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Synthesis and electrochemical characterization of new bulky lithium salts
SOLID STATE
Solid State Ionics 70/7 1 ( 1994) 157-l 62 North-Holland
IONICS
Synthesis and electrochemical characterization of new bulky lithium salts D. Benrabah, J.-Y. Sanchez, D. Deroo and M. Armand Laboratoire d’lonique et d’Electrochimie du Solide URA 1213, CNRS, ENSEEG-INP B.P. 75, 38402 Saint Martin d’H&es, France
Grenoble,
We describe the synthesis of new lithium salts, based on bulky carbanions, substituted by strong electron withdrawing groups together with their electrochemical studies using high molecular weight host polymer poly(oxyethylene) POE. The ionic conductivities of lithium salts-POE complexes have been measured as a function of temperature by impedance spectroscopy. The new lithium salts exhibit a wide electrochemical window stability. Their dissolution in POE (Mw= 5 X 106) provides amorphous polymer electrolytes with high ionic conductivity.
1. Introduction
electrochemical study with the starting TFSMLi are reported.
Despite extensive research on the conductivity of polymer electrolytes [ I,2 1, only a limited number of studies have been devoted to the synthesis and the evaluation of new lithium salts [ 3-6 1. We have therefore undertaken the synthesis of a new family of lithium salts, in order both to increase the ionic conductivity of polymer electrolytes, and to decrease the anionic mobility. The idea is to use bulky multivalent anions whose mobility, due to their charge and their bulkiness, should be reduced. The alcaline salts ( CFJS02),CH-, M+ abbreviated TFSM- M+ should correspond to appreciably dissociated salts. Indeed the negative charge being carried by the soft carbon atom, and the carbanion is substituted by two electron-withdrawing groups. This carbanion induces a nucleophilic substitution and therefore can be attached to various substrates. Thus lithium salts, resulting from the reaction of aromatic diacid chloride, have been synthesized and characterized by several analytical methods such as liquid chromatography, FTIR, 19F RMN and thermal analysis. This paper reports the synthesis and studies of the temperature and ionic conductivity dependence of the new system POE-lithium salts. Here, both the results of the new lithium salt and the comparative 0167-2738/94/$07.00
lithium
salt
2. Experimental 2.1. Synthesis of the new lithium salt (dianion) Dianions have been synthesized by reacting bis(trifluoromethanesulfonyl)methane ( CF3S02)2CHNa abbreviated as TFSMNa with terephthaloyl chloride ( C6H, ( COCI ) *, Aldrich ) and benzene- 1,3disulfonyl chloride ( C6H, (SO,CI),, Fluka). The starting salt TFSMNa has been prepared from the acid form TFSMH (CF,S02)zCH2 whose pKa is close to -1 [7]. First, we prepared the pyridinium salts from solutions of part terephthaloyl chloride to two parts of starting salt TFSMNa in presence of pyridine using anhydrous acetonitrile as solvent. The terephthaloyl chloride was added drop by drop to the homogeneous solution of TFSMNa/pyridine/acetonitrile. The resulting NaCl was removed and the acetonitrile evaporated. Pyridinium salts were transformed into the corresponding lithium salts by adding an excess of lithium hydroxide (or lithium phosphate) to the THF solution and stirring for 48 h at room temperature. After the solvent removal, the lithium salt was washed
0 1994 Elsevier Science B.V. All rights reserved.
