Silyl Bis(trifluoromethane)sulfonimides IL1 and IL2 compd

n

yield (%)

4 5 IL1 IL2

1 9 1 9

91 (92)a 82 (74)a 93 99

Table 2. Evolution of the IL Loading on Stöber SiO2 with (A) and without (B) Triethylamine for n = 1 and n = 9 Alkyl Chain Lengths n 1 9 1 9

a

Note that the values in parentheses refer to the yields under microwave irradiation.

a

NMR spectroscopy and HRMS were carried out to assess the purity of each product (see the Experimental Section). Synthesis of Hybrid Organic/Inorganic Nanoparticles. A functionalized (trialkoxysilyl)imidazolium anchoring unit was used to tether the ILs on the surface of SiO2 nanoparticles. These nanoparticles were synthesized by the Stöber approach to obtain monodispersed particles in size.34 The condensation reaction to anchor the imidazolium-modified trialkoxysilanes onto SiO2 was carried out under an argon atmosphere. This was realized by mixing either IL1 or IL2 with SiO 2 nanoparticles (52 nm diameter) in dry toluene with or without triethylamine (Scheme 2). The choice of using triethylamine in such a nonhydrolytic sol−gel route strengthens the nucleophilic character of the silanol units. This catalyzes the rate of grafting and improves the IL packing for the chemisorption without the use of water.35 The loading has been determined for the two lengths of alkyl chain arms, with and without triethylamine (Table 2). It was deduced from the value of mass loss on TGA and further confirmed by elemental analysis. SiO2−IL2A and SiO2−IL1A were obtained with loadings of 0.17 and 0.10 mmol/g of SiO2. The longer the alkyl chain arm, the lower the IL loading. On the other hand, the loading is noticeably more important when triethylamine is not used. FT-IR in diffuse reflectance mode was carried out to gain characterization insight for the surface of the hybrid materials. The comparison between pure Stöber SiO2 nanoparticles, SiO2−IL1A, and SiO2−IL2A is gathered in Figure 1. The two bands at 1660 and 1570 cm−1 are ascribed to the CC and CN stretching vibrations of the imidazolium ring. The three bands at 2959, 2930, and 2857 cm−1 are due to the C−H stretching from the CH3 and CH2 alkyl chains.36 The shoulder appearing at 1470 cm−1 corresponds to the deformation of the imidazolium moiety. Finally, the spectrum of the different samples never reveals the presence of isolated silanol groups at 3650 cm−1, suggesting the success of the grafting process of the

particle size (⌀, nm) 52 52 52 52

loadinga (mmol/g) b

0.13 (0.10) 0.08 (0.17)b 0.27 0.16

grafted SiO2 SiO2−IL1A SiO2−IL2A SiO2−IL1B SiO2−-IL2B

Determined by TGA. bDetermined by elemental analysis.

Figure 1. FT-IR spectra in diffuse reflectance mode (DRIFT) of compounds SiO2−IL1A (green curve) and SiO2−IL2A (red curve) and pristine silica (blue curve).

ILs on SiO2. The different values and assignments are given in Table 3. The thermal stability (and loading) of the (trialkoxysilyl)imidazolium bis(trifluoromethane)sulfonimide on SiO2 was determined by TGA under air (Figure 2). The thermal degradation proceeds in two steps. The first loss, from room temperature to ca. 150 °C, corresponds to narrow weight losses of 0.96% for SiO2−IL1A and 1.74% SiO2−IL2A ascribed to adsorbed water, in good agreement with the combined mass spectrometry analysis of the gas released. The second loss, between 400 and 600 °C, corresponds to more predominant weight losses of 6.54%

Scheme 2. Grafting of (Trialkoxysilyl)imidazolium Bis(trifluoromethane)sulfonimides

D


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Chemistry of Materials Table 3. FT-IR Absorption Band Positions (cm−1) and Assignments for SiO2−IL1A and SiO2−IL2A Compared to Pristine SiO2 pristine SiO2

SiO2−IL1A/SiO2−IL2A

ν(imidazolium ring) ν(CN)imidazolium ν(CC)imidazolium ν(C−H)CH2

1470 1570 1660 2857

ν(C−H)CH3

2930 2959

ν(Si−O−H)

