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XPS study of lithium surface after contact with lithium-salt doped polymer electrolytes
Electrochimica Acta 46 (2001) 1595– 1603 www.elsevier.nl/locate/electacta
XPS study of lithium surface after contact with lithium-salt doped polymer electrolytes Iqbal Ismail 1, Akihiro Noda, Atsushi Nishimoto, Masayoshi Watanabe * Department of Chemistry and Biotechnology, Yokohama National Uni6ersity, Hodogaya-ku, Yokohama 240 -8501, Japan Received 7 August 2000; received in revised form 3 October 2000
Abstract X-ray photoelectron spectroscopy (XPS) is used to probe the surface layer and element composition of Li-metal electrodes before and after contact with polymer electrolytes containing LiN(SO2CF3)2 (LiTFSI) or LiBF4. Native film on as-received metallic lithium was composed of Li2CO3/LiOH in the outer layer and Li2O in the inner layer. LiF was formed during lithium contact with electrolyte due to reaction between the native film and impurities in the electrolyte. The polymer electrolyte containing LiTFSI yielded a very thin film with limited porosity in the inner layers, which was reflected in the limited amplitude dependence of complex impedance spectra. LiBF4 mixed with polymer resulted in a thicker film with high porosity, as was postulated from the greater influence of the amplitude of the oscillation level. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: XPS; Polymer electrolyte; Lithium electrode; Passivation layer; Lithium polymer battery
1. Introduction In order to improve the cyclability of lithium anode secondary batteries, the rechargeability of the lithium electrode needs to be improved. In other words, the compatibility of electrolyte with lithium metal must be enhanced, which means that a stable, low impedance interfacial layer is formed upon contact. It has been well known that the rechargeability problem is associated with the reactivity of the electrode when attached with electrolytes. It is proposed that the type of electrolyte is the main factor in controlling the nature of the passivation layer at the contact. Furthermore, investigations on the lithium anode suggest that the den-
* Corresponding author. E-mail address: [email protected] (M. Watanabe). 1 Permanent address: Department of Chemistry, King Abdul Aziz University, PO Box 9028, Jeddah 21413, Saudi Arabia.
dritic film formed on the anode during the cycling process strongly influences its rechargeability. Due to the presence of organic and inorganic compositions at the surface of lithium metal after contact with electrolyte, surface analyses will complement the electrochemical studies. Beside several other techniques (e.g. FTIR [1], SEM [2]) X-ray photoelectron spectroscopy (XPS) has been widely used for studying the surface reaction of lithium electrodes. In this study, we report the surface analyses of lithium electrode before and after contact with network polymer, NP, (prepared from mono-acrylated and triacrylated copolymer of ethylene oxide and propylene oxide) electrolyte membrane containing lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) or lithium tetrafluoroborate (LiBF4). The effect of different kinds of lithium salt on the electrode electrolyte interface has been investigated using electrochemical and surface analysis methods. Models for the passivation layer are proposed, depending on the lithium salt incorporated in the membrane.
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 7 5 8 - 1
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I. Ismail et al. / Electrochimica Acta 46 (2001) 1595–1603
2. Experimental
2.1. Materials and preparation Mixtures of two different oligomers (0.5 g of each) were used as macromonomers; mono-acrylate (MA) and tri-acrylate (TA) of copolymers of ethylene oxide and propylene oxide (both kindly supplied by Dai-ichi Kogyo Seiyaku Co.) for polymer electrolyte preparation [3,4]. The molecular structures are shown in Scheme 1. LiTFSI (kindly supplied by IREQ, dried at 180°C, under reduced pressure for 24 h) or LiBF4, (Tomiyama Chemicals, battery grade), and 2,2dimethoxy-2-phenyl acetophenone (Ciba Geigy, 500 ppm based on the weight of macromonomers) as photo initiator, together with the mixture of macromonomers, were dissolved in dehydrated acetonitrile (Kanto Chem-
ical Co. Inc.). The mixture was left to stir until a homogeneous clear solution was obtained. The viscous solution was spread between two glass plates separated by a polytetrafluoroethylene spacer of 0.5 mm thick, before it was irradiated with UV light (250 W high pressure Hg lamp, UI-501C Ushio Electric Inc.) for 5 min. Residual acetonitrile was evaporated under vacuum at room temperature. Transparent and flexible polymer electrolyte membranes were obtained, which were then used in electrochemical measurements followed by XPS analyses.
2.2. Electrochemical measurements Polymer electrolyte films were cut into disks of 13 mm diameter and sandwiched between two symmetrical lithium electrodes (Honjo Metal Co.) of 0.2 mm thick, and the whole was placed in a sealed cell and stored in a constant temperature (60°C) oven. After one week of storage time, amplitude dependence measurements were carried out by using an AC impedance analyzer (Hewlett– Packard 4192A) over 5 Hz– 13 MHz frequency range.
2.3. Lithium surface analysis by XPS
Scheme 1. Structures of macromonomers for polymer electrolyte preparation.