158
D. Benrabah et al. /Synthesis and electrochemical characterization ofnew bulky lithium salts
with anhydrous methylene chloride, dried at 70°C and stored in glove box under argon. 2.2. Preparation of POE complexes The electrolytes based on POE (Mw = 5 x 106, POE Aldrich) and lithium salts were prepared in a glove box under argon atmosphere by mixing the appropriate ratio of polymer and salt in dry acetonitrile and then stirring at room temperature for few hours. The conventional solvent casting method, using a glass ring on PTFE plate in a dry box, provides homogeneous films. 2.3. Electrochemical and physical characterization methods 2.3.1. Gel permeation chromatography (GPC) The new lithium salts were characterized by GPC. These analyses were carried out using a Waters 590 GPC, equipped with a differential refractometer Waters 410 and a Waters 745 B “Data Module”. The analyses were performed using THF as solvent, previously filtered and degassed through two “ 100 A ultrastyragel columns” and used at a flow rate of 0.7 mQ/mn. The resulting separation of different reagents and solvents was useful. Although, this method is generally used to characterize the molecular weight distribution of polymers, it appears very sensitive to low molecular weight variation and notably very efficient to separate the different reagents and salts. 2.3.2. Infrared spectroscopy The infrared spectra, run as a KBr disk, were carried out to identify the structure of various compounds. The different spectra were recorded on a Nicolet 7 10 FTIR system coupled to a microcomputer. The spectra were collected at room temperature over the 4000-400 cm-’ range by averaging 200 scans with a resolution of 4 cm-‘. 2.3.3. Thermal analysis Physical characterizations included differential scanning calorimetry (DSC) and thermogravimetry (TGA). Differential scanning calorimetry (DSC) and thermogravimetry (TGA) were conducted with a STA 409 Netzsch analyser. Thermal analyses were
performed under helium. The samples were first heated from - 120°C to 150°C at a heating rate of 10°C min- ‘, quenched or cooled to - 120°C and then heated again up to 150°C. 2.3.4. Conductivity measurements The ionic conductivity of the polymer-salt mixtures were determined by impedance spectroscopy with blocking electrodes, using a HP 4192 A analyzer over the frequency range 13 MHz-5 Hz under dynamic vacuum. The cell was handled in a dry-box when the samples were being sandwiched between two stainless steel electrodes, after which it was submitted to heating-cooling cycles. The conductivity was measured at various temperatures, after an equilibrium time of one hour at each temperature. 2.3.5. Cyclic vo1tammetr.y The electrochemical stability of lithium salts has been checked by voltamperometry in two variety of cells (micro and macroelectrodes). Microelectrodes were employed in this work because of their many special advantages for the study of polymeric systems [ 8-101. One of the main advantages of microelectrodes is that the measured currents are very weak, making them useful for the highly resistive polymer electrolyte solvents where macroelectrodes encounter large uncompensated IR effects. The electrochemical stability window was determined using a two-electrode configuration cell [ 111 with a glass sealed metallic microdisc of platinum (a=25 l.trn) nickel (0=50 pm) or copper (0 = 100 urn) as working electrode. Lithium metallic foil pressed on a stainless steel collector became the auxiliary electrode, and could be used as the reference electrode, with no detectable perturbation of its potential, since low currents were measured. The experiments were carried out in argon at 80°C with scanning rates of 12 and 20 mV/s. The voltammetric scan and the current measurement were made using a laboratory built potentiostat. The applied voltage range can reach +4 V with a current reading accuracy of 10 picoamps. Since the working electrode surface is not refreshed like a mercury drop electrode, a single scan is made for each measurement, and this electrode is polished prior to each measurement. The macroelectrode device is represented on the next column:
D. Benrabah et al. /Synthesis and electrochemical characterization of new bulky lithium salts
Polymer electrolyte
I
.,
..,
. .
