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Electrochemical characterization of plasticized polyelectrolyte based on lithium-N(4-sulfophenyl) maleimide
PERGAMON
Electrochimica Acta 44 (1999) 2287±2296
Electrochemical characterization of plasticized polyelectrolyte based on lithium-N(4-sulfophenyl) maleimide Wu Xu, Kok Siong Siow *, Zhiqiang Gao, Swee Yong Lee Department of Chemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Republic of Singapore Received 20 July 1998
Abstract Thermal behavior and electrochemical properties of propylene carbonate (PC) plasticized polyelectrolyte based on poly[lithium-N(4-sulfophenyl) maleimide-co-methoxy oligo(oxyethylene) methacrylate] have been investigated. This PC plasticized polyelectrolyte with a PC content of 54.5 wt% has an ionic conductivity of 1.8 10ÿ5 S cmÿ1 at ambient temperature and a wide electrochemical window. Passivation phenomena occurring at the interface between the PC plasticized polyelectrolyte and a Li electrode were particularly evaluated and discussed. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Plasticized polyelectrolyte; Ionic conductivity; Lithium interface; Impedance spectroscopy
1. Introduction Polymer electrolytes have been studied extensively in the last two decades [1±5]. It has been found that ionic conductivities of single ionic conductive polymer electrolytes without any organic solvents are too low (about 10ÿ8010ÿ6 S cmÿ1) for them to be applicable in practical uses. A quick and convenient way to improve the conductivities of these polymer electrolytes is to add some organic solvents, i.e. plasticizers, with high polarity, high boiling point, low viscosity and high stability with electrolytes and electrodes. The increase in ionic conductivity can be one to two orders of magnitude, which makes the polymer electrolytes suitable for lithium rechargeable batteries. However, for the development of lithium batteries based on polymer electrolytes, high ionic transport properties at ambient temperature are not the only parameters in selecting a proper polymer electrolyte because good mechanical properties, a wide electrochemical stability window and especially low resistance of
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the Li/polymer electrolyte interface and interface stability are also required to assure good cyclability and long-life to the batteries. In this paper, we report the thermal and electrochemical characteristics of a propylene carbonate (PC) plasticized polyelectrolyte based on a novel type of comblike, nearly alternating copolyether, poly[lithiumN(4-sulfophenyl) maleimide-co-methoxy oligo(oxyethylene) methacrylate] with 12 oxyethylene repeating units in the oligoether side chain [abbreviated as P(LiSMOE12)] [6], whose structure is shown in Scheme 1. The Li/polyelectrolyte interface stability over time is also investigated by impedance spectroscopy and the results are included here.
2. Experimental 2.1. Material and sample preparations The comblike, nearly alternating copolyether electrolyte, poly[lithium-N(4-sulfophenyl) maleimide-co-methoxy oligo(oxyethylene) methacrylate] with the side chain of 12 oxyethylene repeating units, [P(LiSMOE12)], was synthesized as mentioned in our
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 3 4 8 - X
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The stability of the lithium/polyelectrolyte interface was investigated by monitoring the time dependence of the impedance of cells with P(LiSMOE12)-PC between lithium symmetric non-blocking electrodes, utilizing the same instrument used in the case of the conductivity measurements except for a dierent frequency range, from 1 MHz to 10 mHz. 2.4. Cyclic voltammetry measurements Scheme 1. Structure of the nearly alternating polyelectrolyte P(LiSMOE12).
previous report [6]. The solution of P(LiSMOE12) in methanol was cast on a Te¯on dish and the solvent was allowed to evaporate at room temperature. The ®lm obtained was dried thoroughly in a vacuum oven at about 808C for 3 days to remove residual moisture. The ®lm disks were impregnated with dierent amounts of re-distilled PC according to the required composition of PC in the plasticized polyelectrolyte, ranging from 9.1 to 61.5 wt% and kept in a glove box ®lled with puri®ed argon for at least one day in order for the PC to become well-distributed in the electrolyte. For convenience, the PC plasticized poly[lithiumN(4-sulfophenyl) maleimide-co-methoxy oligo(oxyethylene) methacrylate] was abbreviated as P(LiSMOE12)PC. 2.2. Thermal analyses Dierential scanning calorimetric (DSC) analyses of these P(LiSMOE12)-PC electrolytes were carried out using a DuPont Instrument 2200 Thermal Analyzer. Weighed samples (about 15 mg) were encapsulated in aluminum pans and heated at 108C minÿ1 in the temperature range of ÿ140 to 1408C under a nitrogen atmosphere. The glass transition temperature (Tg) was taken as the midpoint temperature of the baseline shift observed from the second scan. 2.3. Impedance measurements The ionic conductivity plots of the P(LiSMOE12)-PC electrolytes were determined by measuring the temperature dependence of the impedance spectra of cells formed by sandwiching the given sample between two stainless steel blocking electrodes. The measurements were performed using a Zahner Electrik Impedance Measurement Unit (IM6) interfaced with a personal computer, over a frequency range from 1 MHz to 1 Hz. A 5 mV ac amplitude was used and ten points/decade were taken. A thermostatic bath with 0.18C precision was used to control the temperature.
The kinetics of the lithium deposition±stripping process from P(LiSMOE12)-PC electrolytes and the electrochemical stability window of these plasticized electrolytes were evaluated by cyclic voltammetric (CV) curves obtained at 258C in a three-electrode cell having a stainless steel working electrode, lithium counter and lithium reference electrodes. The cyclic voltammetry measurements were run and controlled by using a computer interfaced 273A PAR potentiostat/galvanostat. All the cells were prepared and sealed in an argon®lled glove box. 3. Results and discussion 3.1. Thermal characteristics The DSC curves of these P(LiSMOE12)-PC electrolytes with dierent PC content are shown in Fig. 1. It is seen that these PC plasticized P(LiSMOE12) electrolytes show two glass transitions in the studied temperature range, one is below ÿ508C and the other is around the ambient temperature. As mentioned in our previous report [6], P(LiSMOE12) is a copolymer with two glass transitions. The ®rst glass transition (Tg1) at about ÿ57.08C is attributed to the oligoether side chain and the second one (Tg2) around 32.38C is assigned to the main chain of the polymer host. Therefore, the two glass transitions (Tg1 and Tg2) in the PC plasticized P(LiSMOE12) electrolytes are, respectively, assigned to the oligoether side chain and the main chain of the copolymer. On the other hand, it is indicated from Fig. 1 that the addition of plasticizer PC into P(LiSMOE12) leads to the disappearance of the crystalline phase (Tm around 08C) existing in the pure copolymer P(LiSMOE12). In addition, the presence of PC is also responsible for the decrease of the two glass transitions temperatures (Tg1 and Tg2) in respect to those of the nearly alternating copolymer electrolyte without plasticizer, P(LiSMOE12). The in¯uence of plasticizer content on the two glass transitions temperatures is shown in Fig. 2. With increasing PC content in the electrolyte, both Tg1 and Tg2 decrease. In the studied PC content range, Tg1 decreases from ÿ57.08C for non-PC electrolyte to ÿ90.28C for a PC content of 61.5 wt%, while Tg2
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Fig. 1. DSC curves of PC plasticized P(LiSMOE12) electrolytes. PC content in the electrolyte: (a) none; (b) 9.1%; (c) 18.7%; (d) 28.6%; (c) 37.5%; (f) 44.4%; (g) 54.5% and (h) 61.5%.
