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The electrochemical behaviour of the lithium electrode in organic aprotic media in the presence of decaline
THE ELECTROCHEMICAL BEHAVIOUR OF THE LITHIUM ELECTRODE IN ORGANIC APROTIC MEDIA IN THE PRESENCE OF DECALINE K. WIESENER, U. ECKOLDT and D. RAHNER Dresden University
of Technology, Department of Chemistry, Mommsenstralje G.D.R. (Receiued 9
January
13, Dresden 8027,
1989)
Abstract-The anodic dissolution and cathodic deposition of lithium is strongly influenced by the reductior or formation of surface layers, the reduction of the electrolyte, chemical side reactions and adsorption processes in the presence of some special additives used within the liquid electrolyte solution. The protective surface layer at lithium immersed in liquid organic aprotic media undergoes during polarization dynamic alteration. A repititive destruction and formation process of the protective layer takes place. Independent of the electrolyte solution used, during cathodic polarization a sharp decrease of the measured resistance was observed. Distinct differences in the properties of the lithium surface are also obvious during anodic polarization. In the presence of decaiine the formed surface film is strongly influenced by the hydrocarbon. This surface film determines the electrochemical properties of the lithium electrode, possessing a better cycling performance in the first chargedischarge cycles.
1. INTRODUCTION A lot of efforts has been undertaken to improve the cycleability of the lithium electrode. The cycleability depends decisively on the nature of the lithium deposition. A dendritic one strongly influences its durability. So far, the existing experimental data concerning the cycling behaviour of lithium indicate that the improvement of its cycleability in aprotic organic media can be achieved by choosing stable electrolytes, using additives in the electrolyte, using lithium alloys, modifying the lithium surface by solid electrolytes possessing Li+ conductivity or cycling under me-chanical pressure. According to Shishikura et al.[l] 2-methyl-tetrahydrofurane (2-Me-THF) seems to be the only stable solvent used in lithium cycling. In spite of the chemical reactivity of tetrahydrofurane (THF) regarding lithium it is often used as a component to blend 2-MeTHF. According to our investigations, THF is able to modify the passivating surface layer on the lithium electrode in case of adding an amount of 1 to 10 ~01% to the base solvent. In contact with propylencarbonate (PC) lithium is not so stable as in 2-Me-THF. However, it is assumed by Besenhard et a1.[2] to be worthy for exploitation, if one succeeds in improving the deposition of lithium and retarding the reduction of the solvent by additives to the electrolyte. In this investigation, we have examined the electrochemical behaviour of lithium in aprotic organic media in the presence of decaline as a special additive for improving the cycleability of the lithium electrode.
solutions were prepared by dissolving anhydrous LiC104 (Merck) and LiAsF, (Ventron) in the purified solvents. The latter ones were carefully dehydrated before use: the PC (Merck) by a two-fold vacuum distillation followed by passing through a molecular sieve type 4A, the 2-Me-THF (Merck) and THF (VEB Laborchemie Apolda) by refluxing over metallic sodium and potassium followed by a vacuum distillation procedure. Potentiostatic measurements have been carried out with compact lithium electrodes (0.5 cm’), supported on a nickel collector, in glass vessels at room temperature (293-295 K). In all cases pure lithium supported on a nickel collector served as the counter electrode. All potentials refer to the Li/Li* couple and are corrected for the iR-drop. The resistance R was estimated by analysing the oscillographic current traces caused by superimposing potentiostatic rectangular pulses of small amplitudes (S-10 mV; 250 Hz). The impedance measurements were carried out with an ac impedance measurement system, Model 368. The system was used in the potentiostatic mode at the open circuit potential of the lithuim electrode (diameter 2 mm). The electrode impedance was determined using the lock-in amplifier technique in the high frequence range (105-5 Hz, five points per decade) and the fast fourier transform technique in the low frequence range (1O-1O-3 Hz).