159
- Auxiliary electrode (stainless steel)
Conductor lamella
Work’ (Pl liererence electrode (Lithium)
3.2. Electrochemical stability
3. Results and discussion 3. I. Synthesis of CJf4(COTFSMLi)2
As was described in the experimental section, the new lithium salt was obtained by nucleophile reaction between a diacid chloride and the starting carbanion, according to scheme I:
COCI
0
COTFSMPyr’
+ Z(CF,SOp)zCH’Na’
Pyridine ACelOnltrlle
+ 2 NaCl
0 COCI SO,CF, Li*
i
I
“”
CF,S+02
We described the electrochemical stability window of the starting salt TFSMLi [ 5 ] in the recent paper. A comparative electrochemical stability study of TFSMLi was carried out with both micro and macroelectrode. Fig. 2 shows the recording cyclic macrovoltamperogramm of the electrolyte containing TFSMLi (O/Liz 10) at 6O”C, using platinium as the working electrode on a potential range +0.2 to 4.5 V versus Li/Li+. This voltammogram exhibits a peak at + 3.1 V on the second scan during the oxidation. This peak corresponds to the hydrogen previously produced during the reduction scan of POE/ TFSMLi. Indeed the remaining proton of TFSMLi is acidic and can be reduced in hydrogen during the reduction scan. ( CF3S02)&H-Li+
+ Li
‘SO&F,
Scheme I
The reaction was followed by (GPC) chromatography which shows the disappearance of the sodium salt TFSMNa and a single well-defined peak corresponding to the pyridinium salt. The lithium salt peak also appears as a single peak at a different retention time following the addition of lithium hydroxide. The infra-red studies were focused mainly on the 3030-l 140 cm-’ region which contains the SOz, CF3, CsH5 and CO stretching bands of the lithium salts. The IR spectra, run as a KBr disk, exhibited peaks related to the dianion structure (fig. 1).
Moreover comparative microvoltammetry studies performed on the same complex at 8o”C, using a copper microelectrode, did not reveal any lithium stripping and it can therefore be supposed that this remaining slightly acid proton should react rapidly with the freshly reduced lithium (fig. 3 ). On the other hand, the cyclic microvoltammetry performed on a POE/ 1-4 C6H4 ( COTFSMLi)2 complex, using first a copper (fig. 4) then a platinum (fig. 5) microelectrode, at the same temperature and concentration conditions, shows a wider electrochemical window O-3.9 V versus Li. The plating/stripping of lithium can be observed using the copper microelectrode. With the platinum one we observed
160
D. Benrabah et al. /Synthesis and electrochemical charactenzation ofnew bulky lithium salts
Fig. 1. FTIR absorption
spectrum
of the new lithium salt l-4 C,H,( COTFSMLi
)*.
0.2
5 E/V
vs.Li-
/ Li
Fig. 2. Voltammogram of the electrochemical POE,-TFSMLi complex at 60°C on a platinum 0=9km.Scanrate=lmV/s.
stability of the macroelectrode
three peaks during the oxidation scan which might be attributed respectively to the oxidation of lithium resulting in the formation of a lithium/platinum alloy-Li3Ptz and an LiPt, intermetallic compounds which are successively oxidized during the anodic scan. 3.3. Thermal stability A comparative thermogravimetric study of our new salts, carried out in helium, from room temperature at a heating rate of lVC/min, shows that the decomposition starts around 270°C. All the new salts
Fig. 3. Voltammogram of the lithium plating-stripping process from the POE,-TFSMLi complex at 80°C on a copper microelectrode 0= 100 pm. Scan rate= 12 mV/s.
exhibit a thermal stability which is more than enough, with respect to their application in secondary lithium batteries. Table 1 lists the melting points and the temperatures of decomposition of the salts determined from DSC analysis. The complexes POE/ l-4 C6H4 ( COTFSMLi)2 are amorphous for an O/ Li ratio < 12, and then exhibits a crystalline phase. The related melting points were found to increase with the salt concentration. The polymer electrolytes whose salt concentration ranges from O/Li = 4 to O/ Li=24, exhibits a glass transition and a single endotherm, at between 27 and 42°C for samples with
D. Benrabah et al. /Synthesis and electrochemical characterization of new bulky bthium salts
161
Table 2 Glass transition temperatures and melting temperatures of several samples POE.-C6H4(COTFSMLi), determined from DSC. (Heating rate 10 K min- ’ ) O/Li
T, (“C)
Tr ( 1st heating)
Tr (2nd heating)
8 10
-36 -39
t *
*
12 16 24 32
-41 -33 - 14 *
28 30 52 44
21 28 38 42
l
E i V vs. Li*/Li”
Fig. 4. Voltammogram of the lithium plating-stripping process from the POE,/l-4 CsH,(COTFSMLi)2 complex at 80°C on a copper microelectrode 0 = 100 urn. Scan rate = 12 mV/s.