decreases from 32.3 to 22.48C. The above results demonstrate the obvious plasticization eect of PC on P(LiSMOE12). It is also implied that with the addition of more and more PC into P(LiSMOE12), the P(LiSMOE12)-PC system forms a relatively homogeneous mixture. Gray [7] and Cameron et al. [8] have studied the PC plasticized PEO±LiX complexes. They proposed that there existed separate ionic pathways through the plasticizer in the PEO matrix and that the ionic motion was decoupled from that of the polymeric solvent. Xu et al. [9] observed the separate phases of crystalline PEO and liquid g-butyrolactone (BL) when studying the in¯uence of compatibility of plasticizers with polymeric matrix PEO on the ionic conduction using polar
microscopic photography, which supports the proposal of Cameron et al. [8]. In PEO±LiX-plasticizer systems where the plasticizers are PC, BL, etc., PC or BL extracts salt from the PEO±LiX complex and forms separate domains which serve as the ionic conducting path or channel with high ionic conductivity. On the other hand, Xu et al. [9] also reported that the plasticizer 2,5-di(methyl diglycol)-1,4;3,6-dianhydrous sorbitol ether (abbreviated as DGS) formed a homogeneous system with PEO/EO2PSLi (lithium methoxy di(ethyleneoxy) phenylsulfonate) because of the good compatibility of DGS with PEO. Lee et al. [10] synthesized a new plasticizer by modifying propylene carbonate, 2keto-4-(2,5,8,11-tetraoxadodecyl)-1,3-dioxolane (MC3). They found that the addition of MC3 into the PEO/ LiCF3SO3 complex formed a homogeneous mixture in both the crystalline and amorphous state and increased the ratio of amorphous to crystalline states in the mixture. This was due to the increased compatibility of MC3 with PEO that accrued from the ethylene oxide side chain. In the present case, since the eect of lithium N(4-sulfophenyl) maleimide on the oligo(oxyethylene), this nearly alternating copolymer has a higher polarity than pure PEO has, which in turn improves the compatibility of the copolymer P(LiSMOE12) with the plasticizer PC and favors the formation of a homogeneous system of P(LiSMOE12)PC. In fact, the plasticizer PC added into P(LiSMOE12) may form nanoscaled `pockets' of PC embedded in the polyelectrolyte [11]. 3.2. Ionic conductivity
Fig. 2. The in¯uence of PC content on the two glass transitions temperatures (Tg1 and Tg2) of the plasticized P(LiSMOE12)-PC electrolytes.
Fig. 3 shows the in¯uence of PC content in the plasticized polyelectrolyte on the isothermal ionic conductivity. The ionic conductivity increases with increasing
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Fig. 3. The eect of PC content on the isothermal ionic conductivity of P(LiSMOE12)PC electrolytes.
Fig. 4. Temperature dependence of ionic conductivity of P(LiSMOE12)-PC electrolytes with dierent PC content.
PC content, which re¯ects the normal in¯uence of plasticizer content on the ionic conductivity of polymer solid electrolytes. With increasing PC content in the system of P(LiSMOE12)-PC, the salt content eectively decreases. However, the number of charge carriers in the polyelectrolyte may increase within a certain range of PC content due to the strong ion-pair dissociation eect of PC on the sulfonic lithium salt. In addition, both Tg1 and Tg2 of the polyelectrolyte decrease with increasing PC content and decreasing salt content, especially Tg1 decreases drastically. Thus, the microscopic viscosity of the electrolyte decreases and the segmental movements of polymer macromolecules can be enhanced. Therefore, the ionic conductivity increases with increasing PC content in the electrolyte. The ionic transport is dependent on a cooperative eect of the polymer and PC. Once the PC content is higher than 54.5 wt%, the ionic conductivity of the P(LiSMOE12)-PC electrolytes does not increase any further. The ionic conductivity increases from 4.1 10ÿ8 to 1.8 10ÿ5 S cmÿ1 at 258C when PC content increases from 0 to 54.5 wt%. It is shown that the ionic conductivity can be improved by a magnitude of more than two orders as compared with the unplasticized polyelectrolyte. The plots of logarithmic ionic conductivity against the reciprocal temperature in the range from 10 to 808C for the P(LiSMOE12)-PC electrolytes with dierent PC contents are illustrated in Fig. 4. The experimental results show that with increasing PC content, the temperature dependence of ionic conductivity changes gradually from a curve to an in¯ected straight line and ®nally to a complete straight line. The temperature dependence of ionic conductivity for the P(LiSMOE12)-PC electrolyte with a PC content of 9.1 wt% shows a curve which resembles that of the
unplasticized P(LiSMOE12), implying that the ionic transport in this plasticized polyelectrolyte is still mainly dependent on the segmental movements of polymer macromolecules. On the other hand, the correlation between the ionic conductivity and the reciprocal temperature for the P(LiSMOE12)-PC electrolyte with a PC content of 18.7 and 28.6 wt% show an in¯ected straight line with a turning point at the temperature near 308C where the second glass temperature Tg2 is located. With the increase of PC content, the higher polarity of PC compared to that of the oligoether side chain leads PC to extract more conducting lithium ions from the co-ordination to the oligoether oxygen atoms into the nanoscaled PC `pockets' and favors the transport of free ions in PC because the activation energy for ion transport in PC is lesser than that through the segmental movements of polymer macromolecules. However, the PC content in these electrolytes is not sucient to extract all salt ions from the co-ordination to the oligoether oxygen atoms. Thus, the second glass transition of the polymer would aect the ionic transport, which leads to an in¯ection of the correlation between ionic conductivity and temperature. When the PC content in the plasticized polyelectrolyte exceeds 37.5 wt%, the sucient PC can extract nearly all conducting ions into the nanoscaled PC `pockets' and favors the ion carriers to transport it. The temperature dependence of ionic conductivity, therefore, shows a complete straight line. The straight lines of the temperature dependence of ionic conductivity demonstrate that the ionic conduction in the highly plasticized polyelectrolyte follows the Arrhenius behavior. It is dierent from that in the unplasticized P(LiSMOE12) or the electrolyte with low PC content, where the ion transport depends on the segmental movements of polymer macromolecules in
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the amorphous region and follows the free volume model. It is revealed that with increasing PC content in the electrolyte, the ionic conductive behavior in the electrolyte changes gradually from non-Arrhenius to Arrhenius type. 3.3. Lithium deposition±stripping process and electrochemical stability window The kinetics of the lithium deposition±stripping process, Li eÿ $ Li
1
from P(LiSMOE12)-PC electrolyte samples on stainless steel substrate was determined by cyclic voltammetry (CV) at 258C, as described in Section 2. Fig. 5 shows a typical CV result obtained in a three-electrode cell with a PC content of 54.5 wt% in P(LiSMOE12)-PC electrolyte in the potential range from +2.0 to ÿ0.5 V at a scan rate of 10 mV sÿ1. A single cathodic (deposition) peak and a single corresponding anodic (stripping) peak are found. The anodic peak of lithium stripping is not well de®ned as compared with that in many geltype electrolytes [12]. The peak potential dierence between the anodic and the cathodic peaks is as large as 1.4 V, implying that this lithium deposition±stripping process is irreversible. Due to the much lower ionic conductivity of P(LiSMOE12)-PC than that of other gel-type electrolytes, the anodic peak appears at a potential more positive than that in other gel-type electrolytes and the peak currents are just in the range of 70 mA cmÿ2. To evaluate the electrochemical stability window of the P(LiSMOE12)-PC electrolyte, a cell with stainless steel as working electrode and lithium as counter and
Fig. 5. Cyclic voltammetry at 258C of the lithium deposition± stripping process from a P(LiSMOE12)-PC electrolyte on a stainless steel substrate with a lithium reference. Scan rate: 10 mV sÿ1.
Fig. 6. Cyclic voltammetric response at 258C of a P(LiSMOE12)-PC electrolyte on a stainless steel electrode with a lithium reference. Scan rate: 10 mV sÿ1.
reference electrodes was assembled. The CV result of a typical P(LiSMOE12)-PC electrolyte shown in Fig. 6 does not indicate any obvious electrochemical reactions in the potential ranging between 0 and 5 V. The nucleation loop over 5.3 V suggests that a new phase has formed due to the oxidization of the electrolyte. An electrochemical window greater than 4.5 V is an important feature for a polymer electrolyte because it renders polymer electrolytes suitable in many lithium based batteries applications. This result suggests that the P(LiSMOE12)-PC electrolyte indeed has a good electrochemical stability window. 3.4. Stability of lithium electrode surface To investigate the compatibility of the polyelectrolyte with electrode materials and in an attempt to determine the stability of the lithium/P(LiSMOE12)-PC electrolyte interface over time, a detailed impedance analysis of a symmetric cell of the type Li/ P(LiSMOE12)-PC/Li, stored and tested under open circuit conditions at room temperature, was carried out. Fig. 7 shows the ac impedance spectra recorded at progressively longer storage time at 258C under open circuit conditions. It is seen that the overall ac impedance response is composed of an incomplete semicircle (or arc) in the high frequency range, a distorted semicircle in the middle frequency range and a spur in the low frequency range. The related Cole±Cole (ÿZIM versus ZRE) plots over time illustrate an increase in amplitude for the high frequency arc and an irregular variation in amplitude for the distorted middle frequency semicircle. Since no signi®cant aging process in the PC plasticized polyelectrolyte with stainless steel blocking electrodes was observed in such a storage period, which is shown in Fig. 8 reporting the resistivity (r) value of P(LiSMOE12)-PC electrolyte over time at 258C, thus the expansion of the high frequency arc
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Fig. 8. Variation of the resistivity (r) of P(LiSMOE12)-PC with storage time in a SS/P(LiSMOE12)-PC/SS cell stored and tested at 258C. PC content in the electrolyte is 54.5 wt%.
