fast ion conduction

fast ion conduction

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Fast Ion Conduction in Layer-By-Layer Polymer Films

Dean M. DeLongchamp and Paula T. Hammond*

Department of Chemical Engineering, Massachusetts Institute of Technology, 7 Massachusetts Avenue, Cambridge, Massachusetts 02139

Received September 20, 2002. Revised Manuscript Received January 7, 2003

The layer-by-layer (LBL) deposition technique has been applied to the design of polymer electrolyte films appropriate for electrochemical applications such as sensors and electrochromic cells. In this work, we describe the properties of three LBL polymer electrolyte systems assembled from cationic layers of linear poly(ethylene imine) (LPEI), with anionic layers of Nafion, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS), and poly- (acrylic acid) (PAA). The assembly behavior of these systems was carefully examined, and ionic conductivitywas determinedusing impedancespectroscopy.The influencesof assembly conditions and water plasticization on ion conduction were elaborated. Room-temperature ionic conductivity greater than 10-5 S/cm can be achieved within LPEI/PAMPS and LPEI/ PAA films, which is 2 orders of magnitude greater than the highest values previously described in LBL films. By manipulating a unique assembly mechanism, high ionic conductivity can be achieved in LPEI/PAMPS films at low plasticizer concentrations. Also, the addition of lithium triflate salt to fully constructed films increases ionic conductivity at low plasticizer concentrations. The evaluation of performance based on assembly conditions clearly reveals the contributions of different materials and morphologies, suggesting themes for further ionic conductivity improvement.

Introduction

Solid polymer electrolytes are rapidly attaining notoriety as high-performance materials for a wide range of applications. The industrial manufacture of batteries (includingfuelcells),sensors,and electrochromicdevices increasingly employs solid polymer electrolytes because they feature easier processing, enhanced chemical compatibility, and better mechanical properties over liquid electrolytes.The developmentof these materialsfocuses on increasing ionic conductivity while maintaining favorable mechanical and chemical properties,1 which leads to higher battery power density and faster electrochromics switching speeds. Applying new materials and processing techniques has been a successful strategy for enhancing polymer electrolyte performance.

In recent years the layer-by-layer (LBL) assembly technique has been used to create many new composite materials. In classical LBL processing, substrates are alternately exposed to dilute solutions containing polyelectrolytes of opposite charge. Surface charge is reversed upon each exposure, enabling the controllable deposition of stratified, insoluble polymer complexes as thin films.2 The LBL technique has been extended to materials of many functionalities such as conducting polymers,3 electrochromics,4,5 light emitting systems,6,7 and nanoporousmaterials.8 The LBL techniquecan also use other complexation mechanisms such as hydrogen bonding.9-1 Film growth modulation can be achieved by changing deposition solution ionic strength for “strong” polyelectrolytes,12-14 and changing deposition solution pH for “weak” polyelectrolytes.15,16 In addition, methods have been developed for patterning LBL films on any substrateto achievehighlycomplexmicron-scale architectures.17-19 This capability of combining functional materials on the nanoscale with fine control over film morphology and thickness has attracted strong

* To whom correspondence should be addressed. Phone: 617-258- 7577. Fax: 617-258-5766. E-mail: hammond@mit.edu. (1) Gray, F. M. Polymer Electrolytes; The Royal Society of Chemistry: Cambridge, U.K., 1997. (2) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321-327.

(3) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985-989. (4) Stepp, J.; Schlenoff, J. B. J. Electrochem. Soc. 1997, 144, L155- L157. (5) DeLongchamp,D.;Hammond,P. T.Adv.Mater.2001, 13, 1455- 1459. (6) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B.

R. J. Appl. Phys. 1996, 79, 7501-7509. (7) Wu, A. P.; Lee, J.; Rubner, M. F. Thin Solid Films 1998, 329, 663-667. (8) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes,

A. M.; Rubner, M. F. Langmuir 2000, 16, 5017-5023. (9) Stockton,W. B.; Rubner, M. F. Macromolecules1997, 30, 2717- 2725. (10) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301- 310. (1) Yang, S. Y.; Rubner,M. F. J. Am. Chem. Soc. 2002, 124, 2100- 2101. (12) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. Chem.