3.4. Conductivity of POE/I-4 CJS~(COTFSMLi), electrolytes Several poly( oxyethylene) complexes prepared from the dianion C6H4( COTFSMLi)2 are amor-
d
-1
1
4
3
2
E / V vs. Li+/Li
Fig. 5. Voltammogram of the electrochemical stability of the POE,/ l-4 C6H4( COTFSMLi)2 complex at 80°C on a platinum microelectrode 0 = 2 5 urn. Scan rate = 12 mV/s.
Table 1 Decomposition eral new salts.
temperatures
Compound
TFSMK TriTFSMLi &H,COTFSMLi C6H.,(COTFSMK)2 a) Not determined.
and melting
temperatures
Melting point
Degradation
( T,“C )
(“C)
268 *a) * 250
316 360 270 318
of sev-
temp.
(The same in table 2).
composition of O/Li between 12 and 32 (table 2). At low concentrations the endotherm was present but at higher concentration the endotherm did not appear.
lOOO/T(K-‘)
Fig. 6. Dependence of conductivity versus the reciprocal temperature for complexes POE./ l-4 C6H4 ( COTFSMLi ) 2 for several high concentrations: (a) first heating cycle, (b) first cooling cycle.
D. Benrabah et al. /Synthesis and electrochemical characterization of new bulky lithium salts
conductivity values of the new systems, the triflate and the perchorate obtained with the same POE host polymer (fig. 8 ). The best conductivities, reached for the dianion C6H4( COTFSMLi)* based complexes are about two orders of magnitude higher than the values obtained with usual lithium salts such as LiClO, and LiCF,S03.
4. Conclusion
Fig. 7. Dependence of conductivity versus the reciprocal temperature for complexes POE,/ l-4 CsH4(COTFSMLi)2 for two compositions O/Li = 24 and 32 during heating and cooling.
112
1 o-2
97
84
72
60
50
40
30
21
This divalent lithium salt appears as a very promising salt providing both high conductivity values and a good thermal and electrochemical stability. Furthermore, owing the dianonic nature of the salt we might expect an anionic decrease mobility.
T(“C)
c
Acknowledgement The Laboratoire d’Ionique et d’Electrochimie des Solides de Grenoble gratefully acknowledge financial support from Hydro-Quebec within the ACEP project.
lca’ 2.5
, 2.6
’
2.7
I 2.9
3
2.9 1000/T
Fig. 8. Comparison of conductivity ium salts complexes.
’ 3.1
’ 3.2
3.3
3.4
3.5
(K-‘)
levels for the four POE, lith-
phous at room temperature in a wider concentration range than with LiTFSI. Therefore conductivities close to 3x lo-’ S cm-’ are reached at 25”C, while several complexes exhibit a conductivity of 1O-3 S cm-’ around 60°C. The Arrhenius plots of conductivity versus T-’ (fig. 6a, 6b) exhibit a free-volume behaviour in the salt concentration range O/Li = 4 to 10. Fig. 7 summarizes the conductivity behaviour of two electrolyte compositions O/Li=24 and 32 during a heating/cooling cycle. The Arrhenius plots (fig. 7) of conductivity for these samples show two domains, the slope undergoing a sudden change around 45°C with low conductivities. Finally, we compared
References [ I ] G. Goulart, These de Doctorat (INPG, I992 ), [2] F. Alloin, J.-Y. Sanchez and M. Armand, Solid State Ionics 60 (1993) 3. [3] S. Sylla, J.Y. Sanchez, M. Armand, Electrochim. Acta 37 (1992) 1699. [4 ] D. Benrabah, J.-Y. Sanchez, M. Armand, D. Bar& G.G. Gard, J. Chem. Sot. Faraday Trans. 89 ( 1993) 355. [ 51 D. Benrabah, J.-Y. Sanchez and M. Armand, Solid State Ionics 60 ( 1993) 87. [ 61 L.A. Dominey, V.R. Koch and T.J. Blacley, Electrochim. Acta 37 (1992) 1551. [ 71 R.J. Koshar and R.A. Mitsch, J. Org. Chem. 38 ( 1973) 3358. [ 81 L. Geng, A.G. Ewing, J.C. Jemigan and R.W. Murray, Anal. Chem. 58 (1986) 852. [ 91 M.L. Longmire, M. Watanabe, H. Zhang, T.T. Wooster and R.W. Murray, Anal. Chem. 62 ( 1990) 747. [lo] M. Watanabe, M.L. Longmire and R.W. Murray, J. Phys. Chem. 94 (1990) 2614. [ 111 D. Baril, Memoire de Maitrise des Sciences (Universitt de Sherbrooke, Quebec, Canada, 1991).
