- Email: [email protected]
polymer electrolyte interface
Solid State Ionics 118 (1999) 129–133
Using self-assembled monolayers to inhibit passivation at the lithium electrode / polymer electrolyte interface a a a b, Rachel N. Mason , Mike Smith , Thad Andrews , Dale Teeters * a
Department of Chemical Engineering, The University of Tulsa, Tulsa, OK 74104 -3189, USA b Department of Chemistry, The University of Tulsa, Tulsa, OK 74104 -3189, USA
Abstract This work investigates the use of surface chemistry to modify the lithium electrode / polymer electrolyte interface by placing a molecular layer, most likely in the form of a self-assembled monolayer (SAM), of H–(CH 2 ) 32 –(CH 2 –CH 2 –O) 10 –H onto the surface of PEO, poly(ethylene oxide), electrolyte films. It is proposed that the PEO-like ‘‘head’’ of the molecule above preferentially orients itself to absorb onto the PEO electrolyte, leaving the hydrocarbon CH 2 ‘‘tail’’ to self-assemble. SAM placement was confirmed using attenuated total reflection FTIR spectroscopy and wetting studies, and AC impedance measurements were used to investigate the passivating layer development. Extended time period studies of untreated polymer films in contact with lithium exhibited the rapid rise of an interfacial passivating layer whose resistance overtook that of the bulk electrolyte. Similar studies demonstrated that samples with SAMs actually had a small increase in ion conductivity and developed interfacial passivation much slower, supporting the assertion that SAMs could be used to deter the formation of a barrier to Li 1 transport during cycling of lithium polymer batteries. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Self-assembled monolayers; Electrode / electrolyte interface; Passivating layer
1. Introduction Lithium / polymer electrolyte batteries are receiving much attention today because they offer a number of significant advantages both in terms of a high energy density and a large electrochemical window. However, there are technological problems that must be solved before these batteries reach wide commercial utilization. One of the major problems associated with lithium polymer batteries is the formation of a passivating layer at the lithium / poly*Corresponding author. Tel.: 1 1-918-631-3147; fax: 1 1-918631-3404; e-mail: chem [email protected] ]
mer electrolyte interface. Although the mechanism of ion conduction through the bulk matrix of the polymer has received much attention, it has become increasingly obvious that the electrochemical behavior at the electrode / electrolyte interface plays a major role in determining the critical operating parameters of a device, whether the device is a high energy density battery, a fuel cell, or a sensor. There have been numerous studies of processes involving lithium or lithium alloy anodes in contact with electrolytes consisting of lithium salts complexed with poly(ethylene oxide) polymers [1–5]. Work done by Bruce and Krok [3] using AC impedance methods strongly suggested that surface
0167-2738 / 99 / $ – see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00421-4
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R.N. Mason et al. / Solid State Ionics 118 (1999) 129 – 133
layers develop on both electrode interfaces, as indicated by the increasing interfacial impedances. The authors were able to eliminate the possibility of impurity contribution to this interfacial impedance, but did not offer any insight into the nature of the surface layer. In a related study, Hiratani, et al., [4] investigated the interface between a lithium and a solid polymer electrolyte consisting of a thin film (150 mm) of PEO complexed with 9 mole % LiCF 3 SO 3 . These studies also utilized complex impedance spectroscopy and indicated an increase in the interfacial resistance which was attributed to a charge transfer process. The formation of passivating layers can severely affect the performance of polymer electrolyte cells. It has been shown that, with time, interfacial impedance in lithium / polymer electrolytes systems can grow until it is significantly larger than the bulk resistance [3,5]. Therefore, methods of minimizing certain reactions at the interface are advantageous to cell development. One way to approach the passivation problem at the lithium polymer / electrolyte interface is to study and modify the lithium metal surface [6–8]. Most of this work involves chemical reactions forming ROCO 2 OLi compounds on the surface of the lithium anode resulting in a passivating layer that is more conducive to recycling. Work by Takechara, et al. [9] further indicates the possibility of placing a thin polymer film on lithium surfaces to promote a surface more conducive to battery performance. If the lithium surface could be modified for enhanced properties, could the polymer electrolyte surface be modified for better interface behavior too? Modifying the polymer surface would have several potential advantages. A polymer surface is a much easier surface to work with than the extremely reactive lithium surface. At the same time, one can also conduct more chemistry on polymer surfaces because of the variety of functional groups that can be present. If passivation protection could be ‘‘built into’’ the polymer electrolyte surface, a convenient method of enhanced performance would result. This work is concerned with the formation of molecular layers, most likely of the self–assembled monolayer (SAM) type on the polymer electrolyte interface. These molecular films have been formed by adsorption of surface active agents from solution onto
the polymer electrolyte surfaces and appear to have the potential to protect the interface and enhance cell performance.