10.1021/cm020945a C: $25.0 © 203 American Chemical Society Published on Web 02/08/2003 commercial interest in LBL processing, and in fact the technique has been used for consumer products.20

The LBL technique offers unique advantages for the design and developmentof ionicallyconductivefilms for use as solid polymerelectrolytes.LBL polymerfilms are inherentlyamorphoussolidswithmechanicalproperties superior to those of gels and crystalline solids: they do not flow or easily deform, yet they remain flexible. They can be applied very thin and defect-free, which can lead to thinner solid electrolytes in devices, increasing the overall conductance of the electrolyte layer. Unlike rollcasting, solvent-casting, or spin-coating, the LBL technique can uniformly coat nonplanar surfaces, allowing for a diverserange of cell geometries.Finally,LBL films can be easily tailored on the nanometer scale to create composition gradients or surface passivation, incorporating a wide variety of materials to achieve chemical compatibility and high performance.

Despite these advantages, early ionic conductivity results were disappointing. The dielectric and ion conduction properties of LBL films were first investigated by Durstock and Rubner,21 following limited earlierstudieson castpolycation/polyanioncomplexes.2,23 The first LBL investigation evaluated films of poly- (allylamine hydrochloride) (PAH) with poly(styrene sulfonate) (SPS) and poly(acrylic acid) (PAA). These composites demonstrated ionic conductivity with a maximum of 2 10-7 S/cm at room temperature and high hydration,21 whichis too low for most electrochemical applications. The low ionic conductivity of typical electrostatic LBL films can be explained using the general relation

where ó is ionic conductivity, i is the ion type, n is the number of mobile ions, q is the ion charge, and í is the ion mobility. The ion number and mobility are potentially limited by the LBL technique.

The limited number of mobile ions is due to the large extent of polyion pairing and rejection of residual small ions from the LBL film bulk, which is especiallynotable in strong polyion systems such as poly(diallyldimethylammonium chloride) (PDAC)/SPS.24 In general, an electrostatic LBL film cannot contain as many dissociable small counterionsas a neat film of either polyion, which would contain one counterion per monomer unit. In addition, hydrophobic aspects of common model polyelectrolytes such as PAH, PDAC, or SPS limit the potential for residual or added salt to dissolve into the film.

Limited mobility is due to an inherently high crosslink density, which has been shown to decrease ionic conductivity in polyether networks.25 The underlying mechanism of such poor conductivity is the constraint of small-segment polymer dynamics, which are widely recognized as being coupled to ion mobility.26 Furthermore, each ion pair within a LBL film can behave as a “coulomb trap”, slowing migration by temporary association with the migrating ion.

This work addresses the design of LBL films with high ionic conductivity by maximizing small ion concentration and minimizing cross-link density. The first design step was the choice of the polycation linear (polyethyleneimine)(LPEI).LPEI is a hydrophilic,hard Lewis base with high donicity that can solvate hard Lewis acids such as alkali metal cations and has been employed as a polymer scaffold in several polymer electrolytes.27,28 In addition, the cross-link density in LPEI-containing films can be modulated by adjusting assembly pH. LPEI was paired with three polyacids to createthreeLBL polymerelectrolytecomposites(Figure 1). The firstpolyacidchosenwas Nafion(Dupont),which was employed because of its well-knownproton conduction through hydrophilic pores. The second polyanion chosenwas poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS), a hydrophilic sulfonic acid commonly used in electrochromic devices, which was chosen to increase polymer matrix dielectricconstant and plasticity. Finally, to increase the number of mobile counterions, the weak polyelectrolyte PAA was used under acidic conditions so that it would remain partially protonated.15,16,21

These three composites were optimized by varying assembly conditions and assessing effects on film thickness and ionic conductivity.For strongpolyacidsystems LPEI/Nafion and LPEI/PAMPS, the effects of ionic screening of the assembly solution were evaluated. Adding salt to polyelectrolytesystemscauses deposition in thicker,loopylayers,a phenomenontermedscreening enhancedadsorption.12-14,29 The ionicallyscreenedmorphology may be more plastic or less cross-linked than the extended and flat adsorbed state attained in the absence of added small ions. For the weak polyelectrolyte system LPEI/PAA, pH effects were studied. Modulating assembly pH can influence film composition as well as morphology and cross-link density due to changes in degree of ionization with pH.15,16 Using this pH modulation, films with differing relative amounts of LPEI and PAA can be assembled, and relative contributions to ionic conductivity can be assessed.

