Poly(ethylenimine)-Based Polymer Blends as Single-Ion Lithium

May 15, 2014 - Highly conductive solid polymer electrolytes were generated by blending linear poly(ethyleneimine)-graft-poly(ethylene glycol) with lin...
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Poly(ethylenimine)-Based Polymer Blends as Single-Ion Lithium Conductors Robert P. Doyle,† Xiaorui Chen,† Max Macrae,† Abhijit Srungavarapu,† Luis J. Smith,† Manesh Gopinadhan,‡ Chinedum O. Osuji,‡ and Sergio Granados-Focil*,† †

Gustaf H. Carlson School of Chemistry and Biochemistry, Clark University, 950 Main Street, Worcester, Massachusetts 01610, United States ‡ Department of Chemical and Environmental Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06520, United States S Supporting Information *

ABSTRACT: Highly conductive solid polymer electrolytes were generated by blending linear poly(ethyleneimine)-graftpoly(ethylene glycol) with linear poly(ethyleneimine) bearing lithium N-propylsulfonate groups as the lithium source. The effect of polymer backbone structure on Li+ conductivity was determined by comparing a series of blends made from the PEI-based materials with those from polymethacrylate backbone analogues. The use of PEI backbones promoted ion-pair dissociation, stabilized the macromolecular mix and generated blends with ionic conductivities up to 2 orders of magnitude higher than those of the polymethacrylate-based systems. Blends containing the PEI-bound lithium sulfonates exhibited lithium conductivities higher than those measured for PEG doped with lithium bis(trifluoromethyl)sulfonimide. Shifts in the νs(SO3) IR absorption band suggest that the solvation environment for the lithium sulfonates changes with polymer structure. The PEI-based blends are thermally stable up to 200 °C, electrochemically stable in the ±5 V range, and showed unparalleled ionic conductivities (0.4 mS/cm at room temperature and 5 mS/cm at 80 °C) for solvent-free systems with polymer-bound anions.

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INTRODUCTION One of the main thrusts in the design of more efficient lithium rechargeable batteries is to replace the polar organic liquid electrolytes with solid polymer electrolytes, SPEs, to increase the durability, safety and flexibility. The optimal polymer electrolyte must meet certain requirements: (a) High conductivity (σ > 10−4 S/cm) at ambient and subambient temperature, (b) good mechanical strength, (c) appreciable transference number, (d) thermal and electrochemical stabilities, and (e) better compatibility with the electrodes.1−3 So far, the most widely studied SPE systems are derived from aliphatic polyethers such as poly(ethylene glycol) and polypropylene glycol.4−9 Lithium is typically incorporated into the polymer matrix either by doping the macromolecules with a low molecular weight lithium salt or by covalently attaching the anions to the polymer matrix to generate singleion conductors. In both cases, a significant challenge is the incomplete dissociation of the lithium salts (from 1 to 20%). The rest of the groups form tightly bound ion pairs, which limits the effective charge carrier concentration.10−12 An additional complication arises when using low molecular weight lithium salts where both ionic species (cation and anion) contribute to the measured current. In those cases, typically 20−40% of the ionic motion is due to the Li+ diffusion, while the rest is attributed to the anionic current.12−14 The buildup of anions on the anode leads to electro© 2014 American Chemical Society

polarization, lowering the overall cell performance due to high internal resistance, voltage losses, and undesirable side reactions, such as dendritic growth.15,16 Single-ion conductors overcome most of these challenges, but they tend to exhibit lower ionic conductivities than their salt-doped counterparts. The highest lithium conductivities reported for single-ion systems with covalently immobilized anions range between 10−5 and 10−6 S/cm.16,17 These relatively low values are typically attributed to a combination of decreased ionic mobility due to the covalent bonding of the anions to the polymer backbone and to an incomplete dissociation of the lithium salts within the polymer matrix. A potentially fruitful approach consists of modifying the polymer structure to maximize ionic dissociation. Previous reports suggest that incorporation of nitrogen atoms into the polymer backbone can decrease the anion/cation binding energies, favoring ionic dissociation and increasing the concentration of fully separated charge carriers.18−20 Enticed by the possibility of using nitrogen−sulfonate interactions as a way to increase the dissociation constant of polymer-bound lithium salts, we have synthesized a new linear poly(ethyleneimine)-based polyanion, PNS, by covalently Received: November 10, 2013 Revised: April 29, 2014 Published: May 15, 2014 3401