3. RESULTS 3.1. The behaviour electrolytes without
2. EXPERIMENTAL PC/LiClO, (1 M) and 2-Me-THF/THF (2vol%)/ LiAsF, (1 M) were used as electrolyte solutions. These 1277
AND
DISCUSSION
of the lithium additives
electrode
in
3.1.1. Current potential plots. The current potential plots have been measured under quasi-stationary
K. WIESENER et al.
1278
potentiostatic conditions starting in cathodic direction (Fig. la). The hysteresis of the forward and backward curves and the change of the ohmic resistance measured between the working and reference electrode (Fig. lb) give evidence, that a potential dependent surface film strongly influences the electrochemical behaviour of lithium in these electrolyte solutions. The measured resistance R (Fig. 1b) consists of at least two components-the electrolyte resistance R, and a layer resistance R,, the latter one representing the thickness and the conductivity of the formed surface film with protective properties. Assuming no change in the current distribution during our measurements, we can conclude, that the observed changes in the value of the resistance R are attributed to dynamically alterations of the properties of the protective layer. A significant decrease of the measured resistance occurs during cathodic polarization in both electrolyte systems investigated. Under these conditions, the surface film seems to be destroyed or reduced. At high cathodic overvoltages (19 I> 100 mV) the resistance increases again, obviously due to a new layer formation.
In the cathodic reverse direction this layer seems to be removed more or less reversible causing a decrease of the measured resistance again. The cathodically activated surface represents a better lithium deposition rate leading to higher cathodic currents during the measurements in reverse direction. In the 2-Me-THF electrolyte, an anodic polarization causes obviously no significant change of the layer properties-the resistance is nearly constant. The current potential plots show no pronounced hysteresis. However, in the PC electrolyte a significant increase of the layer resistance during the anodic dissolution process is observed. Up to 100 mV in the forward scan the resistance is nearly constant. From this fact it could be assumed that newly deposited lithium in good electronic contact to the base lithium is dissolved. In the potential region of 100 to 200 mV, lithium islands should be dissolved which are separated from the base material by a layer of more and more decreasing electronic conductivity. This becomes evident by the increasing resistance. Only after all newly deposited lithium (V > 200 mV), which is in a sufficient electronic contact with the base lithium, is dissolved, the anodic current arises while the resistance remains constant. Now, it could be assumed
I
-200
-100
0
I
I
01
.I..
100
I.
200
Wmv Fig. 1. (a) Polarization plots and (b) Ohmic resistance R of the lithium deposition and dissolution. (A, A) (Forward, backward), PC/lM LiCIO,; (---, ,,a($rward, backward), ZMeTHF/THF (2 ~ol%)/lM 6’
Electrochemical
bebaviour
of the lithium electrode
1279
On the basis of this model, which is intensively discussed, the corresponding equivalent circuit determines three types of impedances: (i) the conduction impedance defined by the bulk resistance of the layer R, and the geometric capacitance of the layer C,; (ii) the charge transfer impedance, represented by the and the double layer charge transfer resistance R, capacitance Cd,; (iii) the diffusion impedance Z, determined by a definite thickness of the diffusion layer[9]. The impedance diagram in the complex plane obtained at a lithium electrode immersed in PC/LiClO, solution at the open-circuit potential in dependence of storage time is shown in Fig. 2. Contrary to the observations of Thevenin et a[.[ lo] in our experiments we obtained two depressed semicircles in the Cole-Cole plot, where the second one in the region of lower frequencies is strongly depressed. The influence of diffusion processes could not clearly detected. The reason for this observed behaviour may be the amalgam pretreatment of the PC solvent by Thevenin et al., which will lead to a strong decomposition of the solvent. During immersion (Fig. 2b) only the first semicircle changes, but not the second one. According to the model the high frequency arc represents the conduction impedance, influenced by the thickness and composition of the surface film. During the preparation procedure of the lithium electrode in the argon dry
that the base lithium is involved in the dissolution reaction. In the reverse scan, the dissolution process is retarded due to the remaining layer, although the layer resistance is decreased by the anodic dissolution of the base lithium. The reason for this behaviour could be mechanical damage or generally expressed destruction of the formed surface layer. Garreau[3] has shown by impedance measurements, that during anodic polarization of lithium in PC electrolyte one is not able to remove an existing layer, because the diffusion process within the layer persisted in spite of the anodic stripping. 3.1.2. Impedance at the open circuit potential. The process of surface layer formation can be confirmed by impedance measurements. Many reports on the surface layers have been published. Meanwhile one assumes more or less successfully four models to discuss the interface behaviour of the lithium electrode+he solid electrolyte interphase (SEI) rnodel[4, 57, the polymer electrolyte the solid polymer layer interface (PEI) model[%3], (SPL) model[9, 101 and the compact stratified layer (CSL) model[ lo]. Because the surface layer in PC electrolyte consists of inorganic (Li,C03, Li,O) and polymer organic destruction products of the electrolyteC4, 5, 11,121 we will use the SPL model in order to make an attempt to interpret our impedance data.
cl
b
0
10
20 ZyILcm
Fig. 2. Impedance
40
spectrum of the lithium electrode in PC/t M LiCIO, days. Real part corrected for electrolyte resistance,
$0 0
Fig. 3. Layer
30
resistance
(0)
2
r,
6
storage
time
and capacitance
(0)
50
2
a
electrolyte: (a) after 1 day; {b) after 11 frequency in Hz.
10
12
/days
as a function
of the storage
time in PC/lM
LiClO,.
1280
K. WIESENER et al.
box a surface film will be forined. This film causes the relatively high value of the layer resistance immediately after immersion (Fig. 3) or the attack of the electrolyte is so fast and intensive and leads to a thick layer possessing this high resistance. During the storage time within the electrolyte solution the film equilibrates and after some days it starts to grow. The nearly constant yalue of the product RC (Fig. 4), which represents the time constant of this system, leads to the conclusion, that during the immersion time mainly the thickness of the formed layer changes. The charge transfer process (medium frequency arc) seems to be unchanged.
3.2. The behaviour of the lithium presence of decaline
electrode
in the
Cyclic hydrocarbons have been investigated as suitable additives for improvement of the cycleability. In accordance with Besenhard[2] it was found that for instance decaline has a positive influence on the cycleability of the lithium electrode. The time dependence of the electrode potential during cycling is very small. After the addition of decaline to the electrolyte solution a homogeneous lithium deposition is observed. However, it depends significantly on the decaline concentration used (Fig. 5). In the case of decaline the optimal concentration lies in the range of about 1 vol%, higher contents show a negative effect. 3.2.1. Current potential plots. The polarization plots have been measured under the same conditions as before, but in the presence of I ~01% of decaline in the electrolyte solution (Fig. 6). In the decaline containing electrolyte the electrode polarization is lowered in the cathodic as well as in the anodic branch (Fig. 6a). The electrolyte resistance measured by superimposing rectangular potentiostatic pulses is lowered, too (Fig. 6b). This influences the assumption that the formed surface layer in the presence of the hydrocarbon is thinner or its condutivity is higher. The hysteresis of the forward and backward curves in the cathodic branch is not so pronounced like in the absence of decaline. The more smooth and rapid deposition of lithium in the presence of the hydrocarbon obviously retards the formation of the surface layer. In the anodic branch a strong hysteresis effect occurs, especially at potentials above 100 mV. Now, all newly deposited lithium is dissolved and the base lithium is involved in the dissolution reaction. The current potential plot is not constant (dotted line in Fig. 6), the electrode polarization and the resistance rapidly arise and the behaviour of the lithium electrode in the dissolution reaction is similar to that in PC electrolyte without decaline. The lowered polarization effects (Fig. 6a) in solutions with decaline and the nearly constant medium frequency arc in the impedance data (Fig. 7) allow the assumption, that the charge transfer process is not hindered by the adsorbed hydrocarbon. This means, that the behaviour of the hydrocarbon additives should not be discussed in terms of inhibitor effects like in normal corrosion processes. In this case we
3.8
’ 0
, 2
4
1
6
storage
a time
10
12
/days
Fig. 4. Product RC as a function of the storage time. (0) PC/IM LiC104 without decaiine; (0) PC/LiCIO, + 1 ~01% decaline. 150 [
I -I charge
Fig. 5. Time dependence of lithium electrode potential during cycling using various decaline concentrations: IO”’cycle in 2-Me-THF/IXF (10 vol%)/LiPF, (1M); i =0.25rnAct1~~; Q= 10 Ccm --2. (--) Without decaline;(- - -) 1~01% decaline; (-* -*1 5 ~1% decaline.
should expect an increase in the electrode due to surface blocking.
polarization
3.2.2. Impedance at the open circuit potential. The results of the impedance measurements (Fig. 7) show a remarkable influence of the additive on the behaviour of the surface layer. Now, the high frequency arc is much smaller in equal times. The calculated layer resistance R, (Fig. 8) is lowered, when decaline is added to the electrolyte solution. This result confirms the potentiostatic measurements. The calculated geometric capacitance C, (Fig. 8) is much higher in the presence of decaline, but after five days storage this effect vanishes and it seems, that a long time effect for better cycleability can not be expected. The product RC is nearly time independent and has the same value as before. The lowered layer resistance during the first hours of immersion compared with that value observed in the absence of decaline indicates, that the adsorbed hydrocarbon either prevents a fast attack of the electrolyte or modifies the formed layer. From the impedance data it seems too simple to attribute the influence of decaline only to the layer thickness. It is obvious, that decaline strongly influences the process of layer formation, which can be interpreted by at least two factors: (i) the so formed solid electrolyte layer (Li,O, Li,CO,) seems to be thinner and more compact and retards the electrolyte reduction, which is expressed in lower layer resistances
1281
Electrochemical behaviour of the lithium electrode
1 1
-
200
1
I
I
I
100
0
-100
200
ULilmV
Fig. 6. (a) Polarization plots and (b) Ohmic resistance R of the lithium deposition and dissolution. (0), (0) (Forward, backward), PC/lM LiCIO, + 1 ~01% decaline.
N E 0
5
s,o,
5
0 0
IO
20 Z’/ncm
30
LO
2
Fig. 7. Impedance spectrum of the lithium electrode in PC/LiClO, + 1 ~01% decaline electrolyte: (a) after 1 day; (b) after 10 days. Real part corrected for electrolyte resistance, frequency in Hz. and decreased electrode polarization; (ii) the formation of the polymer part of the layer with an increased permittivity is promoted, which could be assumed from the relatively high values of the geometric capacitance during the first days of storage. At present it is difficult to distinguish between these possibilities.
4. CONCLUSION The study of the electrochemical behaviour of the lithium electrode has shown that dynamic alterations of the protective surface layer occur during polarization. In the presence of decaline the surface film is strongly influenced by the adsorbed hydrocarbon,
1282
K. WIESFNER et al. 30
-
0 0
12
,
,
1
,
I
2
4
6
8
10
storage
0 12
time/days
Fig. 8. Layer resistance (0) and capacitance ( l ) as a function of the storage time in PC/l M UCIO, t 1 ~01% decaline electrolyte. possessing a better cycling performance stage of the chargedischarge process.
in the early
Acknowledgement We wish to thank Dr K. Helfricht for helpful assistance in analysing and .discussing the impedance data.
REFERENCES 1. T. Shishikura, H. Konuma, H. Nakamura, Y. Kobayashi and K. Fujita. Proceedings of the 271h Battery Symposium, Osaka (1986). 2. J. 0. Besenhard, J. Giirtler and P. Komenda, in De&emu-Monogr. 109, 315 (1987). 3. M. Garreau, J. Pwr Sources 20, 9 (1987).
4. Y. Geronov, F. Schwager and R. H. Muller, Proceedings of the Workshop on Lithium Nonaqueous Battery Electrochemistry, E.C.S., Vol. S&7 (1980). 5. E. Peled, in Lithium Batteries (Edited by J. P. Gabano), Ch. 3, Academic Press, London (1983). 6. J. Thevenin, J. Pwr Sources 14, 45 (1985). 7. M. Froment, M. Garreau, J. Thevenin and D. Warin, J. Microsc. Spect. Electron. 4, 111 (1979). 8. G. Nazri and R. H. Muller, J. electrochem. Sot. 132.2050 (1985). 9. M. Armand, Solid St. lo&s, 9/10, 745 (1983). IO. J. Thevenin and R. H. Muller, J. electrochem. SK. 134, 273 (1987).
11. G. Eggert and J. Heitbaum, Elecrrochim. Acta 31, 1443 (1986): 12. D. Aurbach, M. L. Daroux, P. W. Faguy, E. Yeager, J. Electrochem. Sot. 134, 1611 (1987).
of Technology, Department of Chemistry, Mommsenstralje G.D.R. (Receiued 9
January
13, Dresden 8027,
1989)
Abstract-The anodic dissolution and cathodic deposition of lithium is strongly influenced by the reductior or formation of surface layers, the reduction of the electrolyte, chemical side reactions and adsorption processes in the presence of some special additives used within the liquid electrolyte solution. The protective surface layer at lithium immersed in liquid organic aprotic media undergoes during polarization dynamic alteration. A repititive destruction and formation process of the protective layer takes place. Independent of the electrolyte solution used, during cathodic polarization a sharp decrease of the measured resistance was observed. Distinct differences in the properties of the lithium surface are also obvious during anodic polarization. In the presence of decaiine the formed surface film is strongly influenced by the hydrocarbon. This surface film determines the electrochemical properties of the lithium electrode, possessing a better cycling performance in the first chargedischarge cycles.
1. INTRODUCTION A lot of efforts has been undertaken to improve the cycleability of the lithium electrode. The cycleability depends decisively on the nature of the lithium deposition. A dendritic one strongly influences its durability. So far, the existing experimental data concerning the cycling behaviour of lithium indicate that the improvement of its cycleability in aprotic organic media can be achieved by choosing stable electrolytes, using additives in the electrolyte, using lithium alloys, modifying the lithium surface by solid electrolytes possessing Li+ conductivity or cycling under me-chanical pressure. According to Shishikura et al.[l] 2-methyl-tetrahydrofurane (2-Me-THF) seems to be the only stable solvent used in lithium cycling. In spite of the chemical reactivity of tetrahydrofurane (THF) regarding lithium it is often used as a component to blend 2-MeTHF. According to our investigations, THF is able to modify the passivating surface layer on the lithium electrode in case of adding an amount of 1 to 10 ~01% to the base solvent. In contact with propylencarbonate (PC) lithium is not so stable as in 2-Me-THF. However, it is assumed by Besenhard et a1.[2] to be worthy for exploitation, if one succeeds in improving the deposition of lithium and retarding the reduction of the solvent by additives to the electrolyte. In this investigation, we have examined the electrochemical behaviour of lithium in aprotic organic media in the presence of decaline as a special additive for improving the cycleability of the lithium electrode.
solutions were prepared by dissolving anhydrous LiC104 (Merck) and LiAsF, (Ventron) in the purified solvents. The latter ones were carefully dehydrated before use: the PC (Merck) by a two-fold vacuum distillation followed by passing through a molecular sieve type 4A, the 2-Me-THF (Merck) and THF (VEB Laborchemie Apolda) by refluxing over metallic sodium and potassium followed by a vacuum distillation procedure. Potentiostatic measurements have been carried out with compact lithium electrodes (0.5 cm’), supported on a nickel collector, in glass vessels at room temperature (293-295 K). In all cases pure lithium supported on a nickel collector served as the counter electrode. All potentials refer to the Li/Li* couple and are corrected for the iR-drop. The resistance R was estimated by analysing the oscillographic current traces caused by superimposing potentiostatic rectangular pulses of small amplitudes (S-10 mV; 250 Hz). The impedance measurements were carried out with an ac impedance measurement system, Model 368. The system was used in the potentiostatic mode at the open circuit potential of the lithuim electrode (diameter 2 mm). The electrode impedance was determined using the lock-in amplifier technique in the high frequence range (105-5 Hz, five points per decade) and the fast fourier transform technique in the low frequence range (1O-1O-3 Hz).
3. RESULTS 3.1. The behaviour electrolytes without
2. EXPERIMENTAL PC/LiClO, (1 M) and 2-Me-THF/THF (2vol%)/ LiAsF, (1 M) were used as electrolyte solutions. These 1277
AND
DISCUSSION
of the lithium additives
electrode
in
3.1.1. Current potential plots. The current potential plots have been measured under quasi-stationary
K. WIESENER et al.
1278
potentiostatic conditions starting in cathodic direction (Fig. la). The hysteresis of the forward and backward curves and the change of the ohmic resistance measured between the working and reference electrode (Fig. lb) give evidence, that a potential dependent surface film strongly influences the electrochemical behaviour of lithium in these electrolyte solutions. The measured resistance R (Fig. 1b) consists of at least two components-the electrolyte resistance R, and a layer resistance R,, the latter one representing the thickness and the conductivity of the formed surface film with protective properties. Assuming no change in the current distribution during our measurements, we can conclude, that the observed changes in the value of the resistance R are attributed to dynamically alterations of the properties of the protective layer. A significant decrease of the measured resistance occurs during cathodic polarization in both electrolyte systems investigated. Under these conditions, the surface film seems to be destroyed or reduced. At high cathodic overvoltages (19 I> 100 mV) the resistance increases again, obviously due to a new layer formation.
In the cathodic reverse direction this layer seems to be removed more or less reversible causing a decrease of the measured resistance again. The cathodically activated surface represents a better lithium deposition rate leading to higher cathodic currents during the measurements in reverse direction. In the 2-Me-THF electrolyte, an anodic polarization causes obviously no significant change of the layer properties-the resistance is nearly constant. The current potential plots show no pronounced hysteresis. However, in the PC electrolyte a significant increase of the layer resistance during the anodic dissolution process is observed. Up to 100 mV in the forward scan the resistance is nearly constant. From this fact it could be assumed that newly deposited lithium in good electronic contact to the base lithium is dissolved. In the potential region of 100 to 200 mV, lithium islands should be dissolved which are separated from the base material by a layer of more and more decreasing electronic conductivity. This becomes evident by the increasing resistance. Only after all newly deposited lithium (V > 200 mV), which is in a sufficient electronic contact with the base lithium, is dissolved, the anodic current arises while the resistance remains constant. Now, it could be assumed
I
-200
-100
0
I
I
01
.I..
100
I.