Solid State Ionics 70/7 1 ( 1994) 157-l 62 North-Holland
IONICS
Synthesis and electrochemical characterization of new bulky lithium salts D. Benrabah, J.-Y. Sanchez, D. Deroo and M. Armand Laboratoire d’lonique et d’Electrochimie du Solide URA 1213, CNRS, ENSEEG-INP B.P. 75, 38402 Saint Martin d’H&es, France
Grenoble,
We describe the synthesis of new lithium salts, based on bulky carbanions, substituted by strong electron withdrawing groups together with their electrochemical studies using high molecular weight host polymer poly(oxyethylene) POE. The ionic conductivities of lithium salts-POE complexes have been measured as a function of temperature by impedance spectroscopy. The new lithium salts exhibit a wide electrochemical window stability. Their dissolution in POE (Mw= 5 X 106) provides amorphous polymer electrolytes with high ionic conductivity.
1. Introduction
electrochemical study with the starting TFSMLi are reported.
Despite extensive research on the conductivity of polymer electrolytes [ I,2 1, only a limited number of studies have been devoted to the synthesis and the evaluation of new lithium salts [ 3-6 1. We have therefore undertaken the synthesis of a new family of lithium salts, in order both to increase the ionic conductivity of polymer electrolytes, and to decrease the anionic mobility. The idea is to use bulky multivalent anions whose mobility, due to their charge and their bulkiness, should be reduced. The alcaline salts ( CFJS02),CH-, M+ abbreviated TFSM- M+ should correspond to appreciably dissociated salts. Indeed the negative charge being carried by the soft carbon atom, and the carbanion is substituted by two electron-withdrawing groups. This carbanion induces a nucleophilic substitution and therefore can be attached to various substrates. Thus lithium salts, resulting from the reaction of aromatic diacid chloride, have been synthesized and characterized by several analytical methods such as liquid chromatography, FTIR, 19F RMN and thermal analysis. This paper reports the synthesis and studies of the temperature and ionic conductivity dependence of the new system POE-lithium salts. Here, both the results of the new lithium salt and the comparative 0167-2738/94/$07.00
lithium
salt
2. Experimental 2.1. Synthesis of the new lithium salt (dianion) Dianions have been synthesized by reacting bis(trifluoromethanesulfonyl)methane ( CF3S02)2CHNa abbreviated as TFSMNa with terephthaloyl chloride ( C6H, ( COCI ) *, Aldrich ) and benzene- 1,3disulfonyl chloride ( C6H, (SO,CI),, Fluka). The starting salt TFSMNa has been prepared from the acid form TFSMH (CF,S02)zCH2 whose pKa is close to -1 [7]. First, we prepared the pyridinium salts from solutions of part terephthaloyl chloride to two parts of starting salt TFSMNa in presence of pyridine using anhydrous acetonitrile as solvent. The terephthaloyl chloride was added drop by drop to the homogeneous solution of TFSMNa/pyridine/acetonitrile. The resulting NaCl was removed and the acetonitrile evaporated. Pyridinium salts were transformed into the corresponding lithium salts by adding an excess of lithium hydroxide (or lithium phosphate) to the THF solution and stirring for 48 h at room temperature. After the solvent removal, the lithium salt was washed
0 1994 Elsevier Science B.V. All rights reserved.