The cell, after storing ( \ 1 month) at 60°C and subsequent temperature dependence measurements (20– 80°C) of AC impedance (data not shown here), was disassembled in a dry argon atmosphere, and the polymer electrolyte membrane film was carefully peeled off from the metallic lithium, and the lithium was mounted on an XPS stage. The depth profiles of the lithium surface for the relevant elements were explored by using Shimadzu-Koratos Electron Spectrometer XSAM-800 with argon ion sputtering (3 kV, 0.25– 0.5 mA). An Mg–Ka line was used as an x-ray source. The etching rate was roughly estimated to be 4 A, /min. The binding energy for each spectrum for lithium foil before contact was calibrated by C 1s peak, corresponding to hydrocarbon adsorbed on the sample surface (285.0 eV). However, due to better sensitivity and selectivity for LiF peaks in F 1s region, all the binding energies for each spectrum for the lithium surface after contact with membranes were calibrated by F 1s at 685.5 eV, corresponding to LiF. XPS analysis was performed after reducing the pressure in the XPS chamber below 10 − 9 torr.
3. Results and discussion
3.1. Electrochemical in6estigation Fig. 1. Time dependence of interfacial resistance for the network polymer complexed with (a) LiTFSI and (b) LiBF4, at 60°C.
Fig. 1 shows time dependence of interfacial resistance between lithium electrode and network polymer elec-
I. Ismail et al. / Electrochimica Acta 46 (2001) 1595–1603
Fig. 2. Oscillation level dependence of impedance diagram for the network polymer complexed with (a) LiTFSI and (b) LiBF4 and sandwiched between two symmetrical Li electrodes at 60°C.
trolytes containing LiTFSI or LiBF4 ([Li]/[O]=0.08) at 60°C. It is obvious here that LiTFSI-NP interface proved to be constant for a long time, whereas LiBF4NP showed increasing interfacial resistance over the period of time. This is believed to be due to the instability of the latter salt. Table 1 indicates data obtained from a Li/LiX-NP/Li (where X: TFSI− or BF− 4 ) electrochemical cell after 1 month of storage. The thickness of the passivation layer, l, was calculated using Eq. (1): l=2yfmaxmrmoARi
(1)
where fmax is the frequency at the maximum of imaginary part of interfacial impedance (-Z¦) in the complex
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impedance diagram, mr is the relative dielectric constant for the passivation layer, which was set at 10 for lithium compounds on the surface [5], mo is the permittivity of vacuum, and A is the electrode area. Although a value of 5 was used [6] for mr for an interfacial film between a highly lithiated cathode material in contact with tetraethylene glycol dimethyl ether, in the case of salted medium a higher permittivity value is more realistic. For simplicity, we assumed that mr remained constant over the life of the interface while the passivation thickness varied. The estimation of film thickness in this work is much smaller than that reported by Sloop and Lerner [7], who estimated the thickness after 1 week of lithium metal contact with poly(ethylene oxide)/LiClO4. This may be due to low water content ( B 20 ppm) in our polymer electrolytes. Passivation film thickness values for lithium after contact with LiTFSI-NP and LiBF4-NP, are lower and higher, respectively, if compared with that calculated for the film formed on Li metal in contact with a linear polyether (tetraethylene glycol dimethyl ether) [8]. This wide variation in film thickness associated with lithium salt type suggests that the polymer host has a less significant role in controlling the interfacial property, and the nature of lithium salt has a greater effect in modifying the structure of the passivation layer. It can be noticed here that the thicker interface for LiBF4-NP is associated with higher resistance, lower capacitance and larger time constant. This greater rise in Ri for LiBF4-NP, compared with the LiTFSI-NP, is another indication of the contrast in the nature of the interface between the two cases. Fig. 2 shows the amplitude dependence of impedance response for (a) LiTFSI-NP and (b) LiBF4-NP. It can be seen here that, in both compositions, the high frequency semi-circle is the same for different amplitudes. The high frequency semi-circle agreed well with that obtained when blocking, stainless steel, electrodes were used (not shown here). This indicates that the resistance for the high frequency semi-circle corresponds to the bulk property of the polymer electrolyte. However, the low frequency semi-circle depicted different behavior for the two salt compositions. LiBF4-NP was remarkably dependent on the amplitude, whereas LiTFSI-NP was not greatly influenced.
3.2. XPS analysis for the nati6e film on the lithium surface Fig. 3 shows the XPS spectra of Li 1s, C 1s, and O 1s regions for the native film on the lithium surface. In these spectra, three lithium compounds are observed, i.e. Li2CO3, LiOH and Li2O. The peak at 55.2 eV in Li 1s region can be assigned to Li2CO3 and/or LiOH,
3.56 1.18
| at 60°C (10−4 S cm−1)
Tg measured after polymer electrolyte preparation.