200
Wmv Fig. 1. (a) Polarization plots and (b) Ohmic resistance R of the lithium deposition and dissolution. (A, A) (Forward, backward), PC/lM LiCIO,; (---, ,,a($rward, backward), ZMeTHF/THF (2 ~ol%)/lM 6’
Electrochemical
bebaviour
of the lithium electrode
1279
On the basis of this model, which is intensively discussed, the corresponding equivalent circuit determines three types of impedances: (i) the conduction impedance defined by the bulk resistance of the layer R, and the geometric capacitance of the layer C,; (ii) the charge transfer impedance, represented by the and the double layer charge transfer resistance R, capacitance Cd,; (iii) the diffusion impedance Z, determined by a definite thickness of the diffusion layer[9]. The impedance diagram in the complex plane obtained at a lithium electrode immersed in PC/LiClO, solution at the open-circuit potential in dependence of storage time is shown in Fig. 2. Contrary to the observations of Thevenin et a[.[ lo] in our experiments we obtained two depressed semicircles in the Cole-Cole plot, where the second one in the region of lower frequencies is strongly depressed. The influence of diffusion processes could not clearly detected. The reason for this observed behaviour may be the amalgam pretreatment of the PC solvent by Thevenin et al., which will lead to a strong decomposition of the solvent. During immersion (Fig. 2b) only the first semicircle changes, but not the second one. According to the model the high frequency arc represents the conduction impedance, influenced by the thickness and composition of the surface film. During the preparation procedure of the lithium electrode in the argon dry
that the base lithium is involved in the dissolution reaction. In the reverse scan, the dissolution process is retarded due to the remaining layer, although the layer resistance is decreased by the anodic dissolution of the base lithium. The reason for this behaviour could be mechanical damage or generally expressed destruction of the formed surface layer. Garreau[3] has shown by impedance measurements, that during anodic polarization of lithium in PC electrolyte one is not able to remove an existing layer, because the diffusion process within the layer persisted in spite of the anodic stripping. 3.1.2. Impedance at the open circuit potential. The process of surface layer formation can be confirmed by impedance measurements. Many reports on the surface layers have been published. Meanwhile one assumes more or less successfully four models to discuss the interface behaviour of the lithium electrode+he solid electrolyte interphase (SEI) rnodel[4, 57, the polymer electrolyte the solid polymer layer interface (PEI) model[%3], (SPL) model[9, 101 and the compact stratified layer (CSL) model[ lo]. Because the surface layer in PC electrolyte consists of inorganic (Li,C03, Li,O) and polymer organic destruction products of the electrolyteC4, 5, 11,121 we will use the SPL model in order to make an attempt to interpret our impedance data.
cl
b
0
10
20 ZyILcm
Fig. 2. Impedance
40
spectrum of the lithium electrode in PC/t M LiCIO, days. Real part corrected for electrolyte resistance,
$0 0
Fig. 3. Layer
30
resistance
(0)
2
r,
6
storage
time
and capacitance
(0)
50
2
a
electrolyte: (a) after 1 day; {b) after 11 frequency in Hz.
10
12
/days
as a function
of the storage
time in PC/lM
LiClO,.
1280
K. WIESENER et al.
box a surface film will be forined. This film causes the relatively high value of the layer resistance immediately after immersion (Fig. 3) or the attack of the electrolyte is so fast and intensive and leads to a thick layer possessing this high resistance. During the storage time within the electrolyte solution the film equilibrates and after some days it starts to grow. The nearly constant yalue of the product RC (Fig. 4), which represents the time constant of this system, leads to the conclusion, that during the immersion time mainly the thickness of the formed layer changes. The charge transfer process (medium frequency arc) seems to be unchanged.
3.2. The behaviour of the lithium presence of decaline
electrode
in the
Cyclic hydrocarbons have been investigated as suitable additives for improvement of the cycleability. In accordance with Besenhard[2] it was found that for instance decaline has a positive influence on the cycleability of the lithium electrode. The time dependence of the electrode potential during cycling is very small. After the addition of decaline to the electrolyte solution a homogeneous lithium deposition is observed. However, it depends significantly on the decaline concentration used (Fig. 5). In the case of decaline the optimal concentration lies in the range of about 1 vol%, higher contents show a negative effect. 3.2.1. Current potential plots. The polarization plots have been measured under the same conditions as before, but in the presence of I ~01% of decaline in the electrolyte solution (Fig. 6). In the decaline containing electrolyte the electrode polarization is lowered in the cathodic as well as in the anodic branch (Fig. 6a). The electrolyte resistance measured by superimposing rectangular potentiostatic pulses is lowered, too (Fig. 6b). This influences the assumption that the formed surface layer in the presence of the hydrocarbon is thinner or its condutivity is higher. The hysteresis of the forward and backward curves in the cathodic branch is not so pronounced like in the absence of decaline. The more smooth and rapid deposition of lithium in the presence of the hydrocarbon obviously retards the formation of the surface layer. In the anodic branch a strong hysteresis effect occurs, especially at potentials above 100 mV. Now, all newly deposited lithium is dissolved and the base lithium is involved in the dissolution reaction. The current potential plot is not constant (dotted line in Fig. 6), the electrode polarization and the resistance rapidly arise and the behaviour of the lithium electrode in the dissolution reaction is similar to that in PC electrolyte without decaline. The lowered polarization effects (Fig. 6a) in solutions with decaline and the nearly constant medium frequency arc in the impedance data (Fig. 7) allow the assumption, that the charge transfer process is not hindered by the adsorbed hydrocarbon. This means, that the behaviour of the hydrocarbon additives should not be discussed in terms of inhibitor effects like in normal corrosion processes. In this case we
3.8
’ 0
, 2
4
1
6
storage
a time
10
12
/days
Fig. 4. Product RC as a function of the storage time. (0) PC/IM LiC104 without decaiine; (0) PC/LiCIO, + 1 ~01% decaline. 150 [
I -I charge
Fig. 5. Time dependence of lithium electrode potential during cycling using various decaline concentrations: IO”’cycle in 2-Me-THF/IXF (10 vol%)/LiPF, (1M); i =0.25rnAct1~~; Q= 10 Ccm --2. (--) Without decaline;(- - -) 1~01% decaline; (-* -*1 5 ~1% decaline.
should expect an increase in the electrode due to surface blocking.
polarization
3.2.2. Impedance at the open circuit potential. The results of the impedance measurements (Fig. 7) show a remarkable influence of the additive on the behaviour of the surface layer. Now, the high frequency arc is much smaller in equal times. The calculated layer resistance R, (Fig. 8) is lowered, when decaline is added to the electrolyte solution. This result confirms the potentiostatic measurements. The calculated geometric capacitance C, (Fig. 8) is much higher in the presence of decaline, but after five days storage this effect vanishes and it seems, that a long time effect for better cycleability can not be expected. The product RC is nearly time independent and has the same value as before. The lowered layer resistance during the first hours of immersion compared with that value observed in the absence of decaline indicates, that the adsorbed hydrocarbon either prevents a fast attack of the electrolyte or modifies the formed layer. From the impedance data it seems too simple to attribute the influence of decaline only to the layer thickness. It is obvious, that decaline strongly influences the process of layer formation, which can be interpreted by at least two factors: (i) the so formed solid electrolyte layer (Li,O, Li,CO,) seems to be thinner and more compact and retards the electrolyte reduction, which is expressed in lower layer resistances
1281
Electrochemical behaviour of the lithium electrode
1 1
-
200
1
I
I
I
100
0
-100
200
ULilmV
Fig. 6. (a) Polarization plots and (b) Ohmic resistance R of the lithium deposition and dissolution. (0), (0) (Forward, backward), PC/lM LiCIO, + 1 ~01% decaline.
N E 0
5
s,o,
5
0 0
IO
20 Z’/ncm
30
LO
2
Fig. 7. Impedance spectrum of the lithium electrode in PC/LiClO, + 1 ~01% decaline electrolyte: (a) after 1 day; (b) after 10 days. Real part corrected for electrolyte resistance, frequency in Hz. and decreased electrode polarization; (ii) the formation of the polymer part of the layer with an increased permittivity is promoted, which could be assumed from the relatively high values of the geometric capacitance during the first days of storage. At present it is difficult to distinguish between these possibilities.
4. CONCLUSION The study of the electrochemical behaviour of the lithium electrode has shown that dynamic alterations of the protective surface layer occur during polarization. In the presence of decaline the surface film is strongly influenced by the adsorbed hydrocarbon,
1282
K. WIESFNER et al. 30
-
0 0
12
,
,
1
,
I
2
4
6
8
10
storage
0 12
time/days
Fig. 8. Layer resistance (0) and capacitance ( l ) as a function of the storage time in PC/l M UCIO, t 1 ~01% decaline electrolyte. possessing a better cycling performance stage of the chargedischarge process.
in the early
Acknowledgement We wish to thank Dr K. Helfricht for helpful assistance in analysing and .discussing the impedance data.
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