2. Experimental The PEO films were made by dissolving 900,000 MW polymer and lithium triflate, (both obtained from Aldrich) in Baker HPLC grade acetonitrile, which had been dried by refluxing over CaH 2 . The LiSO 3 CF 3 was dried in vacuum oven at 1108C overnight. The polymer–triflate solutions were then cast on a poly(tetrafluoroethylene) (PTFE) substrate and the solvent was allowed to evaporate in a high purity argon atmosphere. The resulting films were dried at 808C in a vacuum for two days and then stored in a vacuum dessicator. The thickness of the films ranged from 80 to 120 mm. A semicrystalline wax from Petrolite Specialty Polymers, which could best be described as H–(CH 2 –CH 2 ) 32 –(CH 2 –CH 2 –O–) 10 –H with an average molecular weight of 900, was used to form SAMs on the polymer surface. The wax was dissolved in hexane where it acted like a surfactant resulting in the SAM formation being accomplished via adsorption from solution. A concentrated solution of the wax in Fisher optima grade hexane was prepared and a selected electrolyte film was placed in the solution and allowed to equilibrate for at least 72 h. Shorter equilibration times did not result in good SAM formation. The film was removed and quickly rinsed in pure hexane to remove wax that might deposit from evaporation of the hexane–wax solution. Films not receiving SAMs were soaked in neat hexane for an equal period of time in order to make certain that all films were treated similarly. These films were dried in a vacuum oven again and stored in a vacuum dessicator. Wetting studies were performed using the dynamic Wilhelmy plate technique [10]. This allows for surface analysis by examining the solid–liquid contact angle of the polymer electrolyte films versus different liquids. The Zisman technique [11] was used to calculate critical surface tensions, which are useful in comparing surface properties of solid substrates. All wetting studies were conducted in an argon atmosphere glove box.
R.N. Mason et al. / Solid State Ionics 118 (1999) 129 – 133
Infrared spectra were collected on a Nicolet 510P Fourier Transform Infrared Spectrometer with a DTGS detector at 1 cm 21 spectral resolution. Surface infrared spectra were obtained using the attenuated total reflection (ATR) technique. The ATR spectra were obtained by using a Spectra–Tech Horizontal Contact Sampler ATR with a slurry sample boat type cell and a ZnSe crystal cut at 458. The average depth of infrared radiation penetration into the sample was 0.5 microns. The films to be studied were quickly removed from their dessicator and placed in the bottom of the sample boat trough and sealed with a rubber cover. This ATR sampling arrangement was used so that the films could be protected from water in the atmosphere. Bulk film infrared transmission spectra were collected in the sample compartment of the Nicolet 510P while under a nitrogen purge. An HP 4914-A AC impedance / gain phase analyzer was used to investigate the electrochemistry of lithium symmetrical cells utilizing both films with and without the SAMs. This was done over a range of 40MHz to 100Hz. Test cells for impedance analysis were assembled in a glove bag under argon purge. The bag was attached directly to the vacuum oven drying the films, allowing the oven to be filled with argon as the vacuum was released. The cells were stacked by hand in the following order: plastic insulator, stainless steel lead, lithium, electrolyte film, lithium, stainless steel lead and plastic insulator. The stack was then clamped and inserted into a glass tube that was sealed and placed under vacuum. A water / ethylene glycol bath was used to maintain the cell temperature at 50618C.
131
cial in the characterization of the system. The CF 3 bending mode serves as an internal standard to which the intensities of the two CH 2 modes can be compared. If the intensities of the 720 and 730 cm 21 modes increase relative to the CF 3 mode, one can assume that more crystalline CH 2 is present. Fig. 1 shows infrared spectra for films with and without the SAMs for the surface (ATR data) and for the bulk polymer. In Fig. 1, the poly(ethylene oxide) / lithium triflate film without the molecular layer shows no absorption in the 720–730 cm 21 range, while the ATR FTIR spectrum of the SAM treated films shows significant absorption in this region. The presence of two peaks for films with the adsorbed layer indicates that the wax was successfully placed on the film and that it has a significant ordered crystalline structure. Furthermore, ATR–FTIR data indicates that the wax was placed at the film surface rather than being distributed throughout the bulk of the film. Fig. 1 illustrates this relative difference in wax concentration by comparison of the lithium triflate peak at 760 cm 21 to the 720 and 730 cm 21 CH 2 modes. The lower frequency modes have a much greater relative intensity compared to the 730 cm 21 mode for the ATR surface spectra. This relationship is reversed for the bulk IR spectra, indicating surface adsorption and not migration into the bulk. Wetting studies performed using dynamic Wilhelmy plate technique also helped to characterize the surface by showing the wax molecular layer to be of a relatively uniform concentration across the film
3. Results and discussion By investigating the 700–800 cm 21 region of the infrared spectrum, SAM placement on the films could be confirmed. This is the area where the triflate’s CF 3 bending mode and the surfactant’s CH 2 rocking mode should occur. The CH 2 rocking mode for (CH 2 ) 4 and longer hydrocarbon chains is found in this region and is known to ‘‘split’’ into two peaks appearing at approximately 720 and 730 cm 21 due to the crystal field effects of crystalline polyethylene [12]. The observation of this splitting proves benefi-
Fig. 1. FTIR spectra for polymer films comparing both bulk and surface with and without wax.
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R.N. Mason et al. / Solid State Ionics 118 (1999) 129 – 133
surface. Critical surface tension for the pure untreated PEO / lithium triflate films was found to be 38.0 dynes / cm. Critical surface tension for the SAM-treated films was shown to be 31.7 dynes / cm. This value is much closer to the accepted polyethylene reference value of 31.0 dynes / cm [13] than to that of the film without the molecular layer. The proximity of the critical surface tension suggests that the outermost surface of the SAM-treated electrolyte film closely resembles that of polyethylene, implying a relatively homogeneous coating on the surface by the CH 2 end portion of the wax molecules. Combining the infrared analysis and wetting work, the form of the wax SAM shown in Fig. 2 could be proposed. This structure can be rationalized if one assumes that the amorphous poly(ethylene oxide)like portion of the wax molecule preferentially orients itself to absorb onto the poly(ethylene oxide) electrolyte film. This leaves the polyethylene-like segment of the wax molecule to self-assemble into an ordered layer just above the electrolyte surface. The crystal field splitting of the CH 2 mode observed in the infrared spectrum would indicate that such an orderly arrangement exists, resulting in a crystalline environment. Having successfully formed the molecular layer at the polymer surface, the investigation of its electrochemical effects was conducted. AC impedance data were collected over varying time ranges. Constant temperature studies over time spans on the order of 300 h showed the development of a passivating layer at the electrode / electrolyte interface in the untreated
Fig. 2. Proposed adsorbed molecular layer having a self-assembled monolayer structure.
Fig. 3. AC impedance data for PEO / lithium triflate films @ 508C. Top: Without SAM. Bottom: With SAM
cells. This appears as the formation of a second semi-circle of resistance in the Nyquist plots in Fig. 3. Work performed by Fauteux [14] and Steele, et al. [2], also on lithium symmetrical cells, shows this second area of resistance as well. Both groups attribute this to the formation of a passivating layer. As shown in the upper plot (films without the SAM) of Fig. 3, the resistance of the passivating layer can become larger than the bulk resistance of the system. The lower plot illustrates that treating the electrolyte films with the SAM significantly reduces the rate of formation of the interfacial passivation layer. At 210 h, the second semi-circle has just begun to form for the treated cells (lower plot) whereas it is overtaking that of the bulk resistance in the untreated cells (upper plot). The possible structure of the monolayer discussed previously and shown in Fig. 1 could explain the passivation protection observed. One would expect the hydrocarbon tails, which would be in contact with the lithium metal, to perhaps ‘‘hide’’ the lithium from species that react to form the passivation layer. One would also expect that ion conduction would be inhibited as the ions moving through the bulk electrolyte to the lithium electrode would have to
R.N. Mason et al. / Solid State Ionics 118 (1999) 129 – 133
pass through these hydrocarbon tails. However, data shown in Fig. 3 indicated that adding the SAM did not decrease conductivity. The results shown are typical as films with the adsorbed layer studied at 508C reproducibly have a higher conductivity than those without the molecular layer. Whether this is due to an interfacial phenomenon or to other interactions is not known at the present time. One must assume that the hydrocarbon monolayer shown in Fig. 1 would have defects through which ions could move. It is difficult to explain how the ion conductivity could actually be enhanced, however. Alternate molecular layer structures where the hydrocarbon tails lie flat on the electrolyte surface allowing the PEO portion of the wax molecule to form ion-conducting channels may help to explain increased conductivity. Understanding the structure of this layer and the increase in ion conduction are both of scientific and technological interest and will be investigated.
4. Conclusions Placing molecular layers, which are believed to be self-assembled monolayers, of surface active molecules at the surface of PEO / lithium triflate electrolyte thin films is possible. The presence of this molecular layer would seem to reduce conductivity, but rather enhances it. By placing such a SAM at the surface of these electrolyte films, the formation of passivation at the electrode / electrolyte interface of polymer lithium cells can be inhibited. Further work in this area will delineate the true structure of the molecular layer, will investigate the effect that various waxes with different tail lengths have on molecular layer formation, and will use different
133
techniques for molecular layer placement as well as focusing on an explanation of the conductivity increase resulting from the SAM addition.
Acknowledgements The authors wish to thank the NSF EPSCoR program whose funding through grant OSR-9550478 made this work possible and Petrolite Specialty Polymers for generously providing the wax samples.
References [1] P.R. Sorensen, T. Jacobsen, Electrochim. Acta 17 (1982) 1671. [2] B.C.H. Steele, G.E. Lagos, P.C. Spurdens, C. Forsyth, A.D. Ford, Solid State Ionics 9–10 (1983) 391. [3] P.G. Bruce, F. Krok, Solid State Ionics 36 (1989) 171. [4] M. Hiratani, K. Miyauchi, T. Kudo, Solid State Ionics 28–30 (1988) 1431. [5] P.G. Bruce, F. Krok, Electrochim. Acta 33 (1988) 1669. [6] J.L. Goldman, R.M. Mank, J.H. Young, V.R. Koch, J. Electrochem. Soc. 127 (1980) 1461. [7] V.R. Koch, J.L. Goldman, C.J. Mattos, M. Mulvaney, J. Electrochem. Soc. 129 (1982) 1. [8] K.M. Abraham, J.S. Foos, G.L. Goldman, J. Electrochem. Soc. 131 (1984) 2197. [9] Z. Takehara, Z. Ogumi, K. Kanamura, Y. Uchimoto, in: Proceedings of the Symposium on Lithium Batteries, 1994, vol. 94, The Electrochemical Society, Pennington, New Jersey, p. 13. [10] D. Teeters, J.F. Wilson, M.A. Anderson, D. C Thomas, J. Colloid Int. Sci. 126 (1988) 641. [11] W.A. Zisman, in: Adv. Chem. Ser. No. 43, American Chemical Society, Washington, D.C., 1964. [12] J.L. Koenig, in: Spectroscopy of Polymers, American Chemical Society, Washington, D.C., 1992, p. 105. [13] W.A. Zisman, Ind. Eng. Chem. 55 (1963) 18. [14] D. Fauteux, Solid State Ionics 17 (1985) 133.