3650

are in relation to those obtained by elemental analysis (Table 2). The TFSI− anion decomposes into SO2 (m/z = 64) and CF3 (m/z = 69) fragments as deduced by mass spectrometry analysis. TEM shows the representative morphology of the pristine Stöber SiO2 (Figure 3A). It depicts spherical and well-calibrated particles with a diameter of ca. 50 nm. The particles are completely amorphous as deduced by the only diffuse ring constituting the electron diffraction pattern (Figure 3A, inset). After grafting, with or without triethylamine (IL1A or IL1B), the two routes lead to relatively different outcomes. In the absence of triethylamine, the sample is constituted by ungrafted IL aggregates a few nanometers in size resulting likely from the condensation of a few IL units through the silanol. As part of these aggregates, a hybrid SiO2 core−IL shell is also obtained, and the thickness of the shell is ca. 5 nm regardless of the length of the alkyl chain arm (Figure 3B,C). Such a size suggests that more than a monolayer is formed. This also tends to suggest that the ILs wrap the SiO2 particles when grafted instead of self-standing vertically on the surface. By contrast, when the triethylamine is used, the samples are systematically free of ungrafted aggregates and the IL surface layer becomes more homogeneous while being thinner (Figure 3D,E). This is in good agreement with the determination of the IL loading in which we found 2-fold less ILs in the final product when associating the triethylamine in the grafting procedure. Such a more compact IL layer ensures a continuous percolation network throughout the particles, thus favoring fast lithium ion transfer across the hybrid material. The purity of the resulting SiO2−IL−TFSI hybrid particles is confirmed by using NMR spectroscopy. 1H solid-state NMR spectra of each SiO2−IL− TFSI were recorded and showed the expected peaks. The grafting was confirmed by a set of solid-state MAS NMR correlation experiments (Figure 4). The 1H−29Si HETCOR, using cross-polarization (CP) for magnetization transfer, can be used to correlate the 1H resonances with the chemical shifts of neighboring 29Si spins, as done in a previous study on similar systems (Figure 4A).37 The signal of Si atoms connected to the CH2 peak is very small compared to similar spectra recorded on mesoporous materials, and therefore, the bulk signal is far more

3650

Figure 2. Thermogravimetric analysis of compounds SiO2−IL1A (red curve) and SiO2−IL2A (blue curve).

and 11.34% for SiO2−IL1A and SiO2−IL2A, respectively. It is attributed to the entire degradation of the ILs grafted on SiO2. Such an elevated temperature stresses once again the high robustness of our hybrid materials. This also affords the ability to envision the integration of such a hybrid solid electrolyte into lithium ion battery applications for operating temperatures greater than 50 °C, a range where the liquid counterparts based on carbonates are inadequate. Note that the weight loss values

Figure 3. Transmission electron microscopy images showing (A) the pristine SiO2, (B, C) SiO2−IL1B (without triethylamine), and (D, E) SiO2− IL1A (with triethylamine). E


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of silanol groups and physisorbed water (which can also transfer magnetization to 29Si spins). The correlation spectrum is therefore used to unambiguously identify the 1H chemical shifts of SiOH moieties and physisorbed water. The 1H−13C HETCOR experiment (Figure 4B) showed the expected 1 H−13C cross-peaks stemming from the grafted IL. The presence of a CP signal indicates that the ionic liquid is effectively immobilized and does not undergo free rotational diffusion. A NOESY spectrum has also been recorded using spin diffusion to mediate the magnetization transfer. The slice at 0.93 ppm extracted from the NOESY experiment shows that spin diffusion occurs between the ionic liquid molecule (IL2) and the SiOH groups and physisorbed water from the material, thus clearly settling the effectiveness of grafting between the ionic liquid and the SiO2 nanoparticles. The mobility of Li+ and TFSI− ions is confirmed by PFG 7Li and 19F NMR experiments. By this method, we determined a self-diffusion coefficient of 2.0 × 10−12 m2/s for lithium cation and 1.6 × 10−12 m2/s for TFSI− at T = 87 °C. Interestingly, as opposed to most common solvents for which TFSI− diffuses faster than Li+ owing to a larger solvated sphere, restraining the lithium transference number (tLi+) at best to 0.3 for pure LiTFSI or LiTFSI embedded in polymer poly(vinylidene fluoride−hexafluoropropylene) (PVDF−HFP),42 in this approach aiming at tethering the ionic liquid on SiO2, the lithium cation diffuses faster than the anion. Consequently, the lithium transference number increases significantly to 0.56 (tLi+ = DLi+/ (DLi+ + DTFSI−)).43 This result is ascribed to the hindrance of TFSI− diffusion by the presence of the positively charged imidazolium cation and the immobilization of the ionic liquid onto SiO2.41 Such a value is even slightly greater than the tLi+ = 0.54 reported by Archer et al. for a SiO2−IL−TFSI gel material including as high as 87% LiTFSI.26 Measurements of ionic conductivity and activation energy were carried out using ac impedance spectroscopy on ca. 70% dense pellets at different temperatures. The Bruggeman symmetric medium model was used to correct the conductivity from the electrode’s porosity.44 For this, we considered a continuous interconnection between random pores as has been described by McLachlan et al.45 Figure 5 gathers the evolution of conductivity as a function of the inverse of temperature. Optimized conductivity was obtained by doping the hybrid SiO2−IL−TFSI materials with 16 wt % LiTFSI. This quantity corresponds to a lithium content significantly lower than those reported on the SiO2− and ZrO2−IL−TFSI gels published by Archer et al., who involved in their gels electrolyte contents greater than 77 wt %.23−25 Regardless of the type of hybrid material, the conductivity systematically follows an Arrhenius thermal activation process (Figure 5). The results highlight that not only the length of the butyl chain separating the anchoring group (n = 1 or n = 9) and the imidazole ring but also the grafting methodology, i.e., the use or not of triethylamine, has an impact on the lithium ion conductivity. Indeed, our results suggest that a longer alkyl chain is preferable to assess a better ionic conduction. This enhances the ionic conductivity by a factor of 2−5 while decreasing the hopping activation barrier by ca. 0.2 eV. We hypothesize that the longer alkyl chain improves the molecular packing in the IL layer, consequently reducing the diffusion length for ionic hopping through oxygen of the anchoring group between neighboring sites. Indeed, the absence of Vogel-TammannFulcher (VTF) dependence between ionic conductivity and

Figure 4. MAS NMR spectra recorded on SiO2−IL2 loaded with 16% LiTFSI. (A) 1H−29Si HETCOR spectrum showing the 1H resonance of 1H spins close to 29Si atoms. (B) 1H−13C HETCOR spectrum showing the 1H−13C cross-peaks of the grafted ionic liquid. Note that chemical shifts are very similar to what was obtained for the precursor dissolved in CD3OD. (C) Slice of the 2D NOESY spectrum corresponding to 1H spins undergoing magnetization transfer through spin diffusion with the methyl resonance from the butyl group at 0.93 ppm. The spectrum was fitted with dmfit.38 The two 1H resonances around 2.2 and 3.5 ppm are assigned to isolated silanols and physisorbed water or H-bonded silanols located close to grafted ionic liquid molecules.39−41 The positions of these two resonances are reported in (A) and (B).

intense due to the size of the silica nanoparticles (small proportion of 29Si atoms close to the surface) and the presence F



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REFERENCES

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Figure 5. Evolution of conductivity as a function of the inverse of the temperature for Stöber SiO2−IL−TFSI with Et3N (SiO2−IL1A (orange solid circles), SiO2−IL2A (red solid squares)) and without Et3N (SiO2−IL1B (pink open circles), SiO2−IL2B (times signs inside squares)) compared to the pristine Stöber SiO2 (black solid triangles).

temperature suggests that the segmental motion of the lithium cation along the alkyl chain of the IL has a negligible effect on the total conductivity. Note that the use of triethylamine decreases the conductivity while increasing the activation energy. The best conductivity obtained is for SiO2−IL2B, which reaches at room temperature a value of σ = 8.9 × 10−7 S/cm and at 65 °C a value of σ = 1.0 × 10−4 S/cm. Such values lie in the range of those of benchmark solid polymer electrolytes.46 Note that this ionic conductivity can even be enhanced to σ = 1.3 × 10−6 S/cm at room temperature with an activation energy of 0.89 eV when the size of the SiO2 particles is decreased from ca. 50 to 35 nm (loading 0.23 mmol/g of SiO2).

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CONCLUSIONS We synthesized original hybrid materials based on ionic liquids supported on Stöber SiO2 nanoparticles. We demonstrated for the first time by pulsed field gradient 7Li and 19F NMR experiments that the immobilization on silica improves the lithium transference number by impeding fast mobility of the TFSI− anion. This work also gives further clarification of the existing literature regarding the effectiveness of anchoring such ILs on SiO2 throughout the silanol moieties. As a result, we achieved lithium ion conductivity in the range of 10−6 S/cm at room temperature and 10−4 S/cm at a mild temperature of 60 °C. Our systematic work suggests that lateral conductivity is promoted when using a long alkyl arm chain without triethylamine for grafting to obtain a well-packed thick shell of ILs.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge the technical assistance and fruitful discussions with Dr. Michel Armand and Mathieu Courty. A.N.V.N. and S.D. thank the Conseil Régional de Picardie for financial support. The solid-state NMR experiments were funded by the CNRS and TGIR-RMN. G


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Ionic Liquid Hybrid Nanoparticles for Solid-State Lithium Ion

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