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ethyl-2-oxazoline) (5 kDa) and poly(ethylene glycol) (6, 20, 100 kDa) were purchased from Scientific Polymer Products incorporated. CuCl (99%), CuCl2 (98%), 2,2′-bipyridine (bipy, 99+%), ethyl 2bromoisobutyrate (EBiB, 98%), ethyl chloropropionate (ECP, 97%), DMF, propargyl bromide (metal stabilized, 80% in toluene), α-methlω-hydroxyl PEG (350 Da), and phosphorus(III) bromide were purchased from Alfa Aesar. Dialysis tubing (3.5 kDa MW cutoff) was purchased from Spectrapor and used as directed. Oligo(ethylene glycol) methyl ether methacrylate (EGMA, average Mn = 475 g/mol, Aldrich) was passed over neutral alumina to remove the inhibitor prior to use. 2-Acrylamido-2-methyl-1-propanesulfonic acid (AAMPSA-H, 98%) was converted to lithium form (AAMPSA-Li) by neutralization with LiOH (Aldrich, 98%). Tris(2-dimethylaminoethyl) amine (Me6TREN) was synthesized according to published procedures.23 Synthesis. Synthesis of Poly[oligo(ethylene glycol) Methyl Ether Acrylate] (PEGMA). The polymer was prepared by ATRP according to published procedures.24 [M]/[I] = 50. Conversion: 99.4%. Mn,GPC = 12.7 kDa; PDI = 1.21. 1H NMR (200 MHz, D2O, ppm): δ = 4.06, 3.57, 3.26, 1.80, 0.83. Synthesis of Lithium Poly(acrylamide-2-methyl-1-propanesulfonate) (PAS). The polymer was prepared by ATRP according to published procedures.24 [M]/[I] = 50. Conversion: 91.7%. Mn,GPC = 32.8 kDa; PDI = 1.28. 1H NMR (200 MHz, D2O, ppm): δ = 3.27, 2.00, 1.43. Synthesis of Linear Poly(ethyleneimine) (LPEI). The acid-catalyzed hydrolysis of poly(2-ethyl-2-oxazoline) was followed as previously reported.25−27 In brief, 20 g of PEtOx (202 mmol PEtOx repeat unit) was dissolved in 50 mL of deionized water. To this, 100 mL of 6 M HCl (600 mmol, ∼3 equiv) was added to the reaction mixture. The solution was then heated to 100 °C for 24 h. After cooling, the solution was then adjusted to pH 12 using 150 mL of 12 M potassium hydroxide (1.8 mol). The solid precipitated was collected by vacuum filtration, washed with water and recrystallized from ethanol in 91% yield. Synthesis of Lithium 3-N-Propyl Sulfonate Substituted LPEI (PNS). In a typical reaction, 1 g of LPEI was placed into a 100 mL round-bottom flask, and the polymer was then dissolved in 30 mL of DMF. To this solution, 3.5 g (28.6 mmol, 1.24 equiv) of 1,3-propane sultone was added. Sodium carbonate (6 g, 2.5 equiv) was added to the homogeneous reaction mixture after stirring for 3 h. The reaction was noted as proceeding forward by the rapid evolution of CO2 gas. The reaction mixture was allowed to stir for 16 h at 80 °C. After 16 h, the reaction mixture was acidified using 50 mL of 10% HCl, yielding a light yellow solution. The water was removed by rotatory evaporation until the volume reached 40 mL, and the resulting sodium chloride crystals were filtered before precipitating the polymer into isopropanol (300 mL), yielding an impure white solid. The white solid was collected by filtration and redissolved in 25 mL of deionized water. The material was then purified by dialysis in water for 72 h. Dialysis water was exchanged every 12 h and stopped when the eluent was neutral in pH. The solvent was then removed via rotary evaporation. 63% yield, full substitution. 1H NMR (D2O): δ 2.68 (4H, bs), 2.95 (2H,bs), 1.94 (2H, bs), 3.64 (2H, bs). 13C NMR (D2O): δ 65.31, 53.70, 50.28, 21.14. The acid form of the polymer was exchanged into the lithium form via a cation exchange column. In a typical reaction, 0.5 g (30 mequiv) was dissolved in 5 mL of water. The ion exchange column was composed of 10 equiv of lithium-exchanged Dowex resin. The polymer solution was passed through the column with an excess of water (50 mL). The solvent was removed by rotatory evaporation. Synthesis of α-Methl-ω-bromo PEG (MPEGBr). The α-methl-ωhydroxyl PEG (350 Da MPEG) was transformed into the halideterminated analogue via a simple bromination reaction with PBr3.28 In brief, 20 g (54 mequiv of hydroxyl groups) of 350 MPEG was placed into a 100 mL round-bottom flask and cooled in an ice bath for 30 min prior to addition of PBr3. The addition of PBr3 (5.6 g, 1.1 equiv) was done incrementally over a 40 min period while under ice bath conditions. After addition was complete, the solution was allowed to come to room temperature and stir overnight. The solution was then added to a 100 mL of 10% H2SO4 in ice and allowed to stir for 15 min