234.3 241.4
LiTFSI LiBF4
a
Tg (K) a
Lithium salt mixed with network polymer 0.16 1.41
Ri at 60°C (103 V cm2) 67.3 69.1
Ea (for Arrhenius plot of Ri) (kJ mol−1)
Table 1 Data calculated for lithium foil after contact with network polymer electrolyte membrane for about a month ([Li]/[O]= 0.08)
0.563 0.226
Cdl (mF cm2)
0.9 3.2
~ (10−4 s)
80 200
l (A, )
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Fig. 3. XPS spectra for the native film on as-received Li-foil.
whose intensity decreases with etching time. The peak at 54.0 eV corresponding to Li2O is observed after 30 min of etching time and is detectable up to 90 min of etching time with almost the same intensity. It may be noticed that the thickness of the native film on lithium foil was estimated by Kanamura et al. [9] to be as much as 900 A, . However, due to instrumental limitation in this work we were unable to extend the argon ion sputtering time and thus, no peak for Li metal (at 52.3 eV in Li 1s) was possible to detect. Nevertheless, we do not expect that the native film in our work is that thick, since during cell assembly for electrochemical measurements lithium foil was firmly pressed on the stainless steel substrate in order to break the native film on the lithium surface. A similar treatment was applied prior to XPS measurement for the as-received lithium surface. Fig. 3b shows C 1s peaks for the native film on as-received lithium foil. A sharp peak of hydrocarbon is depicted at 285.0 eV before etching, as usual. Hydrocarbon is believed to exist in the glove box and/or the XPS chamber as a contaminant, which is mainly adsorbed on the outer surface of the sample. The peak at 290.1 eV corresponding to Li2CO3 is observed in the outer layer of the native film and up to 10 min of etching time in a greater intensity, though in the following scans its intensity decreases remarkably. Fig. 3c shows XPS spectra of the O 1s region. A peak at 532.0 eV corresponding to Li2CO3/LiOH is observed, which also decreases after 10 min of etching time. A small peak, but with strong intensity later, appears at 528.3 eV corresponding to Li2O at 10 min of etching time. The above analysis indicates that the carbonate layer is relatively thinner when compared with the oxide layer. These observations for Li2CO3/LiOH and Li2O in Li 1s, C 1s, and O 1s spectra, are in good agreement with each other in terms of etching profile in accordance with their assigned compositions. The native film on the
as-received lithium can be described schematically as in Fig. 4.
3.3. XPS analysis for the lithium surface following contact with polymer electrolytes Fig. 5a – d show the XPS spectra of Li 1s, C 1s, O 1s, and F 1s regions for the lithium surface following contact with LiTFSI-NP. It may be noticed that peak positions are displaced, compared with those of native film spectra. This is because different calibrating peaks are used for the before and after contact samples (see Section 2). In Fig. 5a Li2CO3/LiOH peaks are apparent from 16 min of etching time onward, whereas the peak assigned to Li2O starts to appear from 36 min of etching time. The TFSI anion or polymer electrolyte peak is observed at 286.8, 533, and 689.2 eV in C 1s, O 1s, and F 1s regions, respectively, before etching and decreased thereafter. This organic content at the surface is the undetached part of the electrolyte left while peeling it off from the electrode and/or it is the product of reaction between the electrolyte and metallic lithium. At 16 min of etching time onward, a peak at 283.8 eV in the C 1s spectrum is recorded which corresponds to the carbide. Fig. 5a and d present the LiF peak at 56.0 eV in Li 1s and 685.5 eV at F 1s regions, respectively.
Fig. 4. Schematic illustration for the native film on as-received Li-foil.
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Fig. 5. XPS for Li-foil after 1 month of contact with LiTFSI-NP at 60°C.
In the F 1s spectrum the peak starts to appear clearly at 1 min of etching time and onward, although in the Li 1s spectrum it appears even before etching. In fact, LiF possibly has two sources within the passivation layer: (i) the reaction of HF as a contaminant in the electrolyte with Li2CO3, LiOH and/or Li2O in the native layer, i.e. acid–base reaction (see for example [10]), and (ii) a direct reaction of the TFSI anion with Li metal. A similar reaction to the latter possibility was proposed by Naoi et al. [11] between bis(perfluoroethylsulfonyimide) [N(C2F5SO2)2] anion and metallic Li to produce the polymeric form of the imide anion at the Li surface when it was immersed in a LiN(C2F5SO2)2/propylene carbonate electrolyte.
The above observations together with the electrochemical results (Table 1, Fig. 2) led us to postulate that in the case of the LiTFSI-NP electrolyte, thin and slightly porous layers of various compositions formed at the surface of lithium foil following the contact. It may be considered that the source of Li2CO3/LiOH and Li2O is almost entirely from the native film, since the adsorption of TFSI anion or its polymerized form at the surface inhibits any further reaction between the polymer and lithium, which normally causes the production of carbonate [11]. Residual HF can easily react with Li2CO3, LiOH or Li2O to form LiF, whereas, TFSI anion needs to migrate through the native film to react with the metallic lithium. If the latter step takes place it is not possible to detect the anion only in the
I. Ismail et al. / Electrochimica Acta 46 (2001) 1595–1603
outer layer. Thus, we believe that the TFSI- peak observed on the outer layer is its natural form rather than the polymeric product, and therefore, the only source for LiF is the acid–base reaction between HF and the lithium compounds in the native film. Fig. 6a–d show the XPS peaks of Li 1s, C 1s, O 1s, and F 1s regions for the lithium surface following contact with LiBF4-NP. In Li 1s (Fig. 6a) and F 1s (Fig. 6d) spectra it may be seen that LiF is formed on the surface with increasing intensity in the following spectra as a function of etching time. Li2CO3/LiOH peaks are observed in Li 1s (Fig. 6a) and O 1s (Fig. 6c)
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spectra, i.e. almost from 0 to 36 min of etching. The Li2O composition peak starts to appear at 16 min of etching time in O 1s (Fig. 6c), which showed an increasing trend thereafter. A polymer peak is observed in the outer layer at 286.8 and 533 eV in C 1s (Fig. 6b) and O 1s (Fig. 6c), respectively. No peak for adsorbed BF4 in the F 1s spectra, no seen in Fig. 5, is apparent.