Fig. 7. Impedance responses of a Li/P(LiSMOE12)-PC/Li cell at dierent storage time stored at 258C under open circuit conditions. PC content in the electrolyte is 54.5 wt%.
over time and the variation of the middle frequency semicircle in the Li symmetric cells reveal the growth of a resistive layer, i.e. the passivation layer, on the lithium electrode surface. The phenomenon of the lithium electrode being passivated when in contact with the electrolyte is not surprising. This behavior has been observed and studied in many reports [12±16]. This passivation layer can be thought of as the result of the corrosion reactions of Li electrode in the electrolyte medium, since some of the electrolyte components, e.g. PC, the carbonyl groups on the polymer chains, the residual moisture and impurities in the electrolyte, are known to be able to react with lithium electrode [12, 15±18]. By using a model equivalent circuit which could represent the electrical equivalent of the lithium electrode interface, it is possible to separate the various impedance parameters which contribute to the responses illustrated in Fig. 7. In this paper, the solid-polymerlayer (SPL) model suggested by Thevenin and Muller [18] has been adopted and modi®ed to ®t the impedance data. The SPL model basically describes the Li passivated surface area as consisting of a dispersion of inorganic solid compounds (maybe the products of the reaction of metallic Li with the electrolyte components) in a polymer matrix (perhaps resulting from the Li-initiated solvent polymerization). Therefore, the total impedance of a passivated Li electrode could be taken as being composed of the resistance of the electrolyte ®lm, the impedance of the passivation ®lm and
the impedance of the charge-transfer process across the Li/passivation ®lm/electrolyte interfaces [13, 18]. As a result, the equivalent circuit for lithium symmetric cells should be that of Fig. 9(a), where Rb and Cg represent the bulk resistance of ion transport in the electrolyte and the geometrical capacitance of the electrolyte; Rpf and Cpf represent the resistance and capacitance of the
Fig. 9. (a) Equivalent circuit of a modi®ed solid-polymer layer (M-SPL) model. (b) Typical impedance response of a Li/ P(LiSMOE12)-PC/Li cell stored at 258C with superimposed the three semicircles obtained by ®tting the data with the MSPL equivalent circuit.
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passivation ®lm; Rct and Cdl represent the chargetransfer resistance of lithium cations across the Li/passivation ®lm/electrolyte interfaces and the double layer capacitance of these interfaces and Zw is the Warburg impedance, which is associated with ionic diusion arising from the presence of concentration gradients within the polymer electrolyte. The modi®ed solidpolymer-layer model was abbreviated as M-SPL model. Using the related M-SPL equivalent circuit, the impedance responses can be analyzed by superimposing the best ®t of the dispersion data. Fig. 9(b) shows one of the responses and its ®tted dispersion data. The ®tted curves reveal that the overall responses could eectively be described as a combination of three-dispersion semicircles and a spur, associated with dierent relaxation phenomena. According to the M-SPL circuit, the high frequency arc can be ascribed to the P(LiSMOE12)-PC electrolyte, the distorted semicircle in the middle frequency range to the passivation ®lm and the charge-transfer process and the low frequency spur to the diusion process. On the basis of the equivalent circuit, it is then possible to determine the time evolution of the single circuit parameters. Fig. 10 illustrates the time evolution of the resistance values of a lithium symmetric cell with P(LiSMOE12)-PC electrolyte, the passivation ®lm Rpf, the charge-transfer process Rct and the bulk electrolyte Rb. The values of Rpf and Rct increase sharply within the initial 8 h after the assembly of the lithium cells, then decrease and increase in an irregular way. This phenomenon may be explained as follows: just when the cell is assembled, there are no reactions
Fig. 10. Time evolution of the resistance of the bulk electrolyte (Rb), of the passivation ®lm (Rpf) and of the charge transfer (Rct) of a Li/P(LiSMOE12)-PC/Li cell stored at 258C under open circuit. PC content in the electrolyte is 54.5 wt%.
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occurring between the lithium electrode and the polymer electrolyte components. Therefore, the impedance spectrum should show a standard semicircle in the middle frequency range for the charge-transfer process. With the increase of storage time, the components in the electrolyte react with lithium electrode to form a passivation layer between the electrode and the electrolyte. This leads to the rapid increase of the resistance of the passivation layer, which in turn increases the charge-transfer resistance. The rapid reactions between the electrolyte components and the Li electrode cause an increase in the thickness of the passivation ®lm which surpass the changes of the passivation layer compositions during the initial period until the thickness of the passivation ®lm reaches a certain degree, the morphological modi®cation processes of the passivation ®lm then result in the variation (decrease and increase) of the resistance of Rpf and Rct in an irregular way. It is implied that the nature of the passivation ®lm may progressively change under prolonged storage time. The decrease of Rct after a storage time of 72 h implies that some porous structures may have formed in the passivation ®lm because of the formation of propylene and Li2CO3 by the chemical decomposition of PC in the presence of lithium [19, 20], of polypropylene oxide P(PO) and carbon dioxide by the polymerization of PC [18], and of Li2O, LiOH and hydrogen by the reaction of residual moisture with lithium [21±23]. Thus, the surface area of the passivation ®lm would increase, which in turn causes the decrease of Rpf and Rct. On the other hand, the resistance of the electrolyte Rb also increases with storage time. It is probably due to the cross-linkage of the polymer macromolecules through the free radical couplings which lead to new C±C bonds and by the co-ordination of lithium cations to the oxygen atoms in ethyleneoxy repeating units, as described in our previous report [17]. After 10 days of storage, the value of Rb is nearly twice its original value, implying that the electrolyte is much less conductive now or the nature of the electrolyte has changed substantially. Further information on the characteristics of the lithium passivation process may be obtained by examining the time evolution of the capacitance of the passivation layer, more exactly, the double layer of the interface (Cdl) between the lithium and polymer electrolyte, which is obtained by the relation of Cdl=1/ (2pf *Rct) where f * is the top frequency of the impedance spectrum for the charge-transfer process, as shown in Fig. 11. It is seen that the value of Cdl decreases sharply in the initial storage period and then constantly decreases in a slow way with the increase of storage time. According to the following expression of capacitance,
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Fig. 11. Time evolution of the double layer capacitance (Cdl) at the interface of a Li/P(LiSMOE12)-PC/Li cell stored at 258C under open circuit. PC content in the electrolyte is 54.5 wt%.
C Ae0 er =l
2
where e0 is the dielectric constant of vacuum, er is the relative dielectric constant of the studied ®lm, A is the ®lm surface area and l is the ®lm thickness, the change in Cdl shown in Fig. 11 may be due to an increase in the thickness of the passivation ®lm (lpf) or a variation of the nature of the passivation ®lm (er,pf) or a combination of the two factors since the surface area of the passivation ®lm (A) has a tendency to increase as stated above. Therefore, it is reasonable to assume that the decrease of Cdl within the initial period after the assembly of the cell can most likely be associated with the rapid increase of the passivation ®lm thickness. The above discussion indicates that the passivation process between the interface of lithium electrode and polymer electrolyte P(LiSMOE12) severely aects the interfacial conditions and the chemical nature of the polymer electrolyte. This fact can be explained by assuming a progressive and irreversible variation of the structure of passivation ®lm which may be associated with a modi®cation of its chemical nature. 3.5. Temperature dependence of impedance spectra
Fig. 12. The in¯uence of temperature on the impedance responses for a cell of Li/P(LiSMOE12-PC)/Li stored at room temperature under open circuit conditions for 15 days prior to measurement.
