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Evolution of the electrode–electrolyte interface in a lithium–polymer battery
Solid State Ionics 177 (2006) 141 – 143 www.elsevier.com/locate/ssi
Evolution of the electrode–electrolyte interface in a lithium–polymer battery Anna Teyssot a, Michel Rosso a,*, Renaud Bouchet b, Stephane Lascaud c a
Laboratoire de Physique de la Matiere Condensee, CNRS-Ecole Polytechnique, 91128 Palaiseau Cedex, France b MADIREL, CNRS-Universite de Provence, Centre St Jerome, 13397 Marseille, France c EDF/R&D division, BP 1, F77250, Moret sur Loing, France Received 27 September 2005; accepted 14 October 2005
Abstract Using impedance spectroscopy and in situ optical absorption experiments, we have studied the evolution of a lithium electrode/polymer electrolyte interface during aging and cycling. During a period of 3 – 5 days after assembling a cell, aging has a detrimental effect on the interface: this effect is observed both on the interface impedance and on the optical properties of the electrolyte. After this initial period, optical absorption measured during cycling a cell reveals a concentration evolution in the electrolyte in agreement with theoretical predictions. D 2005 Elsevier B.V. All rights reserved. PACS: 82.45.Qr; 82.45.Wx Keywords: Lithium polymer battery; Lithium dendrite; Impedance spectroscopy; In situ microscopy
1. Introduction The cyclability of lithium batteries with a metallic lithium negative electrode is very sensitive to the state of the lithium/ electrolyte interface: in particular it depends on the composition and morphology of the passivation layer formed at the surface of lithium, and it is severely affected by the formation of dendrites. In order to study the evolution of this interface upon aging and upon cycling, we have performed impedance spectroscopy (IS) and optical absorption (OA) measurements in symmetrical Li/polymer –electrolyte/Li cells. In particular, the OA experiments permit in situ measurements of concentration profiles in the vicinity of the electrode/electrolyte interface. 2. Experiments We have performed experiments in symmetrical Li/ polymer – electrolyte/Li cells cycled under galvanostatic conditions, at 90 -C. The polymer electrolyte consisted of poly(ethylene oxide) (PEO, Mw = 3.105). Two salts were used: LiN(CF3SO2)2 (abbreviated in LiTFSI) [1,2], and Li(CF3 – SO2N – SO2 – C6H4 – NO2) with the nitro group in
* Corresponding author. Tel.: +33 1 69 33 46 67; fax: +33 1 69 33 30 04. E-mail address: [email protected] (M. Rosso). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.10.026
the para position. The latter salt, synthesized by Michel Armand, has good electrochemical properties (similar to LiTFSI) and has an optical absorption band in the range 310– 430 nm. A detailed description of our experimental setup is given in Ref. [3]: this setup allows to capture a map of the light absorbed through a lithium/polymer cell operated at 90 -C. In the present experiments, the light source is an array of diodes emitting at 430 nm. The light absorption is directly related to the salt concentration in the electrolyte. Simultaneