- Email: [email protected]
The mechanism of lithium ion transport in polyacrylonitrile-based polymer electrolytes
__ /__ El
2s
SOLID STATE
ELSEWIER
IONICS
Solid State Ionics 91 (1996) 279-284
The mechanism
of lithium ion transport in polyacrylonitrile-based polymer electrolytes
Biying Huanga, Zhaoxiang
Wang”, Liquan Chen”‘“, Rongjian
Xue”, Fosong Wangb
“Institute of Physics, Academia Sinica, P.O. Box 603, Beijing 100080, People’s Republic of China hChinese Academy of Sciences, Beijing 100864, People’s Republic of China Received
3 December
1995; accepted
1 July 1996
Abstract A dozen polyacrylonitrile (PAN)-based polymer electrolytes containing various plasticizers, have shown ionic conductivities higher than 1 X lo-’ S cm-’ at room temperature. In order to explain the high ionic conduction in these systems, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), Raman scattering and infrared (IR) spectra have been measured systematically. Based on these results, a mechanism of Li’ ran transport in PAN-based polymer electrolytes has been suggested. There are three kinds of Li’ ions: one in the gel state of PAN, the other in solid PAN and the third in the plasticizer. The high ionic conduction is mainly caused by the Li’ ions in the gel state. These Li’ ions are coupled with the C=N group in PAN and the C=O group in the plasticizer. The Li’ ions can jump from one position to the next along a chain, while moving together with the chain. Keywords: Ionic conductivity
- lithium; Polyacrylonitrile;
Polymer
1. Introduction Most research activities on polymer electrolytes have been concentrated on systems related to poly(ethylene oxide) (PEO), following the initial discovery of Wright [l] that PEO formed crystalline complexes with alkali metal salts. The practical application of PEO for batteries has been pioneered by Armand and his collaborators [2]. Compared with PEO-based electrolytes, the PAN-based lithium salt complex has many advantages, such as high conductivity and good mechanical properties at room temperature. Abraham et al. [3] have reported on a *Corresponding author. Tel: (86- 10) 6255-9 13 1; Fax: (86-10) 6256-2605; e-mail: [email protected] 0167-2738/96/$15.00 Copyright PII SO167-2738(96)00445-6
01996
electrolyte
PAN-based polymer electrolyte with a conductivity higher than 1 X 10m3 S cm-’ at room temperature. Huang et al. [4,5] have made systematic studies on PAN-PC-EC-LiClO,. The major results are as follows: optimum conductivity at room temperature is 2.5 X 10m3 S cm-‘, the transference number for a lithium ion is 0.36 and the compatibility with metal lithium is rather good. This PAN-based polymer has been used as the electrolyte in lithium batteries and supercapacitors [6]. The present authors have synthesized a dozen new PAN-based polymer electrolytes with various plasticizers. The details will appear elsewhere [7]. The conductivities at room temperature are given in Table 1. In order to explain the high ionic conduction and to gain an insight into the mechanism of lithium ion transport in PAN-based
Elsevier Science B.V. All rights reserved
280 Table I Conductivities
B. Hung et al. I Solid State Ionics 91 (1996) 279-284
of PAN”-based
electrolytes
at room temperature
Electrolytesh
Conductivity (S cm-‘) at 22°C
20PAN/35EC/40BL/SLiCIO, 20PAN/26EC/35BL/l4DMSO/5LiCIO, 20PAN/25EC/25PC/25DMF/SLiClO~ 20PAN/37.5EC/37.5DMF/5LiC104 20PAN125EC/25PC/25BL/5LiC10, 20PAN/25PC/25DMS0/25DMF/5LiCIO, 20PAN/37.5PC/37.5BL/5LiClO,, 20PAN/37.5PC/37.5DMSO/5LiClO, 20PAN/25PC/25BL/25DMSO/SLiClO, 20PAN/37.5PC/37.5DMF/5LiClO, 20PAN/25PC/25BL/25DMF/5LiClO, 20PAN/37.5EC/37.5SL/5LiClO~ 2OPAN/25PC/25SL/25BL/5LiClO, 20PAN/30EC/32BL/13SL/5LiClO,
2.16X IO-’ 1.18X IO-’ 3.83X 1O-3 3.82~ 1O-3 2.79~ lo-” 2.14x lo-’ 2.34X IO-’ 3.2x lo-’ 4.7x 1o-3 4.04x 1o-3 4.09x lo-” 2.02x 1om3 2.04x 1om9 2.26~ 10-j
carbonate; PC = 1,2-propanediol * EC = ethylene DMF=N,N-dimethylformamide; DMSO= dimethyl SL = sulpholane; BL = 1,4-butyrolactone. h All are in weight ratio.