158
D. Benrabah et al. /Synthesis and electrochemical characterization ofnew bulky lithium salts
with anhydrous methylene chloride, dried at 70°C and stored in glove box under argon. 2.2. Preparation of POE complexes The electrolytes based on POE (Mw = 5 x 106, POE Aldrich) and lithium salts were prepared in a glove box under argon atmosphere by mixing the appropriate ratio of polymer and salt in dry acetonitrile and then stirring at room temperature for few hours. The conventional solvent casting method, using a glass ring on PTFE plate in a dry box, provides homogeneous films. 2.3. Electrochemical and physical characterization methods 2.3.1. Gel permeation chromatography (GPC) The new lithium salts were characterized by GPC. These analyses were carried out using a Waters 590 GPC, equipped with a differential refractometer Waters 410 and a Waters 745 B “Data Module”. The analyses were performed using THF as solvent, previously filtered and degassed through two “ 100 A ultrastyragel columns” and used at a flow rate of 0.7 mQ/mn. The resulting separation of different reagents and solvents was useful. Although, this method is generally used to characterize the molecular weight distribution of polymers, it appears very sensitive to low molecular weight variation and notably very efficient to separate the different reagents and salts. 2.3.2. Infrared spectroscopy The infrared spectra, run as a KBr disk, were carried out to identify the structure of various compounds. The different spectra were recorded on a Nicolet 7 10 FTIR system coupled to a microcomputer. The spectra were collected at room temperature over the 4000-400 cm-’ range by averaging 200 scans with a resolution of 4 cm-‘. 2.3.3. Thermal analysis Physical characterizations included differential scanning calorimetry (DSC) and thermogravimetry (TGA). Differential scanning calorimetry (DSC) and thermogravimetry (TGA) were conducted with a STA 409 Netzsch analyser. Thermal analyses were
performed under helium. The samples were first heated from - 120°C to 150°C at a heating rate of 10°C min- ‘, quenched or cooled to - 120°C and then heated again up to 150°C. 2.3.4. Conductivity measurements The ionic conductivity of the polymer-salt mixtures were determined by impedance spectroscopy with blocking electrodes, using a HP 4192 A analyzer over the frequency range 13 MHz-5 Hz under dynamic vacuum. The cell was handled in a dry-box when the samples were being sandwiched between two stainless steel electrodes, after which it was submitted to heating-cooling cycles. The conductivity was measured at various temperatures, after an equilibrium time of one hour at each temperature. 2.3.5. Cyclic vo1tammetr.y The electrochemical stability of lithium salts has been checked by voltamperometry in two variety of cells (micro and macroelectrodes). Microelectrodes were employed in this work because of their many special advantages for the study of polymeric systems [ 8-101. One of the main advantages of microelectrodes is that the measured currents are very weak, making them useful for the highly resistive polymer electrolyte solvents where macroelectrodes encounter large uncompensated IR effects. The electrochemical stability window was determined using a two-electrode configuration cell [ 111 with a glass sealed metallic microdisc of platinum (a=25 l.trn) nickel (0=50 pm) or copper (0 = 100 urn) as working electrode. Lithium metallic foil pressed on a stainless steel collector became the auxiliary electrode, and could be used as the reference electrode, with no detectable perturbation of its potential, since low currents were measured. The experiments were carried out in argon at 80°C with scanning rates of 12 and 20 mV/s. The voltammetric scan and the current measurement were made using a laboratory built potentiostat. The applied voltage range can reach +4 V with a current reading accuracy of 10 picoamps. Since the working electrode surface is not refreshed like a mercury drop electrode, a single scan is made for each measurement, and this electrode is polished prior to each measurement. The macroelectrode device is represented on the next column:
D. Benrabah et al. /Synthesis and electrochemical characterization of new bulky lithium salts
Polymer electrolyte
I
.,
..,
. .