3.4. Depth profiles of the lithium surface Fig. 7 shows a depth profile for each element in the surface film of lithium as received (a) and after contact
Fig. 6. XPS for Li-foil after 1 month of contact with LiBF4-NP at 60°C.
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Fig. 7. Depth profile for the Li-metal surface (a) as-received, and after contact for 1 month at 60°C with (b) LiTFSI-NP, or (c) LiBF4-NP.
with LiTFSI-NP (b) or LiBF4-NP (c). The amount of each element was estimated by integrating the intensity of the XPS peak and correcting for the cross-section for the ionization of each element. Thus, a quantitative comparison is more or less satisfactory. In the native film on as-received lithium foil (Fig. 7a), the C content is initially high (due to hydrocarbon adsorbed on the surface), but when etching starts, a decreasing tendency with etching time can be observed, whereas O starts with a high content and slightly increases with etching time. This indicates that the presence of Li2CO3/LiOH is in the outer layer and Li2O mainly in the inner part of the native film. In the depth profile of the lithium surface after contact with LiTFSI-NP (Fig. 7b), one can observe that the content of F is slightly smaller at the surface, although increases in a gradual manner with etching time. On the other hand, C is relatively high in the initial stage, with decreasing tendency after 16 min of etching time. It may be noticed that the C content in the lithium surface after contact with LiTFSI-NP is much higher than its content in the lithium surface before contact, which indicates that the source of C is not only the carbonate salt in the native film (compare carbonate peaks in C 1s before and after contact, Figs. 3b and 5b) but the relatively thick adsorbed TFSI layer or polymer electrolyte layer attached on the surface. This assumption is consistent with the XPS spectrum described earlier (Fig. 5). In the case of the lithium surface after contact with LiBF4-NP (Fig. 7c), the presence of F is predominant, although small before etching, but in a higher content when etching starts. Not much difference in C is found before and after contact, but O content decreases noticeably after contact. The content of C and O is relatively small on lithium after contact with LiBF4-NP if compared with that after contact with LiTFSI-NP,
and alternatively, the F content in the former case is much higher. In fact the amount of O in the native film is much higher than the surface after contact with either of the electrolytes. The main reason for this is the presence of LiF in the interface after contact with polymer electrolytes. This indicates that the surface layer on as-received lithium is replaced by LiF to a higher degree following contact with LiBF4-NP, due to HF presence in a higher ratio in LiBF4 as an impurity and/or as a result of decomposition of the lithium salt. The above observations and discussion of the metallic lithium surface after contact with each membrane are summarized schematically in Fig. 8. For further investigation on the morphology of the passivation layer, a time-dependence surface analyses are essential to follow the change in the chemical nature of the film.
4. Conclusions Due to the lower content of HF and the adsorption of TFSI anion and/or the formation of polymeric form of the anion, a thin and stable interface at the LiTFSINP/Li contact seems to be established. This thin film is not resistive, and thus, its contribution to the charge transfer process is limited. On the other hand, the higher content of HF in LiBF4-NP causes a thick interfacial layer and the replacement of the native passivation layer by LiF, which is likely to form voids within and in between the existing layers. Thus, it may be assumed that the interfacial behavior at the electrolyte/ electrode contact in the LiBF4-NP system is not conductive, but high porosity in the interfacial layer mainly maintains the charge transfer process, which ultimately leads to lithium dissolution/deposition at the electrode.
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Fig. 8. Schematic illustration for the surface film formed on the Li-foil after contact with (a) LiTFSI-NP or (b) LiBF4-NP.
Acknowledgements This research was supported in part by Grant-in-Aid for Scientific Research on Priority Areas (A) ‘Molecular Synchronization for Design of New Materials System’ (No. 404/11167234) from the Japanese Ministry of Education, Science, Sports, and Culture, and NEDO International Joint Research Grant. The authors would like to thank Dr. M. Kondoh for taking XPS measurements of the Li samples and Dr. Kiyoshi Kanamura, Tokyo Metropolitan University, for a useful discussion. The authors would also like to thank Dai-ichi Kogyo Seiyaku for supplying them with valuable macromonomers. I.I. would like to thank Japan Petroleum Institute and Petroleum Energy Center (Japan) for financial support.
[2] [3] [4] [5] [6] [7] [8] [9] [10]
References [11] [1] D. Aurbach, B. Markovsky, M.D. Levi, E. Levi, A.
.