carbonate; sulfoxide;
electrolytes, X-ray diffraction (XRD), ‘Li NMR, Raman and IR spectra have been measured and discussed, respectively. All the details will be published separately [‘7-91. Here, we only quote some of the major results to provide a picture of Li+ ion transport in these systems.
-4
5 25
/
I
I
30
35
40
4.5
1000/T (K-')
Fig. 1. Electrical conductivities of PAN-EC-BL-LiClO, trolyte as a function of temperature.
elec-
at room temperature the conductivity increases with salt concentration, reaches a maximum when N / Li = 8 and then decreases with the salt concentration. This phenomena is similar to the case in the PEO-based electrolyte. It means that the lithium ion transport has structural characteristics which are related to the polymer host PAN. In the low concentration range, the LiClO, is totally dissociated. The number of mobile ions rises with the increase of LiClO, concentration, while in the high concentration range the dissociated ions of Lif and ClO, could be combined to form ion pairs. Thus, a peak appears in the conductivity vs. LiClO, concentration curve at room temperature. 2.2. XRD patterns
2. Some major experimental results related to
the mechanism of Li+ ion transport 2.1. Ionic conductivity Fig. 1 shows the conductivity of PAN-EC-BLLiClO, (EC: ethylene carbonate; BL: butyrolactone) as a function of temperature. The curvature of the plots suggested that the data are fitted with the Vogel-Tamman-Fulcher (VTF) equation [g = AT-I12e-E,I(T-To) ] rather well over a broad temperature range. From the plot, the activation energy (E,), around 25”C, was calculated to be 0.31 eV. The fact that the VTF equation can be applied to conductivity implies that, as with viscosity, ionic conductivity in polymers is strongly coupled to the flow behavior of the polymer. The conductivities of PAN-EC-BLLiClO, as a function of salt concentration at room temperature are shown in Fig. 2. It can be seen that
Fig. 3 shows the XRD patterns of PAN-EC-BLSL-LiClO, (SL = sulpholane) and PAN-EC-PCDMF-LiClO, (DMF = dimethyl formamide) compared with pure LiClO, and PAN. It can be seen that the PAN-based lithium salt complex is an amorphous phase. A fully amorphous morphology produces
LogC
(wt%)
Fig. 2. Electrical conductivities of PAN-EC-BL-LiClO, trolyte as a function of salt concentration (in mass ratios).
elec-
281
B. Huung et al. I Solid State tonics 91 (1996) 279-284
:
I
: I. f
(b)
1:
f: :
._._.__ ._^..._ .__.
I
kb
r .-._.--- - .-
.-. *I
1
id) I
Fig. 3. The XRD pattern of: (a) LiCIO,;
(b) PAN; (c) PAN-EC-BL-
greater polymer flow and ionic diffusivity. That is to say that high ionic conductivity can be obtained in amorphous polymers having highly flexible backbones and low glass-transition temperatures. 2.3. IR and Raman spectra In order to determine the existence of interaction between LiClO, and plasticizer or between LiClO, and PAN in PAN-based electrolytes, the Raman and IR spectra of a series of samples, EC, PAN, EC+ LiClO, and PAN+ EC + LiClO,, have been measured and analyzed, respectively.
SL-LiCIO,
and (d) PAN-EC-PC-DMF-LiCIO,.
Fig. 4 shows the Raman spectra of EC containing different contents of LiClO, at room temperature (25°C) in the region of 690 to 750 cm-’ (a) and 850 to 950 cm-’ (b). When LiClO, concentration is high enough, the Raman spectra of EC+LiClO, have shown obvious changes compared with that of pure EC. For instance, the peaks at the 713 cm-’ band for pure EC became a doublet at 713 and 727 cm-’ for LiClO, + EC and the intensity was greatly decreased, while the original 893 cm-i band moved to 903 cm-‘. When the LiClO, content reached 30%, the originally strong 713 cm-’ band became a weak shoulder of the 727 cm-’ band. The changes of other
282
B. Huang et al. I Solid State Ionics 91 (1996)
279-284
between the Li’ ions and the C=O group of the molecule. In both the Raman and IR spectra of the complex of PAN+EC + LiClO, with different mass ratios of PAN to LiClO, (Fig. 5), the most striking feature was the appearance of a weak but obvious shoulder at about 2270 cm-’ on the high frequency side of the symmetric C=N stretch mode of PAN at 2244 cm-‘. Because a shoulder was not observed in either the vibrational spectra of the PAN + EC system or in the vibrational spectrum of the EC+LiClO, system and the ratios of the intensity of the shoulder at 2270 cm -I to the strong C=N stretch peak at 2244 cm-’ increased with the rising mass ratios of LiClO, to wavenumber
cm
-1
(a)
Fig. 4. Raman spectra in the region of 690 to 750 cm-’ (a) and 850 to 950 cm-’ (b), of EC containing different contents of LiCIO,: A - EC; B - 1%; C - 5%; D - 10%; E - 20%; F - 30%.
bands were also as clear. The obvious changes in the band shape, width, relative intensity and vibrational frequency indicated that there is strong interaction between the Li+ ions and the EC molecules. As most of the changes in the Raman spectra were related to the ring and the C=O group of the molecule and the characteristic vibrational frequencies related to the ClO, ions had no obvious changes, it was supposed that the interaction inside the system mainly occurred
wavenumber
cm
-1
(B) Fig. 5. The C=N stretching band of PAN in the electrolyte PAN-EC-LiCIO, system: (A) Raman spectra of (a) PAN and (b) a mixture in mass ratios of EC-PAN-LiCIO,=50:25:25. (B) IR spectra of PAN (a) and EC-PAN-LiCIO, in the mass ratio of EC:PAN:LiCIO, = 60:25: 15 (b) and 60:20:20 (c).
B. Huang et al. I Solid State tonics 91 (1996) 279-284
PAN in the IR spectra, therefore the weak shoulder at 2270 cm-’ should represent the strong interaction between PAN molecules and LiClO,. In our work, no interaction between Cloy and PAN was observed. It indicated that the interaction between LiClO, and PAN mainly occurred between the Lif ions and the C=N group of PAN. From the above results, it can be concluded that, in the PAN-based electrolyte, there are strong interactions not only between Li+ and plasticizer, but between Li+ and PAN also. 2.4.
‘Li NMR spectra
A spectrum of PAN-EC-PC-DMSO-LiClO, (PC = propanediol carbonate; DMSO = dimethyl sulfoxide) at room temperature is shown in Fig. 6. It can be seen that the spectrum was composed of three components: a narrow Gaussian peak, a broad Lorenzian peak and a small peak with chemical shift. In general, the line width of a NMR spectrum is a measurement of ion mobility in the system. The faster the ions, the narrower the line width of the spectrum. In the present system, there are three kinds of Li+ ions which correspond to each of the components in the NMR spectrum. The narrow peak is caused by the fast ion trans-
I
-
I
I
I
0 experiment fit. results
Lorenzian
peak
Gaussian
peak
._ &
16
I
I
I
20
22
24
1
26
Fig. 6. ‘Li NMR spectrum of PAN-EC-PC-DMSO-LiCIO, room temperature (frequency: 34.964 MHz).
26
at
283
port. The broad peak corresponds to the slow mobile ions. The chemical shift comes from the ions in the plasticizer.