In addition to the polymer matrix improvements, an easily dissociable salt with a low lattice energy and delocalizedanion charge (LiCF3SO3)30,31 was added into already-assembled films to provide additional mobile

(20) Winterton, L. C.; Lally, J. M.; Rubner, M. F.; Qiu, Y. In PCT

International Application; (Novartis A.-G., Switz.; Novartis-Erfindungen Verwaltungsgesellschaft m.b.H.); World Intellectual Property Organization: Geneva, Switzerland, 2001; 30 p. (21) Durstock,M. F.;Rubner,M. F. Langmuir2001, 17, 7865-7872. (2) Toyota, S.; Nogami, T.; Mikawa, H. Solid State Ionics 1984, 13, 243-247. (23) Michaels, A. S. Ind. Eng. Chem. 1965, 57,3 2-40. (24) Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184-1192.

(25) LeNest, J. F.; Gandini, A. In Second International Symposium on Polymer Electrolytes; Scrosati, B., Ed.; Elsevier Science Publishing Co., Inc.: New York, 1990; p 129-141. (26) Berthier, C.; Gorecki, W.; Minier, M.; Armand, M. B.; Chabagno, J. M.; Rigaud, P. Solid State Ionics 1983, 1,9 1-95. (27) Chiang,C. K.; Davis,G. T.; Harding,C. A.; Takahashi,T. Solid

State Ionics 1986, 18-9, 300-305. (28) Takahashi,T.; Davis,G. T.; Chiang,C. K.; Harding,C. A. Solid

State Ionics 1986, 18-9, 321-325. (29) Vandesteeg, H. G. M.; Stuart, M. A. C.; Dekeizer, A.; Bijsterbosch, B. H. Langmuir 1992, 8, 2538-2546. (30) Papke, B. L.; Ratner, M. A.; Shriver, D. F. J. Electrochem. Soc. 1982, 129, 1434-1438. (31) Alamgir, M.; Abraham, K. M. Ind. Chem. Libr. 1994, 5,9 3- 136.

all ion types niqiíi (1)

16 Chem. Mater., Vol. 15, No. 5, 2003 DeLongchamp and Hammond

species. Using these comprehensive design strategies, LBL films with strongly enhanced ionic conductivity were developed. Moreover, clear future development directions were ascertained, so that the high ionic conductivity and applicability of these materials might be even further extended.

Experimental Section

Materials. The polycation LPEI (25 0 Mw, Polysciences, Inc.) was used as received, as were polyanions PAA (90 0

Mw, Polysciences), PAMPS (Aldrich), and Nafion 117 (Fluka). The polyelectrolyteswere dissolvedin MilliQ-filtereddeionized water and then pH was adjusted with dilute HCl or NaOH solutions. PAMPS solutions were 0.002 M; PAA and LPEI solutionswere0.020M (polyelectrolyteconcentrationsare with respect to repeat unit). The Nafion 117 solution as received from Fluka was 5% polymer solution in light alcohols (a mixture of isopropyl alcohol, 1-propanol, and methanol in an unspecified ratio), with 10 Da equivalent weight (MW per sulfonic acid group of Nafion 117), and unspecified polymer

Mw. The as-received solution was diluted with MilliQ-filtered deionized water to 0.002 M Nafion (with respect to the 10

Da equivalent weight), for a final solvent alcohol composition of 4.4%. Substrates were 1 in. 2 in. indium-tin oxide (ITO) coated glass purchased from Donelly Applied Films and patternedby DCI,Inc.to formmultiple3-mmITO stripes.ITO film resistancewas measured to be 28 ¿/square after patterning. The ITO substrates were cleaned by ultrasonication in a seriesof solventsincludingdetergent,deionizedMilliQ-filtered water,acetone,methanol,and 1,1,1-trichloroethanefor 15 min each. Immediately before use, the ITO glass substrates were plasma-etched in a Harrick PCD 32G plasma cleaner with oxygen bleed for 5 min.

Assembly. Films were constructed using a modified Carl

Zeiss DS50 programmable slide stainer. Substrates were exposed first to polycation solution for 15 min, followed by 4 min of rinsing in 3 MilliQ water baths, then exposed to polyanion solution for 15 min and rinsed, and then the cycle was repeated for 30 layer pairs. The (LPEI/Nafion)30 system was assembled at pH 4 and films were made with several levels of ionic strength between 0.025 and 0.4 M by adding NaCl to polycation and polyanion solutions. The (LPEI/

PAMPS)30 systemwas also assembledat pH 4 at severallevels of ionic strength between 0.025 and 3.0 M by adding NaCl.