Schechter, M. Moshkovich, Y. Cohen, J. Power Sources 81-82 (1999) 95. K. Kanamura, S. Shiraishi, H. Tamura, Z. Takehara, J. Electrochem. Soc. 141 (1994) 2379. M. Kono, E. Hayashi, M. Watanabe, J. Electrochem. Soc. 146 (1999) 1626. M. Kono, E. Hayashi, M. Nishiura, M. Watanabe, J. Electrochem. Soc. 147 (2000) 2517. K.A. Murugesamoorth, J.R. Owen, Br. Polym. J. 20 (1988) 227. P.G. Bruce, F. Krok, Electrochim. Acta 33 (1988) 1669. S.E. Sloop, M.M. Lerner, J. Electrochem. Soc. 143 (1996) 1292. A. Lisowska-Oleksiak, Solid State Ionics 119 (1999) 205. K. Kanamura, H. Tamura, S. Shiraishi, Z. Takehara, J. Electrochem. Soc. 142 (1995) 340. K. Kanamura, H. Tamura, Z. Takehara, J. Electroanal. Chem. 333 (1992) 127. K. Naoi, M. Mori, Y. Naruoka, W. Lamanna, R. Atanasoski, J. Electrochem. Soc. 146 (1999) 462.
XPS study of lithium surface after contact with lithium-salt doped polymer electrolytes Iqbal Ismail 1, Akihiro Noda, Atsushi Nishimoto, Masayoshi Watanabe * Department of Chemistry and Biotechnology, Yokohama National Uni6ersity, Hodogaya-ku, Yokohama 240 -8501, Japan Received 7 August 2000; received in revised form 3 October 2000
Abstract X-ray photoelectron spectroscopy (XPS) is used to probe the surface layer and element composition of Li-metal electrodes before and after contact with polymer electrolytes containing LiN(SO2CF3)2 (LiTFSI) or LiBF4. Native film on as-received metallic lithium was composed of Li2CO3/LiOH in the outer layer and Li2O in the inner layer. LiF was formed during lithium contact with electrolyte due to reaction between the native film and impurities in the electrolyte. The polymer electrolyte containing LiTFSI yielded a very thin film with limited porosity in the inner layers, which was reflected in the limited amplitude dependence of complex impedance spectra. LiBF4 mixed with polymer resulted in a thicker film with high porosity, as was postulated from the greater influence of the amplitude of the oscillation level. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: XPS; Polymer electrolyte; Lithium electrode; Passivation layer; Lithium polymer battery
1. Introduction In order to improve the cyclability of lithium anode secondary batteries, the rechargeability of the lithium electrode needs to be improved. In other words, the compatibility of electrolyte with lithium metal must be enhanced, which means that a stable, low impedance interfacial layer is formed upon contact. It has been well known that the rechargeability problem is associated with the reactivity of the electrode when attached with electrolytes. It is proposed that the type of electrolyte is the main factor in controlling the nature of the passivation layer at the contact. Furthermore, investigations on the lithium anode suggest that the den-
* Corresponding author. E-mail address: [email protected] (M. Watanabe). 1 Permanent address: Department of Chemistry, King Abdul Aziz University, PO Box 9028, Jeddah 21413, Saudi Arabia.
dritic film formed on the anode during the cycling process strongly influences its rechargeability. Due to the presence of organic and inorganic compositions at the surface of lithium metal after contact with electrolyte, surface analyses will complement the electrochemical studies. Beside several other techniques (e.g. FTIR [1], SEM [2]) X-ray photoelectron spectroscopy (XPS) has been widely used for studying the surface reaction of lithium electrodes. In this study, we report the surface analyses of lithium electrode before and after contact with network polymer, NP, (prepared from mono-acrylated and triacrylated copolymer of ethylene oxide and propylene oxide) electrolyte membrane containing lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) or lithium tetrafluoroborate (LiBF4). The effect of different kinds of lithium salt on the electrode electrolyte interface has been investigated using electrochemical and surface analysis methods. Models for the passivation layer are proposed, depending on the lithium salt incorporated in the membrane.
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 7 5 8 - 1
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I. Ismail et al. / Electrochimica Acta 46 (2001) 1595–1603
2. Experimental
2.1. Materials and preparation Mixtures of two different oligomers (0.5 g of each) were used as macromonomers; mono-acrylate (MA) and tri-acrylate (TA) of copolymers of ethylene oxide and propylene oxide (both kindly supplied by Dai-ichi Kogyo Seiyaku Co.) for polymer electrolyte preparation [3,4]. The molecular structures are shown in Scheme 1. LiTFSI (kindly supplied by IREQ, dried at 180°C, under reduced pressure for 24 h) or LiBF4, (Tomiyama Chemicals, battery grade), and 2,2dimethoxy-2-phenyl acetophenone (Ciba Geigy, 500 ppm based on the weight of macromonomers) as photo initiator, together with the mixture of macromonomers, were dissolved in dehydrated acetonitrile (Kanto Chem-
ical Co. Inc.). The mixture was left to stir until a homogeneous clear solution was obtained. The viscous solution was spread between two glass plates separated by a polytetrafluoroethylene spacer of 0.5 mm thick, before it was irradiated with UV light (250 W high pressure Hg lamp, UI-501C Ushio Electric Inc.) for 5 min. Residual acetonitrile was evaporated under vacuum at room temperature. Transparent and flexible polymer electrolyte membranes were obtained, which were then used in electrochemical measurements followed by XPS analyses.