3. Mechanism
of Li+ ion transport
It can be seen from the above results that the major phase in the PAN-based polymer electrolyte is in the gel state or amorphous state. There are also small amounts of extra PAN and plasticizer. The mobility of lithium ions in the gel state is much faster than in PAN. Thus, the narrow peak in the 7Li NMR spectrum corresponds to the fast ion transport in the gel state. While the slow mobility of Li’ ions in solid PAN was responsible for the broad peak, the chemical shift was caused by the Li+ ions in the plasticizer. It is obvious that the fast ion transport in the gel state of PAN is the major reason for the high conductivity of the PAN-based electrolytes. The line width of the 7Li NMR spectra, as a function of temperature showing the motional narrowing temperature, is around 200 K (see Fig. 7). Lower than 200 K, the spectra are very broad, indicating a small mobility for most lithium ions. When the temperature increases, the narrow peak appears and becomes the dominant component around room temperature. As a result, the ionic conductivity of this electrolyte at room temperature is very high. Fig. 8 is the plot of log( 1 /W) vs. l/T for the PAN-EC-BL-LiClO, electrolyte. From the plot, the activation energy around 25°C was calculated to be 0.28 eV by using the equation log( 1 l W) = - E,l(2.303RT) + log( 1 lB). From the calculated activation energy, it can be seen that the NMR result is smaller but close to the conduction result (see Fig. 1). Then it can be concluded that the process responsible for the observed NMR linewidth narrowing is similar to the process responsible for ionic conductivity. In the equation, the B value represents the extent to which the dipolar broadened line narrows in the limit of infinite temperature. If the process is long range, the dipolar interactions are completely averaged out and the line width should narrow to its lifetime values (B< lo-” Wz). Otherwise, if the motion is short range, the B value is related to the dipolar interaction and becomes very large (B > 10e4 kHz). The B value of our sample at 25°C is about 10m7 kHz. It can be
B. Huang et al. I Solid State Ionics 91 (1996) 279-284
284
50
40
30
20 g
10
3 0
-10
150
200
250
300
400
w
T Fig. 7. The line width as a function
350
of temperature
for PAN-EC-BL-LiCIO,
electrolyte.
These mobile ions jump from one position to the next, while they move together with the chains of PAN in the gel state.
References
22
-2.0
1000/T Fig. 8. The plot of lo& l/W) electrolyte.
(K-t)
vs. l/T for PAN-EC-BL-LiClO,
concluded that the Li+ ion motion in the electrolyte is mainly a long range process. Above results of Raman and IR spectra indicated that Li+ ions of dissociated LiClO, are coupled with both the C=O group in EC and the C=N group in PAN. There are also dipole interactions between EC and PAN through C=O groups and CrN groups. Therefore, the gel state is not a simple mixture of PAN, EC and LiClO,, but a special co-existent state of the three components. Li+ ions are located close to C=O groups of EC and C=N groups of PAN.
[II B.E. Fenton, J.M. Parker and PS? Wright, Polymer 14 (1973) 58. 121M.B. Armand, in: Fast Ion Transport in Solids, eds. P Vashisthta et al. (Elsevier, North-Holland, 1979). 131 K.M. Abraham and M. Alamgir, J. Electrochem. Sot. 137 (1990) 1657. [41 Huang Hong, Chen Liquan, Huang Xuejie and Xue Rongjian, Electrochim. Acta 37 (1992) 1671. PI Rongjian Xue, Hong Huang, M. Menetrier and Liquan Chen, J. Power Sources 43 (1993) 431. WI Xuejie Huang, Liquan Chen and .I. Schoenman, J. Power Sources 43 (1993) 487. [71 Biying Huang, Zhaoxiang Wang, Hong Huang, Rongjian Xue, Liquan Chen and Fosong Wang, to be submitted. P31 Zhaoxiang Wang, Biying Huang, Hong Huang, Liquan Chen, Rongjian Xue and Fosong Wang, Electrochim. Acta 41 (1996) 1443. 191 Biying Huang, Xuejie Huang, Hong Huang, Liquan Chen, Rongjian Xue and Fosong Wang, Proc. 8th Intemat. Meeting on Lithium Batteries l-B-29 (June 16-21, 1996, Nagoya, Japan).
2s
SOLID STATE
ELSEWIER
IONICS
Solid State Ionics 91 (1996) 279-284
The mechanism
of lithium ion transport in polyacrylonitrile-based polymer electrolytes
Biying Huanga, Zhaoxiang
Wang”, Liquan Chen”‘“, Rongjian
Xue”, Fosong Wangb
“Institute of Physics, Academia Sinica, P.O. Box 603, Beijing 100080, People’s Republic of China hChinese Academy of Sciences, Beijing 100864, People’s Republic of China Received
3 December
1995; accepted
1 July 1996
Abstract A dozen polyacrylonitrile (PAN)-based polymer electrolytes containing various plasticizers, have shown ionic conductivities higher than 1 X lo-’ S cm-’ at room temperature. In order to explain the high ionic conduction in these systems, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), Raman scattering and infrared (IR) spectra have been measured systematically. Based on these results, a mechanism of Li’ ran transport in PAN-based polymer electrolytes has been suggested. There are three kinds of Li’ ions: one in the gel state of PAN, the other in solid PAN and the third in the plasticizer. The high ionic conduction is mainly caused by the Li’ ions in the gel state. These Li’ ions are coupled with the C=N group in PAN and the C=O group in the plasticizer. The Li’ ions can jump from one position to the next along a chain, while moving together with the chain. Keywords: Ionic conductivity
- lithium; Polyacrylonitrile;
Polymer
1. Introduction Most research activities on polymer electrolytes have been concentrated on systems related to poly(ethylene oxide) (PEO), following the initial discovery of Wright [l] that PEO formed crystalline complexes with alkali metal salts. The practical application of PEO for batteries has been pioneered by Armand and his collaborators [2]. Compared with PEO-based electrolytes, the PAN-based lithium salt complex has many advantages, such as high conductivity and good mechanical properties at room temperature. Abraham et al. [3] have reported on a *Corresponding author. Tel: (86- 10) 6255-9 13 1; Fax: (86-10) 6256-2605; e-mail: [email protected] 0167-2738/96/$15.00 Copyright PII SO167-2738(96)00445-6
01996
electrolyte
PAN-based polymer electrolyte with a conductivity higher than 1 X 10m3 S cm-’ at room temperature. Huang et al. [4,5] have made systematic studies on PAN-PC-EC-LiClO,. The major results are as follows: optimum conductivity at room temperature is 2.5 X 10m3 S cm-‘, the transference number for a lithium ion is 0.36 and the compatibility with metal lithium is rather good. This PAN-based polymer has been used as the electrolyte in lithium batteries and supercapacitors [6]. The present authors have synthesized a dozen new PAN-based polymer electrolytes with various plasticizers. The details will appear elsewhere [7]. The conductivities at room temperature are given in Table 1. In order to explain the high ionic conduction and to gain an insight into the mechanism of lithium ion transport in PAN-based
Elsevier Science B.V. All rights reserved
280 Table I Conductivities
B. Hung et al. I Solid State Ionics 91 (1996) 279-284
of PAN”-based
electrolytes
at room temperature
Electrolytesh
Conductivity (S cm-‘) at 22°C
20PAN/35EC/40BL/SLiCIO, 20PAN/26EC/35BL/l4DMSO/5LiCIO, 20PAN/25EC/25PC/25DMF/SLiClO~ 20PAN/37.5EC/37.5DMF/5LiC104 20PAN125EC/25PC/25BL/5LiC10, 20PAN/25PC/25DMS0/25DMF/5LiCIO, 20PAN/37.5PC/37.5BL/5LiClO,, 20PAN/37.5PC/37.5DMSO/5LiClO, 20PAN/25PC/25BL/25DMSO/SLiClO, 20PAN/37.5PC/37.5DMF/5LiClO, 20PAN/25PC/25BL/25DMF/5LiClO, 20PAN/37.5EC/37.5SL/5LiClO~ 2OPAN/25PC/25SL/25BL/5LiClO, 20PAN/30EC/32BL/13SL/5LiClO,
2.16X IO-’ 1.18X IO-’ 3.83X 1O-3 3.82~ 1O-3 2.79~ lo-” 2.14x lo-’ 2.34X IO-’ 3.2x lo-’ 4.7x 1o-3 4.04x 1o-3 4.09x lo-” 2.02x 1om3 2.04x 1om9 2.26~ 10-j
carbonate; PC = 1,2-propanediol * EC = ethylene DMF=N,N-dimethylformamide; DMSO= dimethyl SL = sulpholane; BL = 1,4-butyrolactone. h All are in weight ratio.
carbonate; sulfoxide;
electrolytes, X-ray diffraction (XRD), ‘Li NMR, Raman and IR spectra have been measured and discussed, respectively. All the details will be published separately [‘7-91. Here, we only quote some of the major results to provide a picture of Li+ ion transport in these systems.
-4
5 25
/
I
I
30
35
40
4.5
1000/T (K-')
Fig. 1. Electrical conductivities of PAN-EC-BL-LiClO, trolyte as a function of temperature.
elec-
at room temperature the conductivity increases with salt concentration, reaches a maximum when N / Li = 8 and then decreases with the salt concentration. This phenomena is similar to the case in the PEO-based electrolyte. It means that the lithium ion transport has structural characteristics which are related to the polymer host PAN. In the low concentration range, the LiClO, is totally dissociated. The number of mobile ions rises with the increase of LiClO, concentration, while in the high concentration range the dissociated ions of Lif and ClO, could be combined to form ion pairs. Thus, a peak appears in the conductivity vs. LiClO, concentration curve at room temperature. 2.2. XRD patterns
2. Some major experimental results related to
the mechanism of Li+ ion transport 2.1. Ionic conductivity Fig. 1 shows the conductivity of PAN-EC-BLLiClO, (EC: ethylene carbonate; BL: butyrolactone) as a function of temperature. The curvature of the plots suggested that the data are fitted with the Vogel-Tamman-Fulcher (VTF) equation [g = AT-I12e-E,I(T-To) ] rather well over a broad temperature range. From the plot, the activation energy (E,), around 25”C, was calculated to be 0.31 eV. The fact that the VTF equation can be applied to conductivity implies that, as with viscosity, ionic conductivity in polymers is strongly coupled to the flow behavior of the polymer. The conductivities of PAN-EC-BLLiClO, as a function of salt concentration at room temperature are shown in Fig. 2. It can be seen that
Fig. 3 shows the XRD patterns of PAN-EC-BLSL-LiClO, (SL = sulpholane) and PAN-EC-PCDMF-LiClO, (DMF = dimethyl formamide) compared with pure LiClO, and PAN. It can be seen that the PAN-based lithium salt complex is an amorphous phase. A fully amorphous morphology produces
LogC
(wt%)
Fig. 2. Electrical conductivities of PAN-EC-BL-LiClO, trolyte as a function of salt concentration (in mass ratios).
elec-
281
B. Huung et al. I Solid State tonics 91 (1996) 279-284
:
I
: I. f
(b)
1:
f: :
._._.__ ._^..._ .__.
I
kb
r .-._.--- - .-
.-. *I
1
id) I
Fig. 3. The XRD pattern of: (a) LiCIO,;
(b) PAN; (c) PAN-EC-BL-
greater polymer flow and ionic diffusivity. That is to say that high ionic conductivity can be obtained in amorphous polymers having highly flexible backbones and low glass-transition temperatures. 2.3. IR and Raman spectra In order to determine the existence of interaction between LiClO, and plasticizer or between LiClO, and PAN in PAN-based electrolytes, the Raman and IR spectra of a series of samples, EC, PAN, EC+ LiClO, and PAN+ EC + LiClO,, have been measured and analyzed, respectively.
SL-LiCIO,
and (d) PAN-EC-PC-DMF-LiCIO,.