As it is known, with an increase in temperature, the ionic conductivity of a polymer electrolyte will increase. This conclusion can also be con®rmed by the variation of the impedance spectra with temperature. Fig. 12 illustrates a typical relationship between the temperature and the impedance plots for a lithium symmetric cell with a P(LiSMOE12)-PC electrolyte, which had been stored in a dry box at 258C under open circuit for two weeks prior to measurement. With increasing temperature, the amplitude of the distorted semicircle in the middle frequency range diminishes
sharply as well as the decrease in amplitude of the high frequency arc, indicating the decrease of Rb, Rpf and Rct. On the basis of the M-SPL model and its equivalent circuit, the three resistances, Rb, Rpf and Rct, can be obtained from the impedance spectra in Fig. 12 and are evaluated as the function of temperature (Fig. 13). A linear relationship of log R ÿ1 versus reciprocal temperature for all these three resistances can be observed. This is the characteristic behavior of ionic conduction
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The residual moisture in the polyelectrolyte and PC may react with lithium to produce Li2O, LiOH and hydrogen [21±23]. The polymer macromolecules contacting with lithium electrode may be crosslinked through the free radical couplings [15, 17]. Therefore, the passivation ®lm may contain all the products of the unexpected reactions of lithium electrode with PC and the carbonyl groups on the polymer chains.
4. Conclusions
Fig. 13. Temperature dependence of the resistances (Rb, Rpf and Rct) of a Li/P(LiSMOE12)-PC/Li cell after storing at 258C under open circuit conditions for two weeks.
in solid electrolyte, indicating that a solid passivation ®lm has formed between the interface of lithium electrode and the electrolyte. The linear relationship of log R ÿ1 versus 1/T also suggests that the ionic conductions in the bulk electrolyte, the passivation ®lm formed and across the interfaces of lithium/passivation ®lm/electrolyte, all follow the Arrhenius behavior. The discontinuity of the correlation of log Rÿ1 b ±1/T is located between 25±308C which is near the second glass transition temperature of the bulk electrolyte. Since the reactions of lithium electrode and electrolyte components have changed the structure and the chemical nature of the plasticized P(LiSMOE12), the second glass transition of the electrolyte can have an obvious eect on the ionic conduction. Therefore, a broken straight line can be observed for the relationship of log Rÿ1 b 01/T. Based on the linear correlation of log R ÿ101/T for Rpf and Rct, the apparent activation energies for lithium ion transport through the passivation ®lm (Ea,pf) and across the interface of lithium/passivation ®lm (Ea,ct) can be calculated to be 0.42 and 0.69 eV, respectively. An activation energy value of 0.65 eV for the ionic transport of lithium through a LiOH and Li2O passivation ®lm has been reported by Radman for the lithium electrodes in liquid electrolytes containing small amounts of water [23]. Obviously, the value of Ea,pf for the present system (0.42 eV) is less than 0.65 eV, suggesting that the passivation ®lm may be composed not only of LiOH or Li2O, but also other substances. As stated above, the chemical decomposition of PC in the presence of lithium may result in the formation of propylene and Li2CO3 by the chemical decomposition of PC in the presence of lithium [19, 20]. The polymerization of PC could form polypropylene oxide P(PO) and carbon dioxide [18].
Propylene carbonate (PC) has an obvious plasticization eect on P(LiSMOE12). The two glass transition temperatures (Tg1 and Tg2) of P(LiSMOE12) decrease with increasing PC content, while the ionic conductivity increases. The plasticized P(LiSMOE12)-PC electrolyte has a conductivity as high as 1.8 10ÿ5 S cmÿ1 at 258C when the PC content is 54.5 wt% in the ®lm. The ionic conductive behavior changes gradually from a non-Arrhenius type to Arrhenius type with increasing PC content. The P(LiSMOE12)-PC electrolyte shows an irreversible lithium deposition±stripping process but a good electrochemical stability window. The interface stability study of the Li electrode and P(LiSMOE12)-PC electrolyte shows a passivation phenomenon which can be adequately described by the modi®ed solid-polymer layer (M-SPL) model developed for liquid organic electrolyte cells. The time evolution of the impedance data shows a fast growth of the passivation ®lm during the initial period after the cell is assembled and may include a modi®cation of the structure and the chemical nature of the passivation ®lm. The temperature dependence of resistance for Rb, Rpf and Rct indicates that the ionic conductions in the bulk electrolyte, the passivation ®lm formed and across the interfaces of lithium/passivation ®lm/electrolyte, all follow the Arrhenius behavior. The passivation ®lm may contain the products of the reactions of lithium electrode and the plasticizer PC and the polymer matrix.
Acknowledgements A research grant for this work from the National University of Singapore is gratefully acknowledged.
References [1] J.R. MacCallum, C.A. Vincent (Eds.), Polymer Electrolytes Reviews, vol. 1±2, Elsevier, London, 1987/ 1989.
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[2] F.M. Gray (Ed.), Solid Polymer Electrolytes: Fundamental and Technological Applications, VCH, Weinheim and New York, 1991. [3] S. Takeoka, H. Ohno, E. Tsuchida, Polym. Adv. Technol. 4 (1992) 53. [4] R. Koksbang, I.I. Olsen, D. Shackle, in: J.-M. Tarascon (Ed.), Recent Advances In Rechargeable Li Batteries, North-Holland, Amsterdam/Lausanne/New York/ Oxford/Shannon/Tokyo, Solid State Ionics 69(34) (1994) 320. [5] G.C. Farrington, C.A. Vincent (Eds.), Solid Electrolytes Science and Technology, North-Holland, Amsterdam/ Lausanne/New York/Oxford/Shannon/Tokyo, Solid State Ionics 85 (1995). [6] W. Xu, K.S. Siow, Z. Gao, S.Y. Lee, Chem. Mater. 10 (7) (1998) 1951. [7] F.M. Gray, in: J.R. MacCallum, C.A. Vincent (Eds.), Polymer Electrolyte Reviews, vol. 1, Elsevier Applied Science, New York, 1987, p. 139. [8] G.G. Cameron, M.D. Ingram, K. Sarmouk, Eur. Polym. J. 26 (1990) 197. [9] W. Xu, Z.-H. Deng, X.-Z. Zhang, G.-X. Wan, J. Solid State Electrochem. 2 (4) (1998) 257. [10] H.S. Lee, X.Q. Yang, J. McBreen, Z.S. Xu, T.A. Skotheim, Y. Okamoto, J. Electrochem. Soc. 141 (4) (1994) 886.
[11] M. Armand, Solid State Ionics 69 (1994) 309. [12] G.B. Appetecchi, F. Croce, B. Scrosati, Electrochim. Acta 40 (1995) 991. [13] F. Croce, B. Scrosati, J. Power Sources 43±44 (1993) 9. [14] C. Arbizzani, M. Borghini, M. Mastragostino, L. Meneghello, A. Zanelli, Solid State Ionics 72 (1994) 115. [15] S. Zhang, L. Yang, Q. Liu, Solid State Ionics 76 (1995) 127. [16] D.-W. Kim, J.-K. Park, J.-S. Bae, S.-I. Pyun, J. Polym. Sci. B 34 (1996) 2127. [17] W. Xu, K.S. Siow, Z. Gao, S.Y. Lee, Solid State Ionics, 112 (1±2) (1998) 1. [18] J.G. Thevenin, R.H. Muller, J. Electrochem. Soc. 134 (1987) 273. [19] A.N. Dey, B.P. Sullivan, J. Electrochem. Soc. 117 (1970) 222. [20] F.P. Dousek, J. Jansta, L. Riha, J. Electroanal. Chem. 46 (1973) 281. [21] Y. Geronov, F. Schwager, R.H. Muller, J. Electrochem. Soc. 129 (1982) 1422. [22] E. Peled, in: J.P. Gabano (Ed.), Ambient Temperature Lithium Batteries, Academic Press, London, 1983, p. 43. [23] D.M. Radman, Abstract 125, The Electrochemical Society Extended Abstracts, vol. 84-2, New Orleans, LA, Oct. 1984, p. 188.