2.2. Electrochemical measurements Polymer electrolyte films were cut into disks of 13 mm diameter and sandwiched between two symmetrical lithium electrodes (Honjo Metal Co.) of 0.2 mm thick, and the whole was placed in a sealed cell and stored in a constant temperature (60°C) oven. After one week of storage time, amplitude dependence measurements were carried out by using an AC impedance analyzer (Hewlett– Packard 4192A) over 5 Hz– 13 MHz frequency range.
2.3. Lithium surface analysis by XPS
Scheme 1. Structures of macromonomers for polymer electrolyte preparation.
The cell, after storing ( \ 1 month) at 60°C and subsequent temperature dependence measurements (20– 80°C) of AC impedance (data not shown here), was disassembled in a dry argon atmosphere, and the polymer electrolyte membrane film was carefully peeled off from the metallic lithium, and the lithium was mounted on an XPS stage. The depth profiles of the lithium surface for the relevant elements were explored by using Shimadzu-Koratos Electron Spectrometer XSAM-800 with argon ion sputtering (3 kV, 0.25– 0.5 mA). An Mg–Ka line was used as an x-ray source. The etching rate was roughly estimated to be 4 A, /min. The binding energy for each spectrum for lithium foil before contact was calibrated by C 1s peak, corresponding to hydrocarbon adsorbed on the sample surface (285.0 eV). However, due to better sensitivity and selectivity for LiF peaks in F 1s region, all the binding energies for each spectrum for the lithium surface after contact with membranes were calibrated by F 1s at 685.5 eV, corresponding to LiF. XPS analysis was performed after reducing the pressure in the XPS chamber below 10 − 9 torr.
3. Results and discussion
3.1. Electrochemical in6estigation Fig. 1. Time dependence of interfacial resistance for the network polymer complexed with (a) LiTFSI and (b) LiBF4, at 60°C.
Fig. 1 shows time dependence of interfacial resistance between lithium electrode and network polymer elec-
I. Ismail et al. / Electrochimica Acta 46 (2001) 1595–1603
Fig. 2. Oscillation level dependence of impedance diagram for the network polymer complexed with (a) LiTFSI and (b) LiBF4 and sandwiched between two symmetrical Li electrodes at 60°C.
trolytes containing LiTFSI or LiBF4 ([Li]/[O]=0.08) at 60°C. It is obvious here that LiTFSI-NP interface proved to be constant for a long time, whereas LiBF4NP showed increasing interfacial resistance over the period of time. This is believed to be due to the instability of the latter salt. Table 1 indicates data obtained from a Li/LiX-NP/Li (where X: TFSI− or BF− 4 ) electrochemical cell after 1 month of storage. The thickness of the passivation layer, l, was calculated using Eq. (1): l=2yfmaxmrmoARi
(1)
where fmax is the frequency at the maximum of imaginary part of interfacial impedance (-Z¦) in the complex
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impedance diagram, mr is the relative dielectric constant for the passivation layer, which was set at 10 for lithium compounds on the surface [5], mo is the permittivity of vacuum, and A is the electrode area. Although a value of 5 was used [6] for mr for an interfacial film between a highly lithiated cathode material in contact with tetraethylene glycol dimethyl ether, in the case of salted medium a higher permittivity value is more realistic. For simplicity, we assumed that mr remained constant over the life of the interface while the passivation thickness varied. The estimation of film thickness in this work is much smaller than that reported by Sloop and Lerner [7], who estimated the thickness after 1 week of lithium metal contact with poly(ethylene oxide)/LiClO4. This may be due to low water content ( B 20 ppm) in our polymer electrolytes. Passivation film thickness values for lithium after contact with LiTFSI-NP and LiBF4-NP, are lower and higher, respectively, if compared with that calculated for the film formed on Li metal in contact with a linear polyether (tetraethylene glycol dimethyl ether) [8]. This wide variation in film thickness associated with lithium salt type suggests that the polymer host has a less significant role in controlling the interfacial property, and the nature of lithium salt has a greater effect in modifying the structure of the passivation layer. It can be noticed here that the thicker interface for LiBF4-NP is associated with higher resistance, lower capacitance and larger time constant. This greater rise in Ri for LiBF4-NP, compared with the LiTFSI-NP, is another indication of the contrast in the nature of the interface between the two cases. Fig. 2 shows the amplitude dependence of impedance response for (a) LiTFSI-NP and (b) LiBF4-NP. It can be seen here that, in both compositions, the high frequency semi-circle is the same for different amplitudes. The high frequency semi-circle agreed well with that obtained when blocking, stainless steel, electrodes were used (not shown here). This indicates that the resistance for the high frequency semi-circle corresponds to the bulk property of the polymer electrolyte. However, the low frequency semi-circle depicted different behavior for the two salt compositions. LiBF4-NP was remarkably dependent on the amplitude, whereas LiTFSI-NP was not greatly influenced.
3.2. XPS analysis for the nati6e film on the lithium surface Fig. 3 shows the XPS spectra of Li 1s, C 1s, and O 1s regions for the native film on the lithium surface. In these spectra, three lithium compounds are observed, i.e. Li2CO3, LiOH and Li2O. The peak at 55.2 eV in Li 1s region can be assigned to Li2CO3 and/or LiOH,
3.56 1.18
| at 60°C (10−4 S cm−1)
Tg measured after polymer electrolyte preparation.