Using self-assembled monolayers to inhibit passivation at the lithium electrode / polymer electrolyte interface a a a b, Rachel N. Mason , Mike Smith , Thad Andrews , Dale Teeters * a
Department of Chemical Engineering, The University of Tulsa, Tulsa, OK 74104 -3189, USA b Department of Chemistry, The University of Tulsa, Tulsa, OK 74104 -3189, USA
Abstract This work investigates the use of surface chemistry to modify the lithium electrode / polymer electrolyte interface by placing a molecular layer, most likely in the form of a self-assembled monolayer (SAM), of H–(CH 2 ) 32 –(CH 2 –CH 2 –O) 10 –H onto the surface of PEO, poly(ethylene oxide), electrolyte films. It is proposed that the PEO-like ‘‘head’’ of the molecule above preferentially orients itself to absorb onto the PEO electrolyte, leaving the hydrocarbon CH 2 ‘‘tail’’ to self-assemble. SAM placement was confirmed using attenuated total reflection FTIR spectroscopy and wetting studies, and AC impedance measurements were used to investigate the passivating layer development. Extended time period studies of untreated polymer films in contact with lithium exhibited the rapid rise of an interfacial passivating layer whose resistance overtook that of the bulk electrolyte. Similar studies demonstrated that samples with SAMs actually had a small increase in ion conductivity and developed interfacial passivation much slower, supporting the assertion that SAMs could be used to deter the formation of a barrier to Li 1 transport during cycling of lithium polymer batteries. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Self-assembled monolayers; Electrode / electrolyte interface; Passivating layer
1. Introduction Lithium / polymer electrolyte batteries are receiving much attention today because they offer a number of significant advantages both in terms of a high energy density and a large electrochemical window. However, there are technological problems that must be solved before these batteries reach wide commercial utilization. One of the major problems associated with lithium polymer batteries is the formation of a passivating layer at the lithium / poly*Corresponding author. Tel.: 1 1-918-631-3147; fax: 1 1-918631-3404; e-mail: chem [email protected] ]
mer electrolyte interface. Although the mechanism of ion conduction through the bulk matrix of the polymer has received much attention, it has become increasingly obvious that the electrochemical behavior at the electrode / electrolyte interface plays a major role in determining the critical operating parameters of a device, whether the device is a high energy density battery, a fuel cell, or a sensor. There have been numerous studies of processes involving lithium or lithium alloy anodes in contact with electrolytes consisting of lithium salts complexed with poly(ethylene oxide) polymers [1–5]. Work done by Bruce and Krok [3] using AC impedance methods strongly suggested that surface
0167-2738 / 99 / $ – see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00421-4
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R.N. Mason et al. / Solid State Ionics 118 (1999) 129 – 133
layers develop on both electrode interfaces, as indicated by the increasing interfacial impedances. The authors were able to eliminate the possibility of impurity contribution to this interfacial impedance, but did not offer any insight into the nature of the surface layer. In a related study, Hiratani, et al., [4] investigated the interface between a lithium and a solid polymer electrolyte consisting of a thin film (150 mm) of PEO complexed with 9 mole % LiCF 3 SO 3 . These studies also utilized complex impedance spectroscopy and indicated an increase in the interfacial resistance which was attributed to a charge transfer process. The formation of passivating layers can severely affect the performance of polymer electrolyte cells. It has been shown that, with time, interfacial impedance in lithium / polymer electrolytes systems can grow until it is significantly larger than the bulk resistance [3,5]. Therefore, methods of minimizing certain reactions at the interface are advantageous to cell development. One way to approach the passivation problem at the lithium polymer / electrolyte interface is to study and modify the lithium metal surface [6–8]. Most of this work involves chemical reactions forming ROCO 2 OLi compounds on the surface of the lithium anode resulting in a passivating layer that is more conducive to recycling. Work by Takechara, et al. [9] further indicates the possibility of placing a thin polymer film on lithium surfaces to promote a surface more conducive to battery performance. If the lithium surface could be modified for enhanced properties, could the polymer electrolyte surface be modified for better interface behavior too? Modifying the polymer surface would have several potential advantages. A polymer surface is a much easier surface to work with than the extremely reactive lithium surface. At the same time, one can also conduct more chemistry on polymer surfaces because of the variety of functional groups that can be present. If passivation protection could be ‘‘built into’’ the polymer electrolyte surface, a convenient method of enhanced performance would result. This work is concerned with the formation of molecular layers, most likely of the self–assembled monolayer (SAM) type on the polymer electrolyte interface. These molecular films have been formed by adsorption of surface active agents from solution onto
the polymer electrolyte surfaces and appear to have the potential to protect the interface and enhance cell performance.