Fig. 4 shows the Raman spectra of EC containing different contents of LiClO, at room temperature (25°C) in the region of 690 to 750 cm-’ (a) and 850 to 950 cm-’ (b). When LiClO, concentration is high enough, the Raman spectra of EC+LiClO, have shown obvious changes compared with that of pure EC. For instance, the peaks at the 713 cm-’ band for pure EC became a doublet at 713 and 727 cm-’ for LiClO, + EC and the intensity was greatly decreased, while the original 893 cm-i band moved to 903 cm-‘. When the LiClO, content reached 30%, the originally strong 713 cm-’ band became a weak shoulder of the 727 cm-’ band. The changes of other
282
B. Huang et al. I Solid State Ionics 91 (1996)
279-284
between the Li’ ions and the C=O group of the molecule. In both the Raman and IR spectra of the complex of PAN+EC + LiClO, with different mass ratios of PAN to LiClO, (Fig. 5), the most striking feature was the appearance of a weak but obvious shoulder at about 2270 cm-’ on the high frequency side of the symmetric C=N stretch mode of PAN at 2244 cm-‘. Because a shoulder was not observed in either the vibrational spectra of the PAN + EC system or in the vibrational spectrum of the EC+LiClO, system and the ratios of the intensity of the shoulder at 2270 cm -I to the strong C=N stretch peak at 2244 cm-’ increased with the rising mass ratios of LiClO, to wavenumber
cm
-1
(a)
Fig. 4. Raman spectra in the region of 690 to 750 cm-’ (a) and 850 to 950 cm-’ (b), of EC containing different contents of LiCIO,: A - EC; B - 1%; C - 5%; D - 10%; E - 20%; F - 30%.
bands were also as clear. The obvious changes in the band shape, width, relative intensity and vibrational frequency indicated that there is strong interaction between the Li+ ions and the EC molecules. As most of the changes in the Raman spectra were related to the ring and the C=O group of the molecule and the characteristic vibrational frequencies related to the ClO, ions had no obvious changes, it was supposed that the interaction inside the system mainly occurred
wavenumber
cm
-1
(B) Fig. 5. The C=N stretching band of PAN in the electrolyte PAN-EC-LiCIO, system: (A) Raman spectra of (a) PAN and (b) a mixture in mass ratios of EC-PAN-LiCIO,=50:25:25. (B) IR spectra of PAN (a) and EC-PAN-LiCIO, in the mass ratio of EC:PAN:LiCIO, = 60:25: 15 (b) and 60:20:20 (c).
B. Huang et al. I Solid State tonics 91 (1996) 279-284
PAN in the IR spectra, therefore the weak shoulder at 2270 cm-’ should represent the strong interaction between PAN molecules and LiClO,. In our work, no interaction between Cloy and PAN was observed. It indicated that the interaction between LiClO, and PAN mainly occurred between the Lif ions and the C=N group of PAN. From the above results, it can be concluded that, in the PAN-based electrolyte, there are strong interactions not only between Li+ and plasticizer, but between Li+ and PAN also. 2.4.
‘Li NMR spectra
A spectrum of PAN-EC-PC-DMSO-LiClO, (PC = propanediol carbonate; DMSO = dimethyl sulfoxide) at room temperature is shown in Fig. 6. It can be seen that the spectrum was composed of three components: a narrow Gaussian peak, a broad Lorenzian peak and a small peak with chemical shift. In general, the line width of a NMR spectrum is a measurement of ion mobility in the system. The faster the ions, the narrower the line width of the spectrum. In the present system, there are three kinds of Li+ ions which correspond to each of the components in the NMR spectrum. The narrow peak is caused by the fast ion trans-
I
-
I
I
I
0 experiment fit. results
Lorenzian
peak
Gaussian
peak
._ &
16
I
I
I
20
22
24
1
26
Fig. 6. ‘Li NMR spectrum of PAN-EC-PC-DMSO-LiCIO, room temperature (frequency: 34.964 MHz).
26
at
283
port. The broad peak corresponds to the slow mobile ions. The chemical shift comes from the ions in the plasticizer.