attaching propylsulfonate motifs to nitrogen atoms in each repeat unit of linear poly(ethyleneimine), LPEI. The effect of the nitrogen incorporation on salt dissociation was probed via FTIR and 7Li NMR spectroscopy; our data is consistent with an increase in the fraction of dissociated ions in samples containing LPEI backbones compared to those of the polyacrylamide-based analogues.21,22 In addition to the polymer synthesis, we report the preparation and ion transport properties of highly conductive SPEs based on a series of polymer blends made from LPEIgraf t-PEG, PNE, as a lithium solvating medium and PNS, as a PEI-bound lithium-ion source. The primary drive of this work is to quantify the effect of nitrogen incorporation on Li+ ion transport by comparing the ionic conductivity of PNE/PNS blends with those of materials made from polyacrylamide- or polymethacrylate-based analogues. The use of PEI backbones generated blends with ionic conductivities up to 2 orders of magnitude higher than those of the polymethacrylate-based systems. The PEI-based SPEs were thermally stable up to 200 °C and showed no electrochemical degradation in the ±5 V range. The observed ionic conductivities for these blends, 4 × 10−4 S/cm, at room temperature, and 5 × 10−3 S/cm, at 80 °C, are higher than those measured for PEG doped with lithium bis(trifluoromethyl)-sulfonimide and have not been reported before for SPEs with polymer-bound anions.

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METHODS AND MATERIALS

All 1H and 13C NMR measurements were made using either 200 MHz Varian Mercury or 600 MHz Varian Inova spectrometers. Solvents used in the measurements were either D2O or CDCl3 (99%), depending on the solubility of the polymer, and purchased from Cambridge Isotope Laboratory. The 7Li NMR experiments were conducted on a Varian Unity INOVA 400 MHz spectrometer using a Doty Scientific variable temperature pulsed field gradient NMR probe with a single z-gradient at a frequency of 155.36 MHz. The data were collected at a sample temperature of 70 °C. Infrared spectra were recorded using a PerkinElmer Spectrum 100 spectrometer using either sodium chloride plates or a Miracle Pro ATR attachment. GPC measurements were done at 50 °C in 0.1% (w/v) LiBr in DMF as eluent using an Agilent system composed of a series 200-LC pump, a 356-LC RI detector, two PolarGel-M columns (MW range 2 kDa− 2000 kDa), and PEO molecular weight standards. Thermogravimetric analysis (TGA) was carried out using a TA Instruments TGA Q500 thermogravimetric analyzer with a heating rate of 10 °C/min from room temperature to 600 °C under N2 atmosphere. DSC analysis was performed on a PerkinElmer Jade 6 DSC analyzer at 10 °C/min under N2 atmosphere. Electrochemical impedance data was obtained using a Solartron 1287 potentiostat/1260 frequency response analyzer in the 0.1 Hz−200 kHz range. Cyclic voltammetry was performed using the same potentiostat/FRA system. SAXS was collected on a pinhole collimated (1.2 mm beam diameter in the sample plane) Rigaku SMAX3000 instrument configured with Cu Kα radiation (1.542 Å) produced by a microfocus source. The area detector having a resolution of 1024 pixels × 1024 pixels was located at a distance of 85 cm from the sample center, allowing access to a range of scattering vectors from 0.015 to 0.2 Å−1. The resulting diffraction patterns were calibrated with Silver behenate standard with d-spacing of 58.38 Å. Azimuthally integrated scattered intensity against scattering vector q, where q = (4π/λ) × sin θ, with 2θ the scattering angle, was obtained using MATLAB routines (Rigaku). Samples for DSC, CV, 7Li NMR, SAXS and impedance measurements were packed in an inert atmosphere glovebox and sealed before transferring them to the instruments. All electrochemical measurements were performed under vacuum to ensure an anhydrous environment. All materials were used as received unless otherwise stated. Sodium azide (95%), lithium bis(trifluoromethyl sulfonamide) (LiTfSI, 98%), and 1,3-propane sultone (99%) were purchased from TCI. Poly(23402

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to convert remaining phosphorus(III) bromide to phosphoric and hydrobromic acids and minimize potential hydrolysis of the newly formed MPEG-Br. The aqueous solution was extracted three times with 50 mL of dichloromethane. The organic layers were combined and sequentially washed with 100 mL saturated sodium carbonate and water. The aqueous layer was then dried with magnesium sulfate prior to filtration and removal of solvent. (83% yield) 1H NMR (CDCl3): δ 3.28 (3H, s), 3.60 (30H, m). Synthesis of LPEI-graf t-methylPEG (PNE). In a typical reaction, 1 g (23 mmol repeat units) of LPEI was placed into a 100 mL roundbottom flask. The polymer was then dissolved into 30 mL of DMF. To this solution, 10.5 g (25 mmol, 1.1 equiv) of MPEGBr was added to the reaction mixture. The homogeneous solution was allowed to stir for several hours before sodium carbonate (6 g, 2.5 equiv) was added to the reaction mixture. The reaction was noted as proceeding forward by the evolution of CO2 gas. The reaction was then allowed to proceed for 72 h in an 80 °C oil bath. The reaction mixture was filtered and precipitated into diethyl ether (250 mL). The material was then redissolved in 30 mL of methanol and reprecipitated three additional times from methanol into diethyl ether, yielding a white precipitate. The polymer was then dried by air and then vacuum. Isolated yield of 90%, full substitution. 1 H NMR (D2O): δ 3.14 (4H, bs), 3.29 (3H, bs), 3.61 (28H, bs), 3.89 ppm (2H, bs). 13C NMR (D2O): δ 71.91, 69.76, 64.41, 62.98, 58.28, 52.48, 43.45, 36.79. Synthesis of LPEI with mPEG and Lithium N-Propylsulfonate Pendant Groups (PNE-r-PNS). In a typical reaction, 1 g (23 mequiv) of LPEI was placed into a 100 mL round-bottom flask; the polymer was then dissolved in 30 mL of DMF. To this solution, 0.94 g (7.75 mmol, 0.5 equiv) of 1,3-propane sultone was added. The sodium carbonate (6 g, 2.5 equiv) was added after the formation of a homogeneous solution. The solution was then allowed to stir in an 80 °C oil bath for 16 h to ensure complete conversion. Afterward, 6.6 g (16 mmol) of MPEGBr was added to the reaction vessel. The reaction was stirred at 80 °C for 24 h. Afterward the solution acidified using 20 mL of 10% HCl, yielding a light yellow solution. The resulting solution was filtered and the filtrate was precipitated into diethyl ether (150 mL), yielding an impure orange solid. The acid form of the polymer was exchanged into the lithium form via a cation exchange column. In a typical reaction, 0.5 g (60 mequiv) was dissolved in 5 mL of water. The ion exchange column was composed of 10 equiv of lithiumexchanged Dowex resin. The polymer solution was passed through the column with an excess of water (50 mL). The solvent was removed by rotatory evaporation. 60% yield, full substitution. 1H NMR (D2O): δ 3.14 (8H, bs), 3.29 (6H, bs), 3.61 (56H, bs), 3.89 ppm (4H, bs), 2.95 (1H, bs), 1.94 (1H, bs), 3.64 (1H, bs). 13C NMR (D2O): δ 71.91, 69.76, 65.31, 64.41, 62.98, 58.28, 53.70, 52.48, 50.28 43.45, 36.79, 21.14. Synthesis of PEGMA-r-PAAMPSA Random Copolymer (PEGMA-rPAS). EGMA (0.698 g, 1.469 mmol), AAMPSA_Li (0.198 g, 0.929 mmol) and DMF (6 mL) were mixed in a flask and degassed by argon for 30 min. The mixture was immersed into an oil bath at 80 °C. After 5 min, AIBN (15.75 mg, 0.0959 mmol) in a small amount of DMF was injected into the flask and stirred for 16 h. Full conversion was confirmed by NMR. The mixture was then dialyzed (Spectrumlabs, MWCO = 3500 Da) against water for 24 h. The polymer solution was dried under reduced pressure and in the vacuum oven overnight. 1H NMR (200 MHz, D2O, ppm): δ = 4.06, 3.57, 3.26, 2.02, 1.80, 1.40, 0.83. Blend Preparation. The MPEG used in the grafting technique had an average of eight repeat units per chain. This was used as the basis for determining the doping ratios used in the salt blends. All materials were dried in a 50 °C vacuum oven for 72 h before transference into an oxygen/water-free glovebox. In a typical blend preparation, 1 g (2.6 mmol) of LPEI-g-MPEG and 0.095 g (0.33 mmol) of LiTfSI were added to a 1 dram (3.7 mL) vial inside a glovebox. The vial was removed and the materials was dissolved in 2 mL of water until a homogeneous solution was achieved. The solution was then air-dried overnight before placement in a 50 °C vacuum oven for 72 h. The dried blend was transferred into the