2. Experimental The PEO films were made by dissolving 900,000 MW polymer and lithium triflate, (both obtained from Aldrich) in Baker HPLC grade acetonitrile, which had been dried by refluxing over CaH 2 . The LiSO 3 CF 3 was dried in vacuum oven at 1108C overnight. The polymer–triflate solutions were then cast on a poly(tetrafluoroethylene) (PTFE) substrate and the solvent was allowed to evaporate in a high purity argon atmosphere. The resulting films were dried at 808C in a vacuum for two days and then stored in a vacuum dessicator. The thickness of the films ranged from 80 to 120 mm. A semicrystalline wax from Petrolite Specialty Polymers, which could best be described as H–(CH 2 –CH 2 ) 32 –(CH 2 –CH 2 –O–) 10 –H with an average molecular weight of 900, was used to form SAMs on the polymer surface. The wax was dissolved in hexane where it acted like a surfactant resulting in the SAM formation being accomplished via adsorption from solution. A concentrated solution of the wax in Fisher optima grade hexane was prepared and a selected electrolyte film was placed in the solution and allowed to equilibrate for at least 72 h. Shorter equilibration times did not result in good SAM formation. The film was removed and quickly rinsed in pure hexane to remove wax that might deposit from evaporation of the hexane–wax solution. Films not receiving SAMs were soaked in neat hexane for an equal period of time in order to make certain that all films were treated similarly. These films were dried in a vacuum oven again and stored in a vacuum dessicator. Wetting studies were performed using the dynamic Wilhelmy plate technique [10]. This allows for surface analysis by examining the solid–liquid contact angle of the polymer electrolyte films versus different liquids. The Zisman technique [11] was used to calculate critical surface tensions, which are useful in comparing surface properties of solid substrates. All wetting studies were conducted in an argon atmosphere glove box.
R.N. Mason et al. / Solid State Ionics 118 (1999) 129 – 133
Infrared spectra were collected on a Nicolet 510P Fourier Transform Infrared Spectrometer with a DTGS detector at 1 cm 21 spectral resolution. Surface infrared spectra were obtained using the attenuated total reflection (ATR) technique. The ATR spectra were obtained by using a Spectra–Tech Horizontal Contact Sampler ATR with a slurry sample boat type cell and a ZnSe crystal cut at 458. The average depth of infrared radiation penetration into the sample was 0.5 microns. The films to be studied were quickly removed from their dessicator and placed in the bottom of the sample boat trough and sealed with a rubber cover. This ATR sampling arrangement was used so that the films could be protected from water in the atmosphere. Bulk film infrared transmission spectra were collected in the sample compartment of the Nicolet 510P while under a nitrogen purge. An HP 4914-A AC impedance / gain phase analyzer was used to investigate the electrochemistry of lithium symmetrical cells utilizing both films with and without the SAMs. This was done over a range of 40MHz to 100Hz. Test cells for impedance analysis were assembled in a glove bag under argon purge. The bag was attached directly to the vacuum oven drying the films, allowing the oven to be filled with argon as the vacuum was released. The cells were stacked by hand in the following order: plastic insulator, stainless steel lead, lithium, electrolyte film, lithium, stainless steel lead and plastic insulator. The stack was then clamped and inserted into a glass tube that was sealed and placed under vacuum. A water / ethylene glycol bath was used to maintain the cell temperature at 50618C.