234.3 241.4
LiTFSI LiBF4
a
Tg (K) a
Lithium salt mixed with network polymer 0.16 1.41
Ri at 60°C (103 V cm2) 67.3 69.1
Ea (for Arrhenius plot of Ri) (kJ mol−1)
Table 1 Data calculated for lithium foil after contact with network polymer electrolyte membrane for about a month ([Li]/[O]= 0.08)
0.563 0.226
Cdl (mF cm2)
0.9 3.2
~ (10−4 s)
80 200
l (A, )
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Fig. 3. XPS spectra for the native film on as-received Li-foil.
whose intensity decreases with etching time. The peak at 54.0 eV corresponding to Li2O is observed after 30 min of etching time and is detectable up to 90 min of etching time with almost the same intensity. It may be noticed that the thickness of the native film on lithium foil was estimated by Kanamura et al. [9] to be as much as 900 A, . However, due to instrumental limitation in this work we were unable to extend the argon ion sputtering time and thus, no peak for Li metal (at 52.3 eV in Li 1s) was possible to detect. Nevertheless, we do not expect that the native film in our work is that thick, since during cell assembly for electrochemical measurements lithium foil was firmly pressed on the stainless steel substrate in order to break the native film on the lithium surface. A similar treatment was applied prior to XPS measurement for the as-received lithium surface. Fig. 3b shows C 1s peaks for the native film on as-received lithium foil. A sharp peak of hydrocarbon is depicted at 285.0 eV before etching, as usual. Hydrocarbon is believed to exist in the glove box and/or the XPS chamber as a contaminant, which is mainly adsorbed on the outer surface of the sample. The peak at 290.1 eV corresponding to Li2CO3 is observed in the outer layer of the native film and up to 10 min of etching time in a greater intensity, though in the following scans its intensity decreases remarkably. Fig. 3c shows XPS spectra of the O 1s region. A peak at 532.0 eV corresponding to Li2CO3/LiOH is observed, which also decreases after 10 min of etching time. A small peak, but with strong intensity later, appears at 528.3 eV corresponding to Li2O at 10 min of etching time. The above analysis indicates that the carbonate layer is relatively thinner when compared with the oxide layer. These observations for Li2CO3/LiOH and Li2O in Li 1s, C 1s, and O 1s spectra, are in good agreement with each other in terms of etching profile in accordance with their assigned compositions. The native film on the
as-received lithium can be described schematically as in Fig. 4.
3.3. XPS analysis for the lithium surface following contact with polymer electrolytes Fig. 5a – d show the XPS spectra of Li 1s, C 1s, O 1s, and F 1s regions for the lithium surface following contact with LiTFSI-NP. It may be noticed that peak positions are displaced, compared with those of native film spectra. This is because different calibrating peaks are used for the before and after contact samples (see Section 2). In Fig. 5a Li2CO3/LiOH peaks are apparent from 16 min of etching time onward, whereas the peak assigned to Li2O starts to appear from 36 min of etching time. The TFSI anion or polymer electrolyte peak is observed at 286.8, 533, and 689.2 eV in C 1s, O 1s, and F 1s regions, respectively, before etching and decreased thereafter. This organic content at the surface is the undetached part of the electrolyte left while peeling it off from the electrode and/or it is the product of reaction between the electrolyte and metallic lithium. At 16 min of etching time onward, a peak at 283.8 eV in the C 1s spectrum is recorded which corresponds to the carbide. Fig. 5a and d present the LiF peak at 56.0 eV in Li 1s and 685.5 eV at F 1s regions, respectively.
Fig. 4. Schematic illustration for the native film on as-received Li-foil.
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Fig. 5. XPS for Li-foil after 1 month of contact with LiTFSI-NP at 60°C.
In the F 1s spectrum the peak starts to appear clearly at 1 min of etching time and onward, although in the Li 1s spectrum it appears even before etching. In fact, LiF possibly has two sources within the passivation layer: (i) the reaction of HF as a contaminant in the electrolyte with Li2CO3, LiOH and/or Li2O in the native layer, i.e. acid–base reaction (see for example [10]), and (ii) a direct reaction of the TFSI anion with Li metal. A similar reaction to the latter possibility was proposed by Naoi et al. [11] between bis(perfluoroethylsulfonyimide) [N(C2F5SO2)2] anion and metallic Li to produce the polymeric form of the imide anion at the Li surface when it was immersed in a LiN(C2F5SO2)2/propylene carbonate electrolyte.