Electrochimica Acta 44 (1999) 2287±2296
Electrochemical characterization of plasticized polyelectrolyte based on lithium-N(4-sulfophenyl) maleimide Wu Xu, Kok Siong Siow *, Zhiqiang Gao, Swee Yong Lee Department of Chemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Republic of Singapore Received 20 July 1998
Abstract Thermal behavior and electrochemical properties of propylene carbonate (PC) plasticized polyelectrolyte based on poly[lithium-N(4-sulfophenyl) maleimide-co-methoxy oligo(oxyethylene) methacrylate] have been investigated. This PC plasticized polyelectrolyte with a PC content of 54.5 wt% has an ionic conductivity of 1.8 10ÿ5 S cmÿ1 at ambient temperature and a wide electrochemical window. Passivation phenomena occurring at the interface between the PC plasticized polyelectrolyte and a Li electrode were particularly evaluated and discussed. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Plasticized polyelectrolyte; Ionic conductivity; Lithium interface; Impedance spectroscopy
1. Introduction Polymer electrolytes have been studied extensively in the last two decades [1±5]. It has been found that ionic conductivities of single ionic conductive polymer electrolytes without any organic solvents are too low (about 10ÿ8010ÿ6 S cmÿ1) for them to be applicable in practical uses. A quick and convenient way to improve the conductivities of these polymer electrolytes is to add some organic solvents, i.e. plasticizers, with high polarity, high boiling point, low viscosity and high stability with electrolytes and electrodes. The increase in ionic conductivity can be one to two orders of magnitude, which makes the polymer electrolytes suitable for lithium rechargeable batteries. However, for the development of lithium batteries based on polymer electrolytes, high ionic transport properties at ambient temperature are not the only parameters in selecting a proper polymer electrolyte because good mechanical properties, a wide electrochemical stability window and especially low resistance of
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the Li/polymer electrolyte interface and interface stability are also required to assure good cyclability and long-life to the batteries. In this paper, we report the thermal and electrochemical characteristics of a propylene carbonate (PC) plasticized polyelectrolyte based on a novel type of comblike, nearly alternating copolyether, poly[lithiumN(4-sulfophenyl) maleimide-co-methoxy oligo(oxyethylene) methacrylate] with 12 oxyethylene repeating units in the oligoether side chain [abbreviated as P(LiSMOE12)] [6], whose structure is shown in Scheme 1. The Li/polyelectrolyte interface stability over time is also investigated by impedance spectroscopy and the results are included here.
2. Experimental 2.1. Material and sample preparations The comblike, nearly alternating copolyether electrolyte, poly[lithium-N(4-sulfophenyl) maleimide-co-methoxy oligo(oxyethylene) methacrylate] with the side chain of 12 oxyethylene repeating units, [P(LiSMOE12)], was synthesized as mentioned in our
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 3 4 8 - X
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The stability of the lithium/polyelectrolyte interface was investigated by monitoring the time dependence of the impedance of cells with P(LiSMOE12)-PC between lithium symmetric non-blocking electrodes, utilizing the same instrument used in the case of the conductivity measurements except for a dierent frequency range, from 1 MHz to 10 mHz. 2.4. Cyclic voltammetry measurements Scheme 1. Structure of the nearly alternating polyelectrolyte P(LiSMOE12).
previous report [6]. The solution of P(LiSMOE12) in methanol was cast on a Te¯on dish and the solvent was allowed to evaporate at room temperature. The ®lm obtained was dried thoroughly in a vacuum oven at about 808C for 3 days to remove residual moisture. The ®lm disks were impregnated with dierent amounts of re-distilled PC according to the required composition of PC in the plasticized polyelectrolyte, ranging from 9.1 to 61.5 wt% and kept in a glove box ®lled with puri®ed argon for at least one day in order for the PC to become well-distributed in the electrolyte. For convenience, the PC plasticized poly[lithiumN(4-sulfophenyl) maleimide-co-methoxy oligo(oxyethylene) methacrylate] was abbreviated as P(LiSMOE12)PC. 2.2. Thermal analyses Dierential scanning calorimetric (DSC) analyses of these P(LiSMOE12)-PC electrolytes were carried out using a DuPont Instrument 2200 Thermal Analyzer. Weighed samples (about 15 mg) were encapsulated in aluminum pans and heated at 108C minÿ1 in the temperature range of ÿ140 to 1408C under a nitrogen atmosphere. The glass transition temperature (Tg) was taken as the midpoint temperature of the baseline shift observed from the second scan. 2.3. Impedance measurements The ionic conductivity plots of the P(LiSMOE12)-PC electrolytes were determined by measuring the temperature dependence of the impedance spectra of cells formed by sandwiching the given sample between two stainless steel blocking electrodes. The measurements were performed using a Zahner Electrik Impedance Measurement Unit (IM6) interfaced with a personal computer, over a frequency range from 1 MHz to 1 Hz. A 5 mV ac amplitude was used and ten points/decade were taken. A thermostatic bath with 0.18C precision was used to control the temperature.
The kinetics of the lithium deposition±stripping process from P(LiSMOE12)-PC electrolytes and the electrochemical stability window of these plasticized electrolytes were evaluated by cyclic voltammetric (CV) curves obtained at 258C in a three-electrode cell having a stainless steel working electrode, lithium counter and lithium reference electrodes. The cyclic voltammetry measurements were run and controlled by using a computer interfaced 273A PAR potentiostat/galvanostat. All the cells were prepared and sealed in an argon®lled glove box. 3. Results and discussion 3.1. Thermal characteristics The DSC curves of these P(LiSMOE12)-PC electrolytes with dierent PC content are shown in Fig. 1. It is seen that these PC plasticized P(LiSMOE12) electrolytes show two glass transitions in the studied temperature range, one is below ÿ508C and the other is around the ambient temperature. As mentioned in our previous report [6], P(LiSMOE12) is a copolymer with two glass transitions. The ®rst glass transition (Tg1) at about ÿ57.08C is attributed to the oligoether side chain and the second one (Tg2) around 32.38C is assigned to the main chain of the polymer host. Therefore, the two glass transitions (Tg1 and Tg2) in the PC plasticized P(LiSMOE12) electrolytes are, respectively, assigned to the oligoether side chain and the main chain of the copolymer. On the other hand, it is indicated from Fig. 1 that the addition of plasticizer PC into P(LiSMOE12) leads to the disappearance of the crystalline phase (Tm around 08C) existing in the pure copolymer P(LiSMOE12). In addition, the presence of PC is also responsible for the decrease of the two glass transitions temperatures (Tg1 and Tg2) in respect to those of the nearly alternating copolymer electrolyte without plasticizer, P(LiSMOE12). The in¯uence of plasticizer content on the two glass transitions temperatures is shown in Fig. 2. With increasing PC content in the electrolyte, both Tg1 and Tg2 decrease. In the studied PC content range, Tg1 decreases from ÿ57.08C for non-PC electrolyte to ÿ90.28C for a PC content of 61.5 wt%, while Tg2
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Fig. 1. DSC curves of PC plasticized P(LiSMOE12) electrolytes. PC content in the electrolyte: (a) none; (b) 9.1%; (c) 18.7%; (d) 28.6%; (c) 37.5%; (f) 44.4%; (g) 54.5% and (h) 61.5%.