131
cial in the characterization of the system. The CF 3 bending mode serves as an internal standard to which the intensities of the two CH 2 modes can be compared. If the intensities of the 720 and 730 cm 21 modes increase relative to the CF 3 mode, one can assume that more crystalline CH 2 is present. Fig. 1 shows infrared spectra for films with and without the SAMs for the surface (ATR data) and for the bulk polymer. In Fig. 1, the poly(ethylene oxide) / lithium triflate film without the molecular layer shows no absorption in the 720–730 cm 21 range, while the ATR FTIR spectrum of the SAM treated films shows significant absorption in this region. The presence of two peaks for films with the adsorbed layer indicates that the wax was successfully placed on the film and that it has a significant ordered crystalline structure. Furthermore, ATR–FTIR data indicates that the wax was placed at the film surface rather than being distributed throughout the bulk of the film. Fig. 1 illustrates this relative difference in wax concentration by comparison of the lithium triflate peak at 760 cm 21 to the 720 and 730 cm 21 CH 2 modes. The lower frequency modes have a much greater relative intensity compared to the 730 cm 21 mode for the ATR surface spectra. This relationship is reversed for the bulk IR spectra, indicating surface adsorption and not migration into the bulk. Wetting studies performed using dynamic Wilhelmy plate technique also helped to characterize the surface by showing the wax molecular layer to be of a relatively uniform concentration across the film
3. Results and discussion By investigating the 700–800 cm 21 region of the infrared spectrum, SAM placement on the films could be confirmed. This is the area where the triflate’s CF 3 bending mode and the surfactant’s CH 2 rocking mode should occur. The CH 2 rocking mode for (CH 2 ) 4 and longer hydrocarbon chains is found in this region and is known to ‘‘split’’ into two peaks appearing at approximately 720 and 730 cm 21 due to the crystal field effects of crystalline polyethylene [12]. The observation of this splitting proves benefi-
Fig. 1. FTIR spectra for polymer films comparing both bulk and surface with and without wax.
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R.N. Mason et al. / Solid State Ionics 118 (1999) 129 – 133
surface. Critical surface tension for the pure untreated PEO / lithium triflate films was found to be 38.0 dynes / cm. Critical surface tension for the SAM-treated films was shown to be 31.7 dynes / cm. This value is much closer to the accepted polyethylene reference value of 31.0 dynes / cm [13] than to that of the film without the molecular layer. The proximity of the critical surface tension suggests that the outermost surface of the SAM-treated electrolyte film closely resembles that of polyethylene, implying a relatively homogeneous coating on the surface by the CH 2 end portion of the wax molecules. Combining the infrared analysis and wetting work, the form of the wax SAM shown in Fig. 2 could be proposed. This structure can be rationalized if one assumes that the amorphous poly(ethylene oxide)like portion of the wax molecule preferentially orients itself to absorb onto the poly(ethylene oxide) electrolyte film. This leaves the polyethylene-like segment of the wax molecule to self-assemble into an ordered layer just above the electrolyte surface. The crystal field splitting of the CH 2 mode observed in the infrared spectrum would indicate that such an orderly arrangement exists, resulting in a crystalline environment. Having successfully formed the molecular layer at the polymer surface, the investigation of its electrochemical effects was conducted. AC impedance data were collected over varying time ranges. Constant temperature studies over time spans on the order of 300 h showed the development of a passivating layer at the electrode / electrolyte interface in the untreated
Fig. 2. Proposed adsorbed molecular layer having a self-assembled monolayer structure.