The above observations together with the electrochemical results (Table 1, Fig. 2) led us to postulate that in the case of the LiTFSI-NP electrolyte, thin and slightly porous layers of various compositions formed at the surface of lithium foil following the contact. It may be considered that the source of Li2CO3/LiOH and Li2O is almost entirely from the native film, since the adsorption of TFSI anion or its polymerized form at the surface inhibits any further reaction between the polymer and lithium, which normally causes the production of carbonate [11]. Residual HF can easily react with Li2CO3, LiOH or Li2O to form LiF, whereas, TFSI anion needs to migrate through the native film to react with the metallic lithium. If the latter step takes place it is not possible to detect the anion only in the
I. Ismail et al. / Electrochimica Acta 46 (2001) 1595–1603
outer layer. Thus, we believe that the TFSI- peak observed on the outer layer is its natural form rather than the polymeric product, and therefore, the only source for LiF is the acid–base reaction between HF and the lithium compounds in the native film. Fig. 6a–d show the XPS peaks of Li 1s, C 1s, O 1s, and F 1s regions for the lithium surface following contact with LiBF4-NP. In Li 1s (Fig. 6a) and F 1s (Fig. 6d) spectra it may be seen that LiF is formed on the surface with increasing intensity in the following spectra as a function of etching time. Li2CO3/LiOH peaks are observed in Li 1s (Fig. 6a) and O 1s (Fig. 6c)
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spectra, i.e. almost from 0 to 36 min of etching. The Li2O composition peak starts to appear at 16 min of etching time in O 1s (Fig. 6c), which showed an increasing trend thereafter. A polymer peak is observed in the outer layer at 286.8 and 533 eV in C 1s (Fig. 6b) and O 1s (Fig. 6c), respectively. No peak for adsorbed BF4 in the F 1s spectra, no seen in Fig. 5, is apparent.
3.4. Depth profiles of the lithium surface Fig. 7 shows a depth profile for each element in the surface film of lithium as received (a) and after contact
Fig. 6. XPS for Li-foil after 1 month of contact with LiBF4-NP at 60°C.
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Fig. 7. Depth profile for the Li-metal surface (a) as-received, and after contact for 1 month at 60°C with (b) LiTFSI-NP, or (c) LiBF4-NP.
with LiTFSI-NP (b) or LiBF4-NP (c). The amount of each element was estimated by integrating the intensity of the XPS peak and correcting for the cross-section for the ionization of each element. Thus, a quantitative comparison is more or less satisfactory. In the native film on as-received lithium foil (Fig. 7a), the C content is initially high (due to hydrocarbon adsorbed on the surface), but when etching starts, a decreasing tendency with etching time can be observed, whereas O starts with a high content and slightly increases with etching time. This indicates that the presence of Li2CO3/LiOH is in the outer layer and Li2O mainly in the inner part of the native film. In the depth profile of the lithium surface after contact with LiTFSI-NP (Fig. 7b), one can observe that the content of F is slightly smaller at the surface, although increases in a gradual manner with etching time. On the other hand, C is relatively high in the initial stage, with decreasing tendency after 16 min of etching time. It may be noticed that the C content in the lithium surface after contact with LiTFSI-NP is much higher than its content in the lithium surface before contact, which indicates that the source of C is not only the carbonate salt in the native film (compare carbonate peaks in C 1s before and after contact, Figs. 3b and 5b) but the relatively thick adsorbed TFSI layer or polymer electrolyte layer attached on the surface. This assumption is consistent with the XPS spectrum described earlier (Fig. 5). In the case of the lithium surface after contact with LiBF4-NP (Fig. 7c), the presence of F is predominant, although small before etching, but in a higher content when etching starts. Not much difference in C is found before and after contact, but O content decreases noticeably after contact. The content of C and O is relatively small on lithium after contact with LiBF4-NP if compared with that after contact with LiTFSI-NP,
and alternatively, the F content in the former case is much higher. In fact the amount of O in the native film is much higher than the surface after contact with either of the electrolytes. The main reason for this is the presence of LiF in the interface after contact with polymer electrolytes. This indicates that the surface layer on as-received lithium is replaced by LiF to a higher degree following contact with LiBF4-NP, due to HF presence in a higher ratio in LiBF4 as an impurity and/or as a result of decomposition of the lithium salt. The above observations and discussion of the metallic lithium surface after contact with each membrane are summarized schematically in Fig. 8. For further investigation on the morphology of the passivation layer, a time-dependence surface analyses are essential to follow the change in the chemical nature of the film.
4. Conclusions Due to the lower content of HF and the adsorption of TFSI anion and/or the formation of polymeric form of the anion, a thin and stable interface at the LiTFSINP/Li contact seems to be established. This thin film is not resistive, and thus, its contribution to the charge transfer process is limited. On the other hand, the higher content of HF in LiBF4-NP causes a thick interfacial layer and the replacement of the native passivation layer by LiF, which is likely to form voids within and in between the existing layers. Thus, it may be assumed that the interfacial behavior at the electrolyte/ electrode contact in the LiBF4-NP system is not conductive, but high porosity in the interfacial layer mainly maintains the charge transfer process, which ultimately leads to lithium dissolution/deposition at the electrode.
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Fig. 8. Schematic illustration for the surface film formed on the Li-foil after contact with (a) LiTFSI-NP or (b) LiBF4-NP.
Acknowledgements This research was supported in part by Grant-in-Aid for Scientific Research on Priority Areas (A) ‘Molecular Synchronization for Design of New Materials System’ (No. 404/11167234) from the Japanese Ministry of Education, Science, Sports, and Culture, and NEDO International Joint Research Grant. The authors would like to thank Dr. M. Kondoh for taking XPS measurements of the Li samples and Dr. Kiyoshi Kanamura, Tokyo Metropolitan University, for a useful discussion. The authors would also like to thank Dai-ichi Kogyo Seiyaku for supplying them with valuable macromonomers. I.I. would like to thank Japan Petroleum Institute and Petroleum Energy Center (Japan) for financial support.
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