decreases from 32.3 to 22.48C. The above results demonstrate the obvious plasticization eect of PC on P(LiSMOE12). It is also implied that with the addition of more and more PC into P(LiSMOE12), the P(LiSMOE12)-PC system forms a relatively homogeneous mixture. Gray [7] and Cameron et al. [8] have studied the PC plasticized PEO±LiX complexes. They proposed that there existed separate ionic pathways through the plasticizer in the PEO matrix and that the ionic motion was decoupled from that of the polymeric solvent. Xu et al. [9] observed the separate phases of crystalline PEO and liquid g-butyrolactone (BL) when studying the in¯uence of compatibility of plasticizers with polymeric matrix PEO on the ionic conduction using polar
microscopic photography, which supports the proposal of Cameron et al. [8]. In PEO±LiX-plasticizer systems where the plasticizers are PC, BL, etc., PC or BL extracts salt from the PEO±LiX complex and forms separate domains which serve as the ionic conducting path or channel with high ionic conductivity. On the other hand, Xu et al. [9] also reported that the plasticizer 2,5-di(methyl diglycol)-1,4;3,6-dianhydrous sorbitol ether (abbreviated as DGS) formed a homogeneous system with PEO/EO2PSLi (lithium methoxy di(ethyleneoxy) phenylsulfonate) because of the good compatibility of DGS with PEO. Lee et al. [10] synthesized a new plasticizer by modifying propylene carbonate, 2keto-4-(2,5,8,11-tetraoxadodecyl)-1,3-dioxolane (MC3). They found that the addition of MC3 into the PEO/ LiCF3SO3 complex formed a homogeneous mixture in both the crystalline and amorphous state and increased the ratio of amorphous to crystalline states in the mixture. This was due to the increased compatibility of MC3 with PEO that accrued from the ethylene oxide side chain. In the present case, since the eect of lithium N(4-sulfophenyl) maleimide on the oligo(oxyethylene), this nearly alternating copolymer has a higher polarity than pure PEO has, which in turn improves the compatibility of the copolymer P(LiSMOE12) with the plasticizer PC and favors the formation of a homogeneous system of P(LiSMOE12)PC. In fact, the plasticizer PC added into P(LiSMOE12) may form nanoscaled `pockets' of PC embedded in the polyelectrolyte [11]. 3.2. Ionic conductivity
Fig. 2. The in¯uence of PC content on the two glass transitions temperatures (Tg1 and Tg2) of the plasticized P(LiSMOE12)-PC electrolytes.
Fig. 3 shows the in¯uence of PC content in the plasticized polyelectrolyte on the isothermal ionic conductivity. The ionic conductivity increases with increasing
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Fig. 3. The eect of PC content on the isothermal ionic conductivity of P(LiSMOE12)PC electrolytes.
Fig. 4. Temperature dependence of ionic conductivity of P(LiSMOE12)-PC electrolytes with dierent PC content.
PC content, which re¯ects the normal in¯uence of plasticizer content on the ionic conductivity of polymer solid electrolytes. With increasing PC content in the system of P(LiSMOE12)-PC, the salt content eectively decreases. However, the number of charge carriers in the polyelectrolyte may increase within a certain range of PC content due to the strong ion-pair dissociation eect of PC on the sulfonic lithium salt. In addition, both Tg1 and Tg2 of the polyelectrolyte decrease with increasing PC content and decreasing salt content, especially Tg1 decreases drastically. Thus, the microscopic viscosity of the electrolyte decreases and the segmental movements of polymer macromolecules can be enhanced. Therefore, the ionic conductivity increases with increasing PC content in the electrolyte. The ionic transport is dependent on a cooperative eect of the polymer and PC. Once the PC content is higher than 54.5 wt%, the ionic conductivity of the P(LiSMOE12)-PC electrolytes does not increase any further. The ionic conductivity increases from 4.1 10ÿ8 to 1.8 10ÿ5 S cmÿ1 at 258C when PC content increases from 0 to 54.5 wt%. It is shown that the ionic conductivity can be improved by a magnitude of more than two orders as compared with the unplasticized polyelectrolyte. The plots of logarithmic ionic conductivity against the reciprocal temperature in the range from 10 to 808C for the P(LiSMOE12)-PC electrolytes with dierent PC contents are illustrated in Fig. 4. The experimental results show that with increasing PC content, the temperature dependence of ionic conductivity changes gradually from a curve to an in¯ected straight line and ®nally to a complete straight line. The temperature dependence of ionic conductivity for the P(LiSMOE12)-PC electrolyte with a PC content of 9.1 wt% shows a curve which resembles that of the
unplasticized P(LiSMOE12), implying that the ionic transport in this plasticized polyelectrolyte is still mainly dependent on the segmental movements of polymer macromolecules. On the other hand, the correlation between the ionic conductivity and the reciprocal temperature for the P(LiSMOE12)-PC electrolyte with a PC content of 18.7 and 28.6 wt% show an in¯ected straight line with a turning point at the temperature near 308C where the second glass temperature Tg2 is located. With the increase of PC content, the higher polarity of PC compared to that of the oligoether side chain leads PC to extract more conducting lithium ions from the co-ordination to the oligoether oxygen atoms into the nanoscaled PC `pockets' and favors the transport of free ions in PC because the activation energy for ion transport in PC is lesser than that through the segmental movements of polymer macromolecules. However, the PC content in these electrolytes is not sucient to extract all salt ions from the co-ordination to the oligoether oxygen atoms. Thus, the second glass transition of the polymer would aect the ionic transport, which leads to an in¯ection of the correlation between ionic conductivity and temperature. When the PC content in the plasticized polyelectrolyte exceeds 37.5 wt%, the sucient PC can extract nearly all conducting ions into the nanoscaled PC `pockets' and favors the ion carriers to transport it. The temperature dependence of ionic conductivity, therefore, shows a complete straight line. The straight lines of the temperature dependence of ionic conductivity demonstrate that the ionic conduction in the highly plasticized polyelectrolyte follows the Arrhenius behavior. It is dierent from that in the unplasticized P(LiSMOE12) or the electrolyte with low PC content, where the ion transport depends on the segmental movements of polymer macromolecules in
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the amorphous region and follows the free volume model. It is revealed that with increasing PC content in the electrolyte, the ionic conductive behavior in the electrolyte changes gradually from non-Arrhenius to Arrhenius type. 3.3. Lithium deposition±stripping process and electrochemical stability window The kinetics of the lithium deposition±stripping process, Li eÿ $ Li
1
from P(LiSMOE12)-PC electrolyte samples on stainless steel substrate was determined by cyclic voltammetry (CV) at 258C, as described in Section 2. Fig. 5 shows a typical CV result obtained in a three-electrode cell with a PC content of 54.5 wt% in P(LiSMOE12)-PC electrolyte in the potential range from +2.0 to ÿ0.5 V at a scan rate of 10 mV sÿ1. A single cathodic (deposition) peak and a single corresponding anodic (stripping) peak are found. The anodic peak of lithium stripping is not well de®ned as compared with that in many geltype electrolytes [12]. The peak potential dierence between the anodic and the cathodic peaks is as large as 1.4 V, implying that this lithium deposition±stripping process is irreversible. Due to the much lower ionic conductivity of P(LiSMOE12)-PC than that of other gel-type electrolytes, the anodic peak appears at a potential more positive than that in other gel-type electrolytes and the peak currents are just in the range of 70 mA cmÿ2. To evaluate the electrochemical stability window of the P(LiSMOE12)-PC electrolyte, a cell with stainless steel as working electrode and lithium as counter and
Fig. 5. Cyclic voltammetry at 258C of the lithium deposition± stripping process from a P(LiSMOE12)-PC electrolyte on a stainless steel substrate with a lithium reference. Scan rate: 10 mV sÿ1.
Fig. 6. Cyclic voltammetric response at 258C of a P(LiSMOE12)-PC electrolyte on a stainless steel electrode with a lithium reference. Scan rate: 10 mV sÿ1.
reference electrodes was assembled. The CV result of a typical P(LiSMOE12)-PC electrolyte shown in Fig. 6 does not indicate any obvious electrochemical reactions in the potential ranging between 0 and 5 V. The nucleation loop over 5.3 V suggests that a new phase has formed due to the oxidization of the electrolyte. An electrochemical window greater than 4.5 V is an important feature for a polymer electrolyte because it renders polymer electrolytes suitable in many lithium based batteries applications. This result suggests that the P(LiSMOE12)-PC electrolyte indeed has a good electrochemical stability window. 3.4. Stability of lithium electrode surface To investigate the compatibility of the polyelectrolyte with electrode materials and in an attempt to determine the stability of the lithium/P(LiSMOE12)-PC electrolyte interface over time, a detailed impedance analysis of a symmetric cell of the type Li/ P(LiSMOE12)-PC/Li, stored and tested under open circuit conditions at room temperature, was carried out. Fig. 7 shows the ac impedance spectra recorded at progressively longer storage time at 258C under open circuit conditions. It is seen that the overall ac impedance response is composed of an incomplete semicircle (or arc) in the high frequency range, a distorted semicircle in the middle frequency range and a spur in the low frequency range. The related Cole±Cole (ÿZIM versus ZRE) plots over time illustrate an increase in amplitude for the high frequency arc and an irregular variation in amplitude for the distorted middle frequency semicircle. Since no signi®cant aging process in the PC plasticized polyelectrolyte with stainless steel blocking electrodes was observed in such a storage period, which is shown in Fig. 8 reporting the resistivity (r) value of P(LiSMOE12)-PC electrolyte over time at 258C, thus the expansion of the high frequency arc
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Fig. 8. Variation of the resistivity (r) of P(LiSMOE12)-PC with storage time in a SS/P(LiSMOE12)-PC/SS cell stored and tested at 258C. PC content in the electrolyte is 54.5 wt%.