159
- Auxiliary electrode (stainless steel)
Conductor lamella
Work’ (Pl liererence electrode (Lithium)
3.2. Electrochemical stability
3. Results and discussion 3. I. Synthesis of CJf4(COTFSMLi)2
As was described in the experimental section, the new lithium salt was obtained by nucleophile reaction between a diacid chloride and the starting carbanion, according to scheme I:
COCI
0
COTFSMPyr’
+ Z(CF,SOp)zCH’Na’
Pyridine ACelOnltrlle
+ 2 NaCl
0 COCI SO,CF, Li*
i
I
“”
CF,S+02
We described the electrochemical stability window of the starting salt TFSMLi [ 5 ] in the recent paper. A comparative electrochemical stability study of TFSMLi was carried out with both micro and macroelectrode. Fig. 2 shows the recording cyclic macrovoltamperogramm of the electrolyte containing TFSMLi (O/Liz 10) at 6O”C, using platinium as the working electrode on a potential range +0.2 to 4.5 V versus Li/Li+. This voltammogram exhibits a peak at + 3.1 V on the second scan during the oxidation. This peak corresponds to the hydrogen previously produced during the reduction scan of POE/ TFSMLi. Indeed the remaining proton of TFSMLi is acidic and can be reduced in hydrogen during the reduction scan. ( CF3S02)&H-Li+
+ Li
‘SO&F,
Scheme I
The reaction was followed by (GPC) chromatography which shows the disappearance of the sodium salt TFSMNa and a single well-defined peak corresponding to the pyridinium salt. The lithium salt peak also appears as a single peak at a different retention time following the addition of lithium hydroxide. The infra-red studies were focused mainly on the 3030-l 140 cm-’ region which contains the SOz, CF3, CsH5 and CO stretching bands of the lithium salts. The IR spectra, run as a KBr disk, exhibited peaks related to the dianion structure (fig. 1).
Moreover comparative microvoltammetry studies performed on the same complex at 8o”C, using a copper microelectrode, did not reveal any lithium stripping and it can therefore be supposed that this remaining slightly acid proton should react rapidly with the freshly reduced lithium (fig. 3 ). On the other hand, the cyclic microvoltammetry performed on a POE/ 1-4 C6H4 ( COTFSMLi)2 complex, using first a copper (fig. 4) then a platinum (fig. 5) microelectrode, at the same temperature and concentration conditions, shows a wider electrochemical window O-3.9 V versus Li. The plating/stripping of lithium can be observed using the copper microelectrode. With the platinum one we observed
160
D. Benrabah et al. /Synthesis and electrochemical charactenzation ofnew bulky lithium salts
Fig. 1. FTIR absorption
spectrum
of the new lithium salt l-4 C,H,( COTFSMLi
)*.
0.2
5 E/V
vs.Li-
/ Li
Fig. 2. Voltammogram of the electrochemical POE,-TFSMLi complex at 60°C on a platinum 0=9km.Scanrate=lmV/s.
stability of the macroelectrode
three peaks during the oxidation scan which might be attributed respectively to the oxidation of lithium resulting in the formation of a lithium/platinum alloy-Li3Ptz and an LiPt, intermetallic compounds which are successively oxidized during the anodic scan. 3.3. Thermal stability A comparative thermogravimetric study of our new salts, carried out in helium, from room temperature at a heating rate of lVC/min, shows that the decomposition starts around 270°C. All the new salts
Fig. 3. Voltammogram of the lithium plating-stripping process from the POE,-TFSMLi complex at 80°C on a copper microelectrode 0= 100 pm. Scan rate= 12 mV/s.
