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The electrical and surface properties of thin film of Li glass solid electrolyte
112
Thin Solid Films, 235 (1993) 112-115
The electrical and surface properties of thin film of Li glass solid electrolyte Dachun Zhao and Xiaoren Pan Department of Physics', East China University of Chemical Technology, PO Box 10, Shanghai 200237 (People's Republic of China)
(Received March 12, 1993; accepted May 14, 1993)
Abstract LiCI-Li20-P2Os-WO3 glass solid electrolyte thin film has been prepared by the vacuum flash evaporation method; the electrical conductivity of the film is 10-4S cm -~ at 360 °C. The temperature and frequency dependence of the conductivity and dielectric constant of the films were obtained; typical complex impedance spectra were obtained for films of various compositions at 200, 360 and 380 °C. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy and a work function meter were also used to analyse the films.
1. Introduction Glass solid electrolytes have been of interest in recent years due to their well-known advantages over their crystalline counterparts, their isotropic properties, flexibility of size and shape, as well as their extended composition range. Lithium being the lightest metal and electropositive [1], many workers are engaged in developing glass solid electrolyte systems with high lithium ion conductivity and these materials show good electrochemical and thermal stability. In particular, it is of interest to prepare them in thin film form and to study their characteristics for application in microbatteries, in electrochromic applications, and so on. Up to now, several materials have been found to exhibit high values of lithium ionic conductivity [2, 3]. This paper reports on the properties of LiC1-Li2OsP 2 0 5 - W O 3 solid electrolyte thin film, and describes the complex impedance of L i C 1 - L i 2 0 - P 2 O s - W O 3 under various conditions of temperature and composition; the relation between these properties and ion migration is also discussed.
2. Experimental details The bulk glass solid electrolyte was prepared from reagent grade LiC1, LiCO3, NH4H2PO4 and WO3 adopting the classical quenching technique, the details of which are described in our previous paper [4]. The L i C 1 - L i 2 0 - P 2 O s - W O 3 glass solid electrolyte thin films were fabricated by the vacuum flash evaporation method: thin film deposition was carried out under a
0040-6090/93/$6.00
vacuum chamber pressure of 10 -3 Pa at an evaporation source temperature of 1300-1600°C; the substrate was heated to remove absorption gas, and finally the substrate temperature was chosen as 250 °C. The films for conductivity studies were coated on glass using an Mo boat; a sandwich structure was obtained by successive deposition of gold electrode and electrolyte. The electrical conductivities of electrons and ions in the film were measured using an HP 4284A with an additional polarization signal in the light of the Wagner method [5]. These measurements were made on the following: glass substrate[Li( -)[solid electrolyte film[Ag(+) and the electronic transference number was calculated: te
o-e
O"e
fie + O"i
O"
Here a is the total conductivity and ae and o"i are the conductivities due to the migration of electrons and ions. The measurement temperature range was from about 20 to 380 °C. ae:tri = 1:1000 for the sample with LiCl:Li20:P2 05 :WO3 = 0.7:0.7:1.0:0.15 (molar ratios). X-ray diffraction (XRD) spectra of the film were obtained using a Rigaku DM4Y/rB X-ray generator. The scanning electron microscopy (SEM) image was observed with a Cambridge S-250KM microscope. Ion migration and the elements on the surface were studied by Auger electron spectroscopy (AES); the sample was bombarded using the electron beam provided by the electron probe of the Auger system and a work function (WF) meter was attached to the AES system.
© 1993 -- Elsevier Sequoia. All rights reserved
D. Zhao, X. Pan / Properties of Li glass solid electrolyte
113
3. Results and discussion 3. I. Electrical properties
tt~
Ionic conductivities in LiC1-Li20-P205-WO 3 glass solid electrolyte thin film prepared by the vacuum flash evaporation method were examined. The materials examined had compositions near stoichiometric 0.7LiCI-0.7Li20-P2Os-0.15WO3, 0.5LiCI-0.7Li20P205-0.5WO3, 0.7LiC1-0.7Li20-P2Os-0.5WO3 and LiC1-0.7LizO-P2Os-0.5WO3. The conductivity of the films was l0 -4 S cm -1 at 360 °C. The activation energy Ea and tr can be calculated from the observed data by using the Arrhenius equation [6]
7 380 C/~./ e~ b
_o 200 °C
3
4
'~"
5
6
7 logf (Hz)
Fig. 2. Changein log a vs. logf
e x p ( - ~-~)
a(T)=ao
where a 0 is the pre-exponential factor, k is the Boltzmann constant and T is the absolute temperature. Ionic conductivity data for three compositions of LiC1-Li20-P2Os-WO3 glass solid electrolyte thin films are plotted as In tr vs. 1 / T in Fig. 1. It can be seen from Fig. 1 that, for all the compositions plots of In a against 1/T are linear in the temperature region 160380 °C; in other words, the temperature dependence of the conductivity follows the Arrhenius equation well. However, below 160 °C or above 380 °C the change in conductivity with temperature change diverges from the Arrhenius linear relation and the conductivity is inclined to increase. This is because lithium glass solid electrolyte is apt to absorb moisture below 160 °C, and above 380 °C the free volume of the glass solid electrolyte system is rearranged, or interface effects arising owing to a partial crystallization in the original noncrystalline basis contribute to the conductivity. The activation energy Ea can be calculated from the observed data; the activation energies for the three compositions FI, F2 and F3 of the solid electrolyte are 0.52,
1.6
2.0
qt~
2.41000/T(K-') --x--
F2
0.56 and 0.61 eV. It is obvious that the conductivity increases with increasing LiC1 content, because the conductivity can be simply given as a = N Z e # , where N is the density of mobile ions with charge Z e , and mobility #. Variation in the conductivity of the films with varying Li content must be attributable to variations in N and #, i.e. an increase in N and # with increasing Li content is expected. The conductivity at various temperatures and frequencies are shown in Fig. 2. It can be seen that tr increases with increase in frequency in the high frequency region and remains constant at low frequencies; this behaviour could be explained by considering the universal dielectric response proposed by Jonscher [7]. The dielectric constant variations with temperature and frequency changes are also given in Fig. 3 and Fig. 4 respectively. Figure 3 presents a plot of log e vs. T (°C), where e is the dielectric constant. An increase in value in clear in the temperature range 200-330 °C; it is considered that the free volume in the glass system changes as the temperature rises and the ion clusters which form the glass network may cause movement. In addition, the lithium ion which was tied near a nonbridge oxide may produce additional movement; this means that the direction of the electric dipole can freely
[
/"x ~`4 _o
f
'
'
'
"_--.--'F, I
,4 ,4
-,4 j J Fig. 1. Change in In tr v s . 1000/T for three compounds: 0 , F 1 (LiC1-0.7Li20-P2Os-0.5WO3); x, F 2 (0.7LiCI-0.7LizO-P2050.5WO3); &, F 3 (0.5LiCI-0.7Li20-P205-0.5WO3).
|
i
,
/
i
200 250 300 T °C Fig. 3. Changein dielectricconstant vs. temperature.
D. Zhao, X. Pan / Properties of Li glass solid electrolyte
114
o
solid electrolytes, while the low frequency spur resulted from an interfacial effect.
I
\
3.2. Surface properties
\ 200oc
I 4.0
I ~ "~'q~l "--j 5.0 6.0 Iogf (Hz)
Fig. 4. Dielectric constant as a function of frequency" at two different temperatures.
change. However, e has a maximum at 330 °C owing to a lot of defections across the crystalline-non-crystalline interface in the glass system, resulting in a migration ratio increase of the lithium ion. Figure 4 shows that the dielectric constant decreases with increasing frequency, and at high frequencies the measured values of e are almost independent. This can be explained by Armstrong and Taylor's view [8]. Armstrong and Taylor considered the high value of e in superionic conductors to be caused by high capacitance contributed by the fixed ions and molecules in the lattice. Typical complex impedance spectra (Fig. 5) were measured for the present samples over a wide range of frequencies. The radii of the spectra decrease, and the semicircle is followed by a spur and at lower frequencies by an inclined straight line. However, at high temperatures ( T > 250 °C) the semicircles completely disappear and only an inclined straight line is visible in Fig. 5(b). The high frequency part of the impedance plots can be related to bulk relaxation processes according to Armstrong and Taylor's model of
X R D spectra show the films were amorphous. Figure 6 shows the relative intensities of Auger peaks as a function of time for LiC1-Li20-P2Os-WO3 glass solid electrolyte. It was found that the intensity of the W and P Auger peaks was unchanged. However, the decrease in the O Auger peak may be due to beam damage; through the cover hole that formed it is quite likely that adsorption oxygen escaped. The increase in the C Auger peak was mainly due to contamination of the high vacuum system. Because the sensitivity of lithium is lower, the Auger signal of lithium is difficult to inspect with certainty. It can be determined from conductivity measurements that lithium ion migration occurs in the glass solid electrolyte. The change in surface topography of the sample can be attributed to ion bombardment. Figure 7 is a scanning electron micrograph of an unbombarded sample; Fig. 8 is a micrograph of the bombarded sample. Moreover, a WF meter (homemade) was used to analyse the sample; the
.=.
I
40
8~0
1~0 time min
Fig. 6. Relative intensities of the Auger peaks for a 0.7LiCI0.7Li20-P2Os-0.15WO 3 sample.
×
(a)
! 20
! 40
| 60
b 80
! 100
120
× l03~
(b)
Fig. 5. Complex impedance spectra for samples at high temperatures: (a) 200 °C; (b) 360 and 380 °C.
3
logR'(~)
D. Zhao, X. Pan / Properties of Li glass solid electrolyte
115
to analyse the sample; the WF of the sample compared with a standard Ag sample is 180 meV.
4. Conclusion
Fig. 7. Topography of the unbombarded sample.
The LiCI-Li20-P2Os-WO3 glass solid electrolyte thin films were prepared by the vacuum flash evaporation method, and were characterized by electrical and surface properties. Conductivity dispersion and dielectric constant variation with frequency were discussed with reference to interfacial and bulk effects. Conductivity changes follow the Arrhenius equation. From the AES and SEM studies it is concluded that ion migration occurs in the surface layer of the glass solid electrolyte and can be detected through changes in the intensity of Auger signals as a function of time. The accumulation or deposition of carriers can be described by a simple equation [9]. Besides, we can conclude that P, W and O form a stable network unit in the solid electrolyte system and Li ÷ migrates through network channels.
References
Fig. 8. Surface of the bombarded sample.
1 D. Ravine, J. Non-Cryst. Solids, 39 (1980) 353. 2 Y. P. Hong, Mater. Res. Bull., 13 (1978) 117. 3 B. A. Bou Kamp and R. A. Huggins, Mater. Res. Bull., 13 (1978) 23. 4 D. C. Zhao, J. Inst. Chem. Technol., I (1993) 84. 5 J. B. Wagner and C. Wagner, J. Chem. Phys., 26(1957) 1597. 6 M. A. Ratner and A. Nitzan, Solid State lonics, 28 (1988) 6. 7 A. K. Jonscher, J. Mater. Sci., 13 (1978) 553. 8 R. D. Armstrong and K. Taylor, J. Electronal. Chem., 63 (1975) 9. 9 X. R. Pan, Y. H. Zu and D. C. Zhao, Proc. 2nd Natl. Conf. on Electron Energy Spectra and Solid Chemistry, Beijing, 1990, p. 72.
Thin Solid Films, 235 (1993) 112-115
The electrical and surface properties of thin film of Li glass solid electrolyte Dachun Zhao and Xiaoren Pan Department of Physics', East China University of Chemical Technology, PO Box 10, Shanghai 200237 (People's Republic of China)
(Received March 12, 1993; accepted May 14, 1993)
Abstract LiCI-Li20-P2Os-WO3 glass solid electrolyte thin film has been prepared by the vacuum flash evaporation method; the electrical conductivity of the film is 10-4S cm -~ at 360 °C. The temperature and frequency dependence of the conductivity and dielectric constant of the films were obtained; typical complex impedance spectra were obtained for films of various compositions at 200, 360 and 380 °C. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy and a work function meter were also used to analyse the films.
1. Introduction Glass solid electrolytes have been of interest in recent years due to their well-known advantages over their crystalline counterparts, their isotropic properties, flexibility of size and shape, as well as their extended composition range. Lithium being the lightest metal and electropositive [1], many workers are engaged in developing glass solid electrolyte systems with high lithium ion conductivity and these materials show good electrochemical and thermal stability. In particular, it is of interest to prepare them in thin film form and to study their characteristics for application in microbatteries, in electrochromic applications, and so on. Up to now, several materials have been found to exhibit high values of lithium ionic conductivity [2, 3]. This paper reports on the properties of LiC1-Li2OsP 2 0 5 - W O 3 solid electrolyte thin film, and describes the complex impedance of L i C 1 - L i 2 0 - P 2 O s - W O 3 under various conditions of temperature and composition; the relation between these properties and ion migration is also discussed.
2. Experimental details The bulk glass solid electrolyte was prepared from reagent grade LiC1, LiCO3, NH4H2PO4 and WO3 adopting the classical quenching technique, the details of which are described in our previous paper [4]. The L i C 1 - L i 2 0 - P 2 O s - W O 3 glass solid electrolyte thin films were fabricated by the vacuum flash evaporation method: thin film deposition was carried out under a
0040-6090/93/$6.00
vacuum chamber pressure of 10 -3 Pa at an evaporation source temperature of 1300-1600°C; the substrate was heated to remove absorption gas, and finally the substrate temperature was chosen as 250 °C. The films for conductivity studies were coated on glass using an Mo boat; a sandwich structure was obtained by successive deposition of gold electrode and electrolyte. The electrical conductivities of electrons and ions in the film were measured using an HP 4284A with an additional polarization signal in the light of the Wagner method [5]. These measurements were made on the following: glass substrate[Li( -)[solid electrolyte film[Ag(+) and the electronic transference number was calculated: te
o-e
O"e
fie + O"i
O"
Here a is the total conductivity and ae and o"i are the conductivities due to the migration of electrons and ions. The measurement temperature range was from about 20 to 380 °C. ae:tri = 1:1000 for the sample with LiCl:Li20:P2 05 :WO3 = 0.7:0.7:1.0:0.15 (molar ratios). X-ray diffraction (XRD) spectra of the film were obtained using a Rigaku DM4Y/rB X-ray generator. The scanning electron microscopy (SEM) image was observed with a Cambridge S-250KM microscope. Ion migration and the elements on the surface were studied by Auger electron spectroscopy (AES); the sample was bombarded using the electron beam provided by the electron probe of the Auger system and a work function (WF) meter was attached to the AES system.
© 1993 -- Elsevier Sequoia. All rights reserved
D. Zhao, X. Pan / Properties of Li glass solid electrolyte
113
3. Results and discussion 3. I. Electrical properties
tt~
Ionic conductivities in LiC1-Li20-P205-WO 3 glass solid electrolyte thin film prepared by the vacuum flash evaporation method were examined. The materials examined had compositions near stoichiometric 0.7LiCI-0.7Li20-P2Os-0.15WO3, 0.5LiCI-0.7Li20P205-0.5WO3, 0.7LiC1-0.7Li20-P2Os-0.5WO3 and LiC1-0.7LizO-P2Os-0.5WO3. The conductivity of the films was l0 -4 S cm -1 at 360 °C. The activation energy Ea and tr can be calculated from the observed data by using the Arrhenius equation [6]
7 380 C/~./ e~ b
_o 200 °C
3
4
'~"
5
6
7 logf (Hz)
Fig. 2. Changein log a vs. logf
e x p ( - ~-~)
a(T)=ao
where a 0 is the pre-exponential factor, k is the Boltzmann constant and T is the absolute temperature. Ionic conductivity data for three compositions of LiC1-Li20-P2Os-WO3 glass solid electrolyte thin films are plotted as In tr vs. 1 / T in Fig. 1. It can be seen from Fig. 1 that, for all the compositions plots of In a against 1/T are linear in the temperature region 160380 °C; in other words, the temperature dependence of the conductivity follows the Arrhenius equation well. However, below 160 °C or above 380 °C the change in conductivity with temperature change diverges from the Arrhenius linear relation and the conductivity is inclined to increase. This is because lithium glass solid electrolyte is apt to absorb moisture below 160 °C, and above 380 °C the free volume of the glass solid electrolyte system is rearranged, or interface effects arising owing to a partial crystallization in the original noncrystalline basis contribute to the conductivity. The activation energy Ea can be calculated from the observed data; the activation energies for the three compositions FI, F2 and F3 of the solid electrolyte are 0.52,
1.6
2.0
qt~
2.41000/T(K-') --x--
F2
0.56 and 0.61 eV. It is obvious that the conductivity increases with increasing LiC1 content, because the conductivity can be simply given as a = N Z e # , where N is the density of mobile ions with charge Z e , and mobility #. Variation in the conductivity of the films with varying Li content must be attributable to variations in N and #, i.e. an increase in N and # with increasing Li content is expected. The conductivity at various temperatures and frequencies are shown in Fig. 2. It can be seen that tr increases with increase in frequency in the high frequency region and remains constant at low frequencies; this behaviour could be explained by considering the universal dielectric response proposed by Jonscher [7]. The dielectric constant variations with temperature and frequency changes are also given in Fig. 3 and Fig. 4 respectively. Figure 3 presents a plot of log e vs. T (°C), where e is the dielectric constant. An increase in value in clear in the temperature range 200-330 °C; it is considered that the free volume in the glass system changes as the temperature rises and the ion clusters which form the glass network may cause movement. In addition, the lithium ion which was tied near a nonbridge oxide may produce additional movement; this means that the direction of the electric dipole can freely
[
/"x ~`4 _o
f
'
'
'
"_--.--'F, I
,4 ,4
-,4 j J Fig. 1. Change in In tr v s . 1000/T for three compounds: 0 , F 1 (LiC1-0.7Li20-P2Os-0.5WO3); x, F 2 (0.7LiCI-0.7LizO-P2050.5WO3); &, F 3 (0.5LiCI-0.7Li20-P205-0.5WO3).
|
i
,
/
i
200 250 300 T °C Fig. 3. Changein dielectricconstant vs. temperature.
D. Zhao, X. Pan / Properties of Li glass solid electrolyte
114
o
solid electrolytes, while the low frequency spur resulted from an interfacial effect.
I
\
3.2. Surface properties
\ 200oc
I 4.0
I ~ "~'q~l "--j 5.0 6.0 Iogf (Hz)
Fig. 4. Dielectric constant as a function of frequency" at two different temperatures.
change. However, e has a maximum at 330 °C owing to a lot of defections across the crystalline-non-crystalline interface in the glass system, resulting in a migration ratio increase of the lithium ion. Figure 4 shows that the dielectric constant decreases with increasing frequency, and at high frequencies the measured values of e are almost independent. This can be explained by Armstrong and Taylor's view [8]. Armstrong and Taylor considered the high value of e in superionic conductors to be caused by high capacitance contributed by the fixed ions and molecules in the lattice. Typical complex impedance spectra (Fig. 5) were measured for the present samples over a wide range of frequencies. The radii of the spectra decrease, and the semicircle is followed by a spur and at lower frequencies by an inclined straight line. However, at high temperatures ( T > 250 °C) the semicircles completely disappear and only an inclined straight line is visible in Fig. 5(b). The high frequency part of the impedance plots can be related to bulk relaxation processes according to Armstrong and Taylor's model of
X R D spectra show the films were amorphous. Figure 6 shows the relative intensities of Auger peaks as a function of time for LiC1-Li20-P2Os-WO3 glass solid electrolyte. It was found that the intensity of the W and P Auger peaks was unchanged. However, the decrease in the O Auger peak may be due to beam damage; through the cover hole that formed it is quite likely that adsorption oxygen escaped. The increase in the C Auger peak was mainly due to contamination of the high vacuum system. Because the sensitivity of lithium is lower, the Auger signal of lithium is difficult to inspect with certainty. It can be determined from conductivity measurements that lithium ion migration occurs in the glass solid electrolyte. The change in surface topography of the sample can be attributed to ion bombardment. Figure 7 is a scanning electron micrograph of an unbombarded sample; Fig. 8 is a micrograph of the bombarded sample. Moreover, a WF meter (homemade) was used to analyse the sample; the
.=.
I
40
8~0
1~0 time min
Fig. 6. Relative intensities of the Auger peaks for a 0.7LiCI0.7Li20-P2Os-0.15WO 3 sample.
×
(a)
! 20
! 40
| 60
b 80
! 100
120
× l03~
(b)
Fig. 5. Complex impedance spectra for samples at high temperatures: (a) 200 °C; (b) 360 and 380 °C.
3
logR'(~)
D. Zhao, X. Pan / Properties of Li glass solid electrolyte
115
to analyse the sample; the WF of the sample compared with a standard Ag sample is 180 meV.
4. Conclusion
Fig. 7. Topography of the unbombarded sample.
The LiCI-Li20-P2Os-WO3 glass solid electrolyte thin films were prepared by the vacuum flash evaporation method, and were characterized by electrical and surface properties. Conductivity dispersion and dielectric constant variation with frequency were discussed with reference to interfacial and bulk effects. Conductivity changes follow the Arrhenius equation. From the AES and SEM studies it is concluded that ion migration occurs in the surface layer of the glass solid electrolyte and can be detected through changes in the intensity of Auger signals as a function of time. The accumulation or deposition of carriers can be described by a simple equation [9]. Besides, we can conclude that P, W and O form a stable network unit in the solid electrolyte system and Li ÷ migrates through network channels.
References
Fig. 8. Surface of the bombarded sample.
1 D. Ravine, J. Non-Cryst. Solids, 39 (1980) 353. 2 Y. P. Hong, Mater. Res. Bull., 13 (1978) 117. 3 B. A. Bou Kamp and R. A. Huggins, Mater. Res. Bull., 13 (1978) 23. 4 D. C. Zhao, J. Inst. Chem. Technol., I (1993) 84. 5 J. B. Wagner and C. Wagner, J. Chem. Phys., 26(1957) 1597. 6 M. A. Ratner and A. Nitzan, Solid State lonics, 28 (1988) 6. 7 A. K. Jonscher, J. Mater. Sci., 13 (1978) 553. 8 R. D. Armstrong and K. Taylor, J. Electronal. Chem., 63 (1975) 9. 9 X. R. Pan, Y. H. Zu and D. C. Zhao, Proc. 2nd Natl. Conf. on Electron Energy Spectra and Solid Chemistry, Beijing, 1990, p. 72.