Fig. 7. Impedance responses of a Li/P(LiSMOE12)-PC/Li cell at dierent storage time stored at 258C under open circuit conditions. PC content in the electrolyte is 54.5 wt%.
over time and the variation of the middle frequency semicircle in the Li symmetric cells reveal the growth of a resistive layer, i.e. the passivation layer, on the lithium electrode surface. The phenomenon of the lithium electrode being passivated when in contact with the electrolyte is not surprising. This behavior has been observed and studied in many reports [12±16]. This passivation layer can be thought of as the result of the corrosion reactions of Li electrode in the electrolyte medium, since some of the electrolyte components, e.g. PC, the carbonyl groups on the polymer chains, the residual moisture and impurities in the electrolyte, are known to be able to react with lithium electrode [12, 15±18]. By using a model equivalent circuit which could represent the electrical equivalent of the lithium electrode interface, it is possible to separate the various impedance parameters which contribute to the responses illustrated in Fig. 7. In this paper, the solid-polymerlayer (SPL) model suggested by Thevenin and Muller [18] has been adopted and modi®ed to ®t the impedance data. The SPL model basically describes the Li passivated surface area as consisting of a dispersion of inorganic solid compounds (maybe the products of the reaction of metallic Li with the electrolyte components) in a polymer matrix (perhaps resulting from the Li-initiated solvent polymerization). Therefore, the total impedance of a passivated Li electrode could be taken as being composed of the resistance of the electrolyte ®lm, the impedance of the passivation ®lm and
the impedance of the charge-transfer process across the Li/passivation ®lm/electrolyte interfaces [13, 18]. As a result, the equivalent circuit for lithium symmetric cells should be that of Fig. 9(a), where Rb and Cg represent the bulk resistance of ion transport in the electrolyte and the geometrical capacitance of the electrolyte; Rpf and Cpf represent the resistance and capacitance of the
Fig. 9. (a) Equivalent circuit of a modi®ed solid-polymer layer (M-SPL) model. (b) Typical impedance response of a Li/ P(LiSMOE12)-PC/Li cell stored at 258C with superimposed the three semicircles obtained by ®tting the data with the MSPL equivalent circuit.
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passivation ®lm; Rct and Cdl represent the chargetransfer resistance of lithium cations across the Li/passivation ®lm/electrolyte interfaces and the double layer capacitance of these interfaces and Zw is the Warburg impedance, which is associated with ionic diusion arising from the presence of concentration gradients within the polymer electrolyte. The modi®ed solidpolymer-layer model was abbreviated as M-SPL model. Using the related M-SPL equivalent circuit, the impedance responses can be analyzed by superimposing the best ®t of the dispersion data. Fig. 9(b) shows one of the responses and its ®tted dispersion data. The ®tted curves reveal that the overall responses could eectively be described as a combination of three-dispersion semicircles and a spur, associated with dierent relaxation phenomena. According to the M-SPL circuit, the high frequency arc can be ascribed to the P(LiSMOE12)-PC electrolyte, the distorted semicircle in the middle frequency range to the passivation ®lm and the charge-transfer process and the low frequency spur to the diusion process. On the basis of the equivalent circuit, it is then possible to determine the time evolution of the single circuit parameters. Fig. 10 illustrates the time evolution of the resistance values of a lithium symmetric cell with P(LiSMOE12)-PC electrolyte, the passivation ®lm Rpf, the charge-transfer process Rct and the bulk electrolyte Rb. The values of Rpf and Rct increase sharply within the initial 8 h after the assembly of the lithium cells, then decrease and increase in an irregular way. This phenomenon may be explained as follows: just when the cell is assembled, there are no reactions
Fig. 10. Time evolution of the resistance of the bulk electrolyte (Rb), of the passivation ®lm (Rpf) and of the charge transfer (Rct) of a Li/P(LiSMOE12)-PC/Li cell stored at 258C under open circuit. PC content in the electrolyte is 54.5 wt%.
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occurring between the lithium electrode and the polymer electrolyte components. Therefore, the impedance spectrum should show a standard semicircle in the middle frequency range for the charge-transfer process. With the increase of storage time, the components in the electrolyte react with lithium electrode to form a passivation layer between the electrode and the electrolyte. This leads to the rapid increase of the resistance of the passivation layer, which in turn increases the charge-transfer resistance. The rapid reactions between the electrolyte components and the Li electrode cause an increase in the thickness of the passivation ®lm which surpass the changes of the passivation layer compositions during the initial period until the thickness of the passivation ®lm reaches a certain degree, the morphological modi®cation processes of the passivation ®lm then result in the variation (decrease and increase) of the resistance of Rpf and Rct in an irregular way. It is implied that the nature of the passivation ®lm may progressively change under prolonged storage time. The decrease of Rct after a storage time of 72 h implies that some porous structures may have formed in the passivation ®lm because of the formation of propylene and Li2CO3 by the chemical decomposition of PC in the presence of lithium [19, 20], of polypropylene oxide P(PO) and carbon dioxide by the polymerization of PC [18], and of Li2O, LiOH and hydrogen by the reaction of residual moisture with lithium [21±23]. Thus, the surface area of the passivation ®lm would increase, which in turn causes the decrease of Rpf and Rct. On the other hand, the resistance of the electrolyte Rb also increases with storage time. It is probably due to the cross-linkage of the polymer macromolecules through the free radical couplings which lead to new C±C bonds and by the co-ordination of lithium cations to the oxygen atoms in ethyleneoxy repeating units, as described in our previous report [17]. After 10 days of storage, the value of Rb is nearly twice its original value, implying that the electrolyte is much less conductive now or the nature of the electrolyte has changed substantially. Further information on the characteristics of the lithium passivation process may be obtained by examining the time evolution of the capacitance of the passivation layer, more exactly, the double layer of the interface (Cdl) between the lithium and polymer electrolyte, which is obtained by the relation of Cdl=1/ (2pf *Rct) where f * is the top frequency of the impedance spectrum for the charge-transfer process, as shown in Fig. 11. It is seen that the value of Cdl decreases sharply in the initial storage period and then constantly decreases in a slow way with the increase of storage time. According to the following expression of capacitance,
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Fig. 11. Time evolution of the double layer capacitance (Cdl) at the interface of a Li/P(LiSMOE12)-PC/Li cell stored at 258C under open circuit. PC content in the electrolyte is 54.5 wt%.
C Ae0 er =l
2
where e0 is the dielectric constant of vacuum, er is the relative dielectric constant of the studied ®lm, A is the ®lm surface area and l is the ®lm thickness, the change in Cdl shown in Fig. 11 may be due to an increase in the thickness of the passivation ®lm (lpf) or a variation of the nature of the passivation ®lm (er,pf) or a combination of the two factors since the surface area of the passivation ®lm (A) has a tendency to increase as stated above. Therefore, it is reasonable to assume that the decrease of Cdl within the initial period after the assembly of the cell can most likely be associated with the rapid increase of the passivation ®lm thickness. The above discussion indicates that the passivation process between the interface of lithium electrode and polymer electrolyte P(LiSMOE12) severely aects the interfacial conditions and the chemical nature of the polymer electrolyte. This fact can be explained by assuming a progressive and irreversible variation of the structure of passivation ®lm which may be associated with a modi®cation of its chemical nature. 3.5. Temperature dependence of impedance spectra
Fig. 12. The in¯uence of temperature on the impedance responses for a cell of Li/P(LiSMOE12-PC)/Li stored at room temperature under open circuit conditions for 15 days prior to measurement.