Fig. 3. AC impedance data for PEO / lithium triflate films @ 508C. Top: Without SAM. Bottom: With SAM
cells. This appears as the formation of a second semi-circle of resistance in the Nyquist plots in Fig. 3. Work performed by Fauteux [14] and Steele, et al. [2], also on lithium symmetrical cells, shows this second area of resistance as well. Both groups attribute this to the formation of a passivating layer. As shown in the upper plot (films without the SAM) of Fig. 3, the resistance of the passivating layer can become larger than the bulk resistance of the system. The lower plot illustrates that treating the electrolyte films with the SAM significantly reduces the rate of formation of the interfacial passivation layer. At 210 h, the second semi-circle has just begun to form for the treated cells (lower plot) whereas it is overtaking that of the bulk resistance in the untreated cells (upper plot). The possible structure of the monolayer discussed previously and shown in Fig. 1 could explain the passivation protection observed. One would expect the hydrocarbon tails, which would be in contact with the lithium metal, to perhaps ‘‘hide’’ the lithium from species that react to form the passivation layer. One would also expect that ion conduction would be inhibited as the ions moving through the bulk electrolyte to the lithium electrode would have to
R.N. Mason et al. / Solid State Ionics 118 (1999) 129 – 133
pass through these hydrocarbon tails. However, data shown in Fig. 3 indicated that adding the SAM did not decrease conductivity. The results shown are typical as films with the adsorbed layer studied at 508C reproducibly have a higher conductivity than those without the molecular layer. Whether this is due to an interfacial phenomenon or to other interactions is not known at the present time. One must assume that the hydrocarbon monolayer shown in Fig. 1 would have defects through which ions could move. It is difficult to explain how the ion conductivity could actually be enhanced, however. Alternate molecular layer structures where the hydrocarbon tails lie flat on the electrolyte surface allowing the PEO portion of the wax molecule to form ion-conducting channels may help to explain increased conductivity. Understanding the structure of this layer and the increase in ion conduction are both of scientific and technological interest and will be investigated.
4. Conclusions Placing molecular layers, which are believed to be self-assembled monolayers, of surface active molecules at the surface of PEO / lithium triflate electrolyte thin films is possible. The presence of this molecular layer would seem to reduce conductivity, but rather enhances it. By placing such a SAM at the surface of these electrolyte films, the formation of passivation at the electrode / electrolyte interface of polymer lithium cells can be inhibited. Further work in this area will delineate the true structure of the molecular layer, will investigate the effect that various waxes with different tail lengths have on molecular layer formation, and will use different
133
techniques for molecular layer placement as well as focusing on an explanation of the conductivity increase resulting from the SAM addition.
Acknowledgements The authors wish to thank the NSF EPSCoR program whose funding through grant OSR-9550478 made this work possible and Petrolite Specialty Polymers for generously providing the wax samples.
References [1] P.R. Sorensen, T. Jacobsen, Electrochim. Acta 17 (1982) 1671. [2] B.C.H. Steele, G.E. Lagos, P.C. Spurdens, C. Forsyth, A.D. Ford, Solid State Ionics 9–10 (1983) 391. [3] P.G. Bruce, F. Krok, Solid State Ionics 36 (1989) 171. [4] M. Hiratani, K. Miyauchi, T. Kudo, Solid State Ionics 28–30 (1988) 1431. [5] P.G. Bruce, F. Krok, Electrochim. Acta 33 (1988) 1669. [6] J.L. Goldman, R.M. Mank, J.H. Young, V.R. Koch, J. Electrochem. Soc. 127 (1980) 1461. [7] V.R. Koch, J.L. Goldman, C.J. Mattos, M. Mulvaney, J. Electrochem. Soc. 129 (1982) 1. [8] K.M. Abraham, J.S. Foos, G.L. Goldman, J. Electrochem. Soc. 131 (1984) 2197. [9] Z. Takehara, Z. Ogumi, K. Kanamura, Y. Uchimoto, in: Proceedings of the Symposium on Lithium Batteries, 1994, vol. 94, The Electrochemical Society, Pennington, New Jersey, p. 13. [10] D. Teeters, J.F. Wilson, M.A. Anderson, D. C Thomas, J. Colloid Int. Sci. 126 (1988) 641. [11] W.A. Zisman, in: Adv. Chem. Ser. No. 43, American Chemical Society, Washington, D.C., 1964. [12] J.L. Koenig, in: Spectroscopy of Polymers, American Chemical Society, Washington, D.C., 1992, p. 105. [13] W.A. Zisman, Ind. Eng. Chem. 55 (1963) 18. [14] D. Fauteux, Solid State Ionics 17 (1985) 133.