glovebox, where it was loaded into the AC impedance cell or, a DSC aluminum pan, and sealed before placement into the impedance analyzer or the DSC instrument. This process was repeated for all polymers and compositions.

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RESULTS/DISCUSSION Synthesis. Synthesis of the PEGMA and PAS homopolymers yielded materials with PDIs and conversions similar to those reported in the literature.24 Synthesis of the LPEI graft copolymers was achieved by combining and adapting several procedures illustrated in Scheme 1.29−31 Scheme 1. Synthesis of Homopolymers

Synthesis of PNS. The zwitterionic nature of the polymer determined our choice of purification method; the most viable option for purification was to dialyze the material in water. Partial leakage of the polymer molecules through the dialysis membrane pores or absorption to the wall of the film decreased the recovery efficiency, lowering the yield to approximately 60%. The use of dialysis membranes with a lower MW cutoff should improve the overall efficiency of the purification process. Blend Stability. The polymer blends made from the LPEIg-PEG (PNE-blend-PNS and PNE-blend-PAS) are waxy solids at room temperature and remained homogeneous throughout the testing temperature range. The absence of any phaseseparated domains was confirmed by SAXS; see Figure S16 in the Supporting Information (SI). In contrast, the blends made from polymethyl acrylate-graf t-PEG (PEGMA-blend-PNS) are viscous liquids and phase separate over a period of a few hours at room temperature. Thermal Analysis. TGA and DSC results for the homopolymers and their blends are summarized in Table 1. PNE is thermally stable up to 225 °C while PNS started to thermally degrade at 140 °C. The thermal stability of the sulfonate-bearing PEI determined the upper limit of the temperature range of our tests at 130 °C. 3403

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Table 1. Thermal Stability and Tg of the Homopolymers and Their Blendsa Sample PNE PNS PEGMA PAS PNE-r-PNS (16:1) PEGMA-r-PAS PNE-blend-PNS (8:1) PNE-blend-PNS (16:1) PNE-blend-PNS (24:1) PNE-blend-PAS (16:1) PEGMA-blend-PNS (16:1)

MW (kDa) b

25 9b 13c 33c 19c 30b

Tg (°C)

Td (°C)d

2 N/Ae −7 N/Ae 3 10 17 9 0 13 −7

227 140 230 186 146 151 150 165 180 150 150

a

The number in parentheses represents the sample’s O:Li ratio. Calculated based on the MW of the precursor LPEI. cCalculated from GPC measurements. dTemperature of 2% weight loss. eSample decomposed before reaching Tg.

b

Figure 2. Conductivity as a function of temperature for LiTFSI-doped PNE (open symbols) and PNE/PNS blends (filled symbols). 6.8k PEG doped with LiTFSI with O/Li = 16 is added for reference. Dashed lines correspond to VTF data fits.

Ionic Conductivity. LPEI-g-MPEG/LiTFSi Blends. As an internal control, we measured the ionic conductivity of three PNE-based SPEs using LiTFSI as the lithium source. Figure 1 illustrates the influence of the O/Li ratio on the ionic conductivity as a function of temperature for PNE doped with LiTFSI.