The (LPEI/PAA)30 system was assembled with polyelectrolyte solutions at pH conditions between 2 and 7. In each trial, the ionicstrengthand pH of the polycationand polyanionsolutions wereequivalent.Thicknessandroughnessmeasurementswere performed with a Tencor P10 profilometerusing a 2-ím stylus and 5 mg of stylus force. Following analysis of film assembly, films for ionic conductivity evaluation were fabricated. (LPEI/

Nafion)30 films were assembled at pH 4 with no NaCl added and with 0.1 M NaCl added to deposition solutions. (LPEI/

PAMPS)30 films were assembled at pH 4 with no NaCl added and with 0.2 M NaCl added to deposition solutions. Finally,

(LPEI/PAA)30 films were assembled with deposition solutions at pH 2 and pH 5.

Test Bed Fabrication. After assembly, films for ionic conductivity evaluation were dried at 110 °C for 24 h, which has been shown to effectively remove water from LBL assembled films.32 The drying was followed by thermal evaporation through a custom designed shadow mask of 2-m wide, 1000 Å thick gold electrodes perpendicular to the 3-m wide patterned ITO stripes. This technique creates 2-electrode test beds of 6 mm2 area in which the LBL film is sandwiched between ITO and gold electrodes. The dimensions allowed 8 such cells per substrate. The cells were profiled to verify the absence of significant gold penetration into the LBL film.

Testing. Following fabrication, the cells were exposed to a controlled-humidityenvironment.First,the cells were exposed to a chamber that contained anhydrous CaSO4 (Drierite) the solid-vapor equilibrium of which controls humidity to ap- proximately 17% relative humidity (RH), as measured by a VWR pen thermometer/hygrometer (all RH measurements (2%) at a room temperature of 25 °C. The chamber was approximately 0.05 m3 and contained a fan recirculating at 0.15 m3/min; equilibrium RH at any humidity level was reached within approximately 5 min with this configuration. The cells were exposed to this relatively dry environment for 7 days. After this equilibration period, ionic conductivity was evaluated within the chamber by impedance spectroscopy. Substrates were accessed by means of electrodes built into the chamber wall.

(32) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621-6623.

Figure 1. Three polymer electrolyte LBL systems assembled and tested in this study. LPEI is shown fully protonated, though it was employed at several extents of ionization (controlled by pH) in this study.

Fast Ion Conduction in LBL Polymer Films Chem. Mater., Vol. 15, No. 5, 2003 1167

Impedance spectroscopy was performed using a Solartron 1260 scanning from 1 MHz to 1 Hz. Because of noise at low frequency and high impedance, the lowest frequency included in analysis was variable and typically greater than 1 Hz; the lowest frequency was chosen so that the measurement would be within the <2% error region of impedance measurement for the instrument. The initial signal amplitude was 10 mV with no bias; amplitude was increased to 100 mV for each sampleto reducenoiseand increasethe effectivemeasurement range. Results at 100 mV amplitude were compared with the earlier 10 mV measurement to ensure no artifacts from increasing amplitude above kT (or approximately 25 mV at 25 °C), which in some cases can cause nonlinearity in the impedance response, especially in the interfacialcomponent.3 Fitting of the impedance results is described in the Results and Discussion section. The absence of any cell shorting, even for rough samples, further substantiated that evaporationdeposited gold did not penetrate the LBL film.

Following impedance spectroscopy, the anhydrous CaSO4 was replaced with a saturated solution of Mg(NO2)2â6H2O (Aldrich) in MilliQ water, the vapor-liquid equilibrium of which controls relative humidity to 52% RH at 25 °C.34 After 7 days the ionic conductivity was again evaluated by imped- ance spectroscopy. Finally, the Mg(NO2)2â6H2O solution was replaced with pure MilliQ water for 100% RH at 25 °C, with

7-day equilibration and subsequent ionic conductivity evaluation. Following the 100% RH measurement, substrates were immersed in MilliQ water overnight and then evaluated “dripping wet” by impedance spectroscopy using an apparatus that maintained a constant slow drip of MilliQ water over the substrate surface, ensuring that the surface was continuously wetted. During this last process the gold electrodes atop the two (LPEI/PAMPS)30 samples delaminated and thus impedance measurements could not be made on these two samples.

LBL films were invested with salt by soaking the films in aqueoussolutionsof LiCF3SO3 overnightandthendryingthem without rinsing. The films were first soaked in a solution of

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