As it is known, with an increase in temperature, the ionic conductivity of a polymer electrolyte will increase. This conclusion can also be con®rmed by the variation of the impedance spectra with temperature. Fig. 12 illustrates a typical relationship between the temperature and the impedance plots for a lithium symmetric cell with a P(LiSMOE12)-PC electrolyte, which had been stored in a dry box at 258C under open circuit for two weeks prior to measurement. With increasing temperature, the amplitude of the distorted semicircle in the middle frequency range diminishes
sharply as well as the decrease in amplitude of the high frequency arc, indicating the decrease of Rb, Rpf and Rct. On the basis of the M-SPL model and its equivalent circuit, the three resistances, Rb, Rpf and Rct, can be obtained from the impedance spectra in Fig. 12 and are evaluated as the function of temperature (Fig. 13). A linear relationship of log R ÿ1 versus reciprocal temperature for all these three resistances can be observed. This is the characteristic behavior of ionic conduction
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The residual moisture in the polyelectrolyte and PC may react with lithium to produce Li2O, LiOH and hydrogen [21±23]. The polymer macromolecules contacting with lithium electrode may be crosslinked through the free radical couplings [15, 17]. Therefore, the passivation ®lm may contain all the products of the unexpected reactions of lithium electrode with PC and the carbonyl groups on the polymer chains.
4. Conclusions
Fig. 13. Temperature dependence of the resistances (Rb, Rpf and Rct) of a Li/P(LiSMOE12)-PC/Li cell after storing at 258C under open circuit conditions for two weeks.
in solid electrolyte, indicating that a solid passivation ®lm has formed between the interface of lithium electrode and the electrolyte. The linear relationship of log R ÿ1 versus 1/T also suggests that the ionic conductions in the bulk electrolyte, the passivation ®lm formed and across the interfaces of lithium/passivation ®lm/electrolyte, all follow the Arrhenius behavior. The discontinuity of the correlation of log Rÿ1 b ±1/T is located between 25±308C which is near the second glass transition temperature of the bulk electrolyte. Since the reactions of lithium electrode and electrolyte components have changed the structure and the chemical nature of the plasticized P(LiSMOE12), the second glass transition of the electrolyte can have an obvious eect on the ionic conduction. Therefore, a broken straight line can be observed for the relationship of log Rÿ1 b 01/T. Based on the linear correlation of log R ÿ101/T for Rpf and Rct, the apparent activation energies for lithium ion transport through the passivation ®lm (Ea,pf) and across the interface of lithium/passivation ®lm (Ea,ct) can be calculated to be 0.42 and 0.69 eV, respectively. An activation energy value of 0.65 eV for the ionic transport of lithium through a LiOH and Li2O passivation ®lm has been reported by Radman for the lithium electrodes in liquid electrolytes containing small amounts of water [23]. Obviously, the value of Ea,pf for the present system (0.42 eV) is less than 0.65 eV, suggesting that the passivation ®lm may be composed not only of LiOH or Li2O, but also other substances. As stated above, the chemical decomposition of PC in the presence of lithium may result in the formation of propylene and Li2CO3 by the chemical decomposition of PC in the presence of lithium [19, 20]. The polymerization of PC could form polypropylene oxide P(PO) and carbon dioxide [18].
Propylene carbonate (PC) has an obvious plasticization eect on P(LiSMOE12). The two glass transition temperatures (Tg1 and Tg2) of P(LiSMOE12) decrease with increasing PC content, while the ionic conductivity increases. The plasticized P(LiSMOE12)-PC electrolyte has a conductivity as high as 1.8 10ÿ5 S cmÿ1 at 258C when the PC content is 54.5 wt% in the ®lm. The ionic conductive behavior changes gradually from a non-Arrhenius type to Arrhenius type with increasing PC content. The P(LiSMOE12)-PC electrolyte shows an irreversible lithium deposition±stripping process but a good electrochemical stability window. The interface stability study of the Li electrode and P(LiSMOE12)-PC electrolyte shows a passivation phenomenon which can be adequately described by the modi®ed solid-polymer layer (M-SPL) model developed for liquid organic electrolyte cells. The time evolution of the impedance data shows a fast growth of the passivation ®lm during the initial period after the cell is assembled and may include a modi®cation of the structure and the chemical nature of the passivation ®lm. The temperature dependence of resistance for Rb, Rpf and Rct indicates that the ionic conductions in the bulk electrolyte, the passivation ®lm formed and across the interfaces of lithium/passivation ®lm/electrolyte, all follow the Arrhenius behavior. The passivation ®lm may contain the products of the reactions of lithium electrode and the plasticizer PC and the polymer matrix.
Acknowledgements A research grant for this work from the National University of Singapore is gratefully acknowledged.
References [1] J.R. MacCallum, C.A. Vincent (Eds.), Polymer Electrolytes Reviews, vol. 1±2, Elsevier, London, 1987/ 1989.
2296
W. Xu et al. / Electrochimica Acta 44 (1999) 2287±2296
[2] F.M. Gray (Ed.), Solid Polymer Electrolytes: Fundamental and Technological Applications, VCH, Weinheim and New York, 1991. [3] S. Takeoka, H. Ohno, E. Tsuchida, Polym. Adv. Technol. 4 (1992) 53. [4] R. Koksbang, I.I. Olsen, D. Shackle, in: J.-M. Tarascon (Ed.), Recent Advances In Rechargeable Li Batteries, North-Holland, Amsterdam/Lausanne/New York/ Oxford/Shannon/Tokyo, Solid State Ionics 69(34) (1994) 320. [5] G.C. Farrington, C.A. Vincent (Eds.), Solid Electrolytes Science and Technology, North-Holland, Amsterdam/ Lausanne/New York/Oxford/Shannon/Tokyo, Solid State Ionics 85 (1995). [6] W. Xu, K.S. Siow, Z. Gao, S.Y. Lee, Chem. Mater. 10 (7) (1998) 1951. [7] F.M. Gray, in: J.R. MacCallum, C.A. Vincent (Eds.), Polymer Electrolyte Reviews, vol. 1, Elsevier Applied Science, New York, 1987, p. 139. [8] G.G. Cameron, M.D. Ingram, K. Sarmouk, Eur. Polym. J. 26 (1990) 197. [9] W. Xu, Z.-H. Deng, X.-Z. Zhang, G.-X. Wan, J. Solid State Electrochem. 2 (4) (1998) 257. [10] H.S. Lee, X.Q. Yang, J. McBreen, Z.S. Xu, T.A. Skotheim, Y. Okamoto, J. Electrochem. Soc. 141 (4) (1994) 886.
[11] M. Armand, Solid State Ionics 69 (1994) 309. [12] G.B. Appetecchi, F. Croce, B. Scrosati, Electrochim. Acta 40 (1995) 991. [13] F. Croce, B. Scrosati, J. Power Sources 43±44 (1993) 9. [14] C. Arbizzani, M. Borghini, M. Mastragostino, L. Meneghello, A. Zanelli, Solid State Ionics 72 (1994) 115. [15] S. Zhang, L. Yang, Q. Liu, Solid State Ionics 76 (1995) 127. [16] D.-W. Kim, J.-K. Park, J.-S. Bae, S.-I. Pyun, J. Polym. Sci. B 34 (1996) 2127. [17] W. Xu, K.S. Siow, Z. Gao, S.Y. Lee, Solid State Ionics, 112 (1±2) (1998) 1. [18] J.G. Thevenin, R.H. Muller, J. Electrochem. Soc. 134 (1987) 273. [19] A.N. Dey, B.P. Sullivan, J. Electrochem. Soc. 117 (1970) 222. [20] F.P. Dousek, J. Jansta, L. Riha, J. Electroanal. Chem. 46 (1973) 281. [21] Y. Geronov, F. Schwager, R.H. Muller, J. Electrochem. Soc. 129 (1982) 1422. [22] E. Peled, in: J.P. Gabano (Ed.), Ambient Temperature Lithium Batteries, Academic Press, London, 1983, p. 43. [23] D.M. Radman, Abstract 125, The Electrochemical Society Extended Abstracts, vol. 84-2, New Orleans, LA, Oct. 1984, p. 188.