3. Mechanism
of Li+ ion transport
It can be seen from the above results that the major phase in the PAN-based polymer electrolyte is in the gel state or amorphous state. There are also small amounts of extra PAN and plasticizer. The mobility of lithium ions in the gel state is much faster than in PAN. Thus, the narrow peak in the 7Li NMR spectrum corresponds to the fast ion transport in the gel state. While the slow mobility of Li’ ions in solid PAN was responsible for the broad peak, the chemical shift was caused by the Li+ ions in the plasticizer. It is obvious that the fast ion transport in the gel state of PAN is the major reason for the high conductivity of the PAN-based electrolytes. The line width of the 7Li NMR spectra, as a function of temperature showing the motional narrowing temperature, is around 200 K (see Fig. 7). Lower than 200 K, the spectra are very broad, indicating a small mobility for most lithium ions. When the temperature increases, the narrow peak appears and becomes the dominant component around room temperature. As a result, the ionic conductivity of this electrolyte at room temperature is very high. Fig. 8 is the plot of log( 1 /W) vs. l/T for the PAN-EC-BL-LiClO, electrolyte. From the plot, the activation energy around 25°C was calculated to be 0.28 eV by using the equation log( 1 l W) = - E,l(2.303RT) + log( 1 lB). From the calculated activation energy, it can be seen that the NMR result is smaller but close to the conduction result (see Fig. 1). Then it can be concluded that the process responsible for the observed NMR linewidth narrowing is similar to the process responsible for ionic conductivity. In the equation, the B value represents the extent to which the dipolar broadened line narrows in the limit of infinite temperature. If the process is long range, the dipolar interactions are completely averaged out and the line width should narrow to its lifetime values (B< lo-” Wz). Otherwise, if the motion is short range, the B value is related to the dipolar interaction and becomes very large (B > 10e4 kHz). The B value of our sample at 25°C is about 10m7 kHz. It can be
B. Huang et al. I Solid State Ionics 91 (1996) 279-284
284
50
40
30
20 g
10
3 0
-10
150
200
250
300
400
w
T Fig. 7. The line width as a function
350
of temperature
for PAN-EC-BL-LiCIO,
electrolyte.
These mobile ions jump from one position to the next, while they move together with the chains of PAN in the gel state.
References
22
-2.0
1000/T Fig. 8. The plot of lo& l/W) electrolyte.
(K-t)
vs. l/T for PAN-EC-BL-LiClO,
concluded that the Li+ ion motion in the electrolyte is mainly a long range process. Above results of Raman and IR spectra indicated that Li+ ions of dissociated LiClO, are coupled with both the C=O group in EC and the C=N group in PAN. There are also dipole interactions between EC and PAN through C=O groups and CrN groups. Therefore, the gel state is not a simple mixture of PAN, EC and LiClO,, but a special co-existent state of the three components. Li+ ions are located close to C=O groups of EC and C=N groups of PAN.
[II B.E. Fenton, J.M. Parker and PS? Wright, Polymer 14 (1973) 58. 121M.B. Armand, in: Fast Ion Transport in Solids, eds. P Vashisthta et al. (Elsevier, North-Holland, 1979). 131 K.M. Abraham and M. Alamgir, J. Electrochem. Sot. 137 (1990) 1657. [41 Huang Hong, Chen Liquan, Huang Xuejie and Xue Rongjian, Electrochim. Acta 37 (1992) 1671. PI Rongjian Xue, Hong Huang, M. Menetrier and Liquan Chen, J. Power Sources 43 (1993) 431. WI Xuejie Huang, Liquan Chen and .I. Schoenman, J. Power Sources 43 (1993) 487. [71 Biying Huang, Zhaoxiang Wang, Hong Huang, Rongjian Xue, Liquan Chen and Fosong Wang, to be submitted. P31 Zhaoxiang Wang, Biying Huang, Hong Huang, Liquan Chen, Rongjian Xue and Fosong Wang, Electrochim. Acta 41 (1996) 1443. 191 Biying Huang, Xuejie Huang, Hong Huang, Liquan Chen, Rongjian Xue and Fosong Wang, Proc. 8th Intemat. Meeting on Lithium Batteries l-B-29 (June 16-21, 1996, Nagoya, Japan).