of LiTFSI-doped PEG with equivalent O:Li ratios. Furthermore, all the blends are amorphous at room temperature and do not show the decrease in ionic transport typically observed below the melting temperature of PEG. To the best of our knowledge, the polymer blend made from PNE and PNS has the highest conductivity for a solvent-free SPE with covalently immobilized anions. Polymer Blends vs Random Copolymers. The effect on ion transport of the spatial distribution of ion-bearing groups and lithium solvating chains was investigated by comparing the ionic conductivity of one polymer blend, PNE-blend-PNS, O/ Li = 16, with an LPEI random copolymer with the same O/Li ratio, PNE-r-PNS. This macromolecule contains a random distribution of PEG chains and propyl sulfonate groups attached to the nitrogen atoms of the LPEI backbone. The conductivity of the random copolymer compared to that of the blend is shown in Figure 3. The data show that there is a significant decrease in conductivity when both ion-bearing (propyl sulfonate) and the ion solvating (PEG) motifs are placed within the same polymer chain. One striking feature is that the glass transition

Figure 1. Conductivity vs 1000/K for LiTFSI doped PNE, effect of O:Li ratio. Dashed lines correspond to VTF data fits.

LiTFSI was added to PNE to produce SPEs with O:Li ratios of 8:1, 16:1 and 24:1. The data in Figure 1 shows that the highest conductivities were obtained for the materials with a 16:1 O:Li ratio. This is consistent with previous reports where the highest conductivities with O:Li ratios ranging between 16 and 20.7,11 Single-Ion Conduction. The polymer blends were made in an identical fashion to the LiTFSI blends. The resulting materials were homogeneous waxy solids. As shown in Figure 2, the ionic conductivity of the blends made from the polyanion, PNS, and the graft polymer, PNE, also reaches a distinct maximum in conductivity at a doping level of 16:1. The ionic conductivity of the polymer blends is 2−8 times lower than that of the LiTFSI-doped PNE; this decrease is consistent with a lack of anion diffusion due to its covalent attachment to the polymer backbone. In spite of this decrease, the polymer blends showed higher ionic conductivity than that

Figure 3. Conductivity as a function of temperature for PNE/PNS blend and random copolymer PNE-r-PNS. 6.8k PEG doped with LiTFSI with O/Li = 16 is added for comparison. Dashed lines correspond to VTF data fits. 3404

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temperatures are similar for both the polymer blend (Tg = 9 °C) and the random copolymer (Tg = 3 °C); a similar trend was observed when calculating the activation energies for conduction from fitting the data to the VTF model (see Table 2). This suggests that polymer matrix mobility is not the main Table 2. Activation Energies and Reference Temperatures Calculated from Data Fits to the VTF Model Name

Ea (kJ/mol)

To (°C)

Tg (°C)

R2

PEG-d-LiTFSI PNE-d-LiTFSI PNE-r-PNS PEGMA-r-PAS PEGMA-blend-PNS PNE-blend-PAS PNE-blend-PNS

27.10 13.05 14.03 19.21 22.83 20.48 20.70

−92.64 −61.28 −68.23 −73.94 −130.08 −74.74 −83.76

−26 −10 3 10 −7 13 9

0.9998 0.9995 0.9683 0.9999 0.9999 0.9999 0.9999

limiting factor for ion transport. The observed contrast in ionic conductivity may be due to a difference in the amount of fully dissociated ions that can exhibit fast diffusion within the polymer matrix. Accurate measurements of these quantities are not trivial and would, by themselves, warrant a separate study. However, if the differences in lithium solvation states are large, they can be qualitatively characterized by FTIR spectroscopy, more specifically by correlating changes in the SO3 symmetric stretching mode, νs(SO3), to polymer structure.20,32,33 The positions of the sulfonate symmetric stretch mode as a function of polymer structure are listed in Table 3. The polymer blend

Figure 4. FTIR spectra and fitted peaks for PNE-blend-PNS and PNEr-PNS. The solid lines are the original spectra, the short-dashed lines are individual fitted peaks, and the long-dashed lines correspond to the peaks ascribed to νs(SO3).