exhibit a thermal stability which is more than enough, with respect to their application in secondary lithium batteries. Table 1 lists the melting points and the temperatures of decomposition of the salts determined from DSC analysis. The complexes POE/ l-4 C6H4 ( COTFSMLi)2 are amorphous for an O/ Li ratio < 12, and then exhibits a crystalline phase. The related melting points were found to increase with the salt concentration. The polymer electrolytes whose salt concentration ranges from O/Li = 4 to O/ Li=24, exhibits a glass transition and a single endotherm, at between 27 and 42°C for samples with
D. Benrabah et al. /Synthesis and electrochemical characterization of new bulky bthium salts
161
Table 2 Glass transition temperatures and melting temperatures of several samples POE.-C6H4(COTFSMLi), determined from DSC. (Heating rate 10 K min- ’ ) O/Li
T, (“C)
Tr ( 1st heating)
Tr (2nd heating)
8 10
-36 -39
t *
*
12 16 24 32
-41 -33 - 14 *
28 30 52 44
21 28 38 42
l
E i V vs. Li*/Li”
Fig. 4. Voltammogram of the lithium plating-stripping process from the POE,/l-4 CsH,(COTFSMLi)2 complex at 80°C on a copper microelectrode 0 = 100 urn. Scan rate = 12 mV/s.
3.4. Conductivity of POE/I-4 CJS~(COTFSMLi), electrolytes Several poly( oxyethylene) complexes prepared from the dianion C6H4( COTFSMLi)2 are amor-
d
-1
1
4
3
2
E / V vs. Li+/Li
Fig. 5. Voltammogram of the electrochemical stability of the POE,/ l-4 C6H4( COTFSMLi)2 complex at 80°C on a platinum microelectrode 0 = 2 5 urn. Scan rate = 12 mV/s.
Table 1 Decomposition eral new salts.
temperatures
Compound
TFSMK TriTFSMLi &H,COTFSMLi C6H.,(COTFSMK)2 a) Not determined.
and melting
temperatures
Melting point
Degradation
( T,“C )
(“C)
268 *a) * 250
316 360 270 318
of sev-
temp.
(The same in table 2).
composition of O/Li between 12 and 32 (table 2). At low concentrations the endotherm was present but at higher concentration the endotherm did not appear.
lOOO/T(K-‘)
Fig. 6. Dependence of conductivity versus the reciprocal temperature for complexes POE./ l-4 C6H4 ( COTFSMLi ) 2 for several high concentrations: (a) first heating cycle, (b) first cooling cycle.
D. Benrabah et al. /Synthesis and electrochemical characterization of new bulky lithium salts
conductivity values of the new systems, the triflate and the perchorate obtained with the same POE host polymer (fig. 8 ). The best conductivities, reached for the dianion C6H4( COTFSMLi)* based complexes are about two orders of magnitude higher than the values obtained with usual lithium salts such as LiClO, and LiCF,S03.
4. Conclusion
Fig. 7. Dependence of conductivity versus the reciprocal temperature for complexes POE,/ l-4 CsH4(COTFSMLi)2 for two compositions O/Li = 24 and 32 during heating and cooling.
112
1 o-2
97
84
72
60
50
40
30
21
This divalent lithium salt appears as a very promising salt providing both high conductivity values and a good thermal and electrochemical stability. Furthermore, owing the dianonic nature of the salt we might expect an anionic decrease mobility.
T(“C)
c
Acknowledgement The Laboratoire d’Ionique et d’Electrochimie des Solides de Grenoble gratefully acknowledge financial support from Hydro-Quebec within the ACEP project.
lca’ 2.5
, 2.6
’
2.7
I 2.9
3
2.9 1000/T
Fig. 8. Comparison of conductivity ium salts complexes.
’ 3.1
’ 3.2
3.3
3.4
3.5
(K-‘)
levels for the four POE, lith-
phous at room temperature in a wider concentration range than with LiTFSI. Therefore conductivities close to 3x lo-’ S cm-’ are reached at 25”C, while several complexes exhibit a conductivity of 1O-3 S cm-’ around 60°C. The Arrhenius plots of conductivity versus T-’ (fig. 6a, 6b) exhibit a free-volume behaviour in the salt concentration range O/Li = 4 to 10. Fig. 7 summarizes the conductivity behaviour of two electrolyte compositions O/Li=24 and 32 during a heating/cooling cycle. The Arrhenius plots (fig. 7) of conductivity for these samples show two domains, the slope undergoing a sudden change around 45°C with low conductivities. Finally, we compared
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