Table 3. IR Absorption Frequencies for the νs(SO3) Mode for the Lithium Containing Polymers and Polymer Blendsa

a

Sample

νs(SO3) (cm−1)

PNS PAS PNE-r-PNS (16:1) PEGMA-r-PAS (14.5) PNE-blend-PNS (16:1) PNE-blend-PAS (16:1) PEGMA-blend-PNS (16:1)

1035 1047 1050 1054 1038 1051 1064

solvation state of the Li+ ions. Though both PNS and PAS contain nitrogen atoms, the PEI-based structure seems to favor lithium solvation better than the polyacrylamide-base material. Efficient ionic solvation is required, but not sufficient, for charge transport; the solvated charges must also be able to easily diffuse through the polymer matrix in order to generate highly conductive materials. Due to their extremely high glass transition temperatures, both PNS and PAS are rigid matrices that exhibited no measurable ion transport. The polymer backbone effect was further explored by preparing a series of low Tg blends analogous to the PNE/PNS materials where one of the components had a PEI-based backbone while the other had a polyacrylamide or polymethacrylate backbone. The ionic conductivity of the different blends is illustrated as a function of temperature in Figure 5. These data revealed that substitution of the LPEI backbone by either a polymethacrylate backbone (PEGMA-blend-PNS) or a polyacrylamide backbone (PNE-blend-PAS) results in lower ionic conductivities compared to the blend where both polymers are made from functionalized LPEI chains (PNEblend-PNS). PEGMA produced the least conductive and less stable polymer blends. Replacement of LPEI-based polyanion, PNS, by the polyacrylamide-based analogue, PAS, produced stable blends with ionic conductivities about 1 order of magnitude lower than those of the PNE/PNS mixture. It is unlikely that these differences are due only to differences in polymer matrix mobility, since the glass transition temperatures and activation energies for these mixtures (see Table 2) do not correlate inversely with the ionic conductivity. Some qualitative insights can be gained from examining the variations of the SO3 symmetric stretching mode, νs(SO3) in the three blends. Figure 6 and Table 3 show that for the blends

The number in parentheses represents the sample’s O:Li ratio.

exhibited a band ascribable to sulfonate groups not closely associated with their lithium counterions (1038 cm−1).21,22 In contrast, the random copolymer showed one broad peak centered at 1050 cm−1 which is consistent with a material where the majority of sulfonate groups are tightly bound to the lithium cations; see Figure 4.21 The absence of an intense IR signal at approximately 1035 cm−1 for the random copolymer indicates that the lithium ions within this material are less efficiently solvated than those present in the polymer blend, thus decreasing the concentration of fully dissociated charge carriers able to exhibit fast diffusion. There is a small peak centered at 1027 cm−1 in the spectrum of PNE-r-PNS, but this absorption is consistent with a PEO bending mode also observed in the PNE homopolymer (see SI Figures S6 and S12). Effect of Polymer Backbone on Conductivity. A comparison of the νs(SO3) frequencies for PNS (1035 cm−1) and PAS (1047 cm−1) indicates that the structure of the polymer bearing the ionic groups has a significant effect on the 3405

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PNE-blend-PAS sample shows an additional band at 1070 cm−1 that may also be attributable to tightly bound lithium sulfonate groups. The qualitative conclusions drawn from the shifts in the νs(SO3) band were corroborated via 7Li NMR experiments. The breadth and line shape of the lithium signal are affected by its ionic mobility. By fitting a broad Gaussian line and a sharp Lorentzian line to the 7Li NMR data, it is possible to extract the relative populations of slow moving (tightly bound) and fast moving (well-solvated) lithium ions; see Table 4. A detailed explanation of the data treatment can be found in the Supporting Information; see Table ST1 and Figure S17. Table 4. Relative Populations of Lithium Ions Extracted from 7Li-NMR Line Shape Fits Figure 5. Conductivity as a function of temperature for polymer blends and random copolymers. 6.8k PEG doped with LiTFSI with O/ Li = 16 is added for comparison. Dashed lines correspond to VTF data fits.

Sample

Low mobility Li+

High mobility Li+

PEGMA-blend-PNS PNE-blend-PAS PNE-blend-PNS

72% 25%

28% 75% 100%

In order to explore the effect of chemical structure on the solvation state of lithium sulfonates within copolymer matrices, we compared the ionic conductivity and νs(SO3) bands of two random copolymers, PNE-r-PNS and PEGMA-r-PAS, with similar O/Li ratio; see Figure 7. Both random copolymers

Figure 6. FTIR spectra and fitted peaks for the polymer blends. The solid lines are the original spectra, the short-dashed lines are individual fitted peaks, and the long-dashed lines correspond to the peaks ascribed to νs(SO3).

Figure 7. FTIR spectra and fitted peaks for the random copolymers. The solid lines are the original spectra, the short-dashed lines are individual fitted peaks, and the long-dashed lines correspond to the peaks ascribed to νs(SO3).

containing PNE the νs(SO3) band shifts from 1038 to 1050 cm−1 when changing the polymer backbone from LPEI to polyacrylamide. This indicates that, even when mixed with the same PEG-bearing material, the nature of the ion solvation remains different and produces less well-solvated ions in the PNE/PAS blend compared to the PNE/PNS-blend. Interestingly, the blend containing PEGMA and PNS does not show an IR band that can be easily ascribed to the sulfonate groups, but it shows a band at 1064 cm−1 that could be attributable to very poorly solvated, tightly bound, lithium sulfonate groups. The

showed a similar ionic conductivity dependence on temperature, but the material made from methacrylate and acrylamide units (PEGMA-r-PAS) showed ionic conductivity values 2 orders of magnitude lower than those of the LPEI-based analogue (PNE-r-PNS). This is consistent with a lower concentration of fully dissociated charge carriers in the PEGMA-r-PAS matrix. However, the νs(SO3) band was 3406

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centered at 1050 cm−1 for PNE-r-PNS and at 1054 cm−1 for PEGMA-r-PAS, suggesting that, in both materials, the ionic groups are similarly solvated. The differences in the observed conductivity are likely due to the differences in molecular weight; PEGMA-r-PAS has a degree of polymerization of approximately 400, while all the other copolymers and homopolymers have degrees of polymerization of approximately 50. Similarly, a difference in molecular weight of 1 order of magnitude decreased the observed ionic conductivity for PNE-LiTFSI blends by about a factor of 50; see Supporting Information, Figure S18. Since the solvation state is less affected by chemical structure in the random copolymers compared to the homopolymer blends, the differences in solvation may be linked to the nature of the interface between the sulfonate-bearing and the PEGfunctionalized segments in blends. Such an interfacial effect has been observed before for miscible PMMA/PEO blends where exceptionally fast-moving PEO “liquid-like” chains have been found to accumulate next to the PMMA/PEO interface.34,35 A detailed study of the interfacial nature within these PNE−PNS blends is beyond the scope of this report, but this is the subject of a separate manuscript in preparation. Drawing quantitative conclusions from the IR data is a tempting but potentially misleading endeavor. A significant number of vibration modes occur in the νs(SO3) frequency range, complicating the accurate integration and deconvolution of isolated absorption bands. The data presented here is intended to qualitatively illustrate the differences in lithium solvation between the LPEI-based and the polyacrylamidebased polyanions and to stimulate a more extensive study of the backbone/ion interactions within these materials. In addition to high conductivity and chemical and thermal stability, any attractive candidate for use as a solid polymer electrolyte must also exhibit electrochemical stability within a −5 V to 5 V window. Cyclic voltammetry measurements on the PNE/PNS blends showed no oxidative processes at room temperature and at 100 °C in the ±5 V range; see Figure S19 in the Supporting Information. The electrochemical stability and unparalleled ionic conductivity of these solvent- and plasticizerfree SPEs makes an extension of this approach particularly attractive.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra, SAXS data, 7Li NMR spectra, along with a detailed data interpretation, and cyclic voltammetry plot for PNE-blend-PNS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors equally. All authors have given approval to the final version of the manuscript. Funding

Funding was provided through Clark University Start-Up funds. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Mr. Joshua Boykin for his efforts in maintaining the glovebox for the duration of this project.

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ABBREVIATIONS PEI, poly(ethyleneimine); LPEI, linear poly(ethyleneimine); DMF, N,N-dimethylformamide; P(EtOx), poly(2-ethyl-2-oxazoline); equiv, molar equivalents; MPEG-Br, α-methyl-ωbromo poly(ethylene glycol)

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REFERENCES

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CONCLUSIONS

Straightforward covalent modification of LPEI backbones has generated highly miscible PEG-grafted lithium solvating matrices and propylsulfonate-bearing LPEI. Blends of these two materials have produced solid lithium conducting matrices exhibiting ionic conductivities higher than those previously reported for anion-immobilized, solvent and plasticizer-free solid polymer electrolytes. FTIR analysis of the νs(SO3) band, and 7Li NMR experiments suggest that the use of LPEI as the polymer backbone for these ion transporting matrices allows for more efficient solvation of the cation, leading to increases in ionic conductivity. A continuation of this work focused on measuring the concentration of fast-diffusing, charge carriers as a function of polymer backbone structure is currently underway and is expected to lead to a new generation of single-ion SPEs with properties well beyond the minimum requirements for use as electrode spacers in lithium-ion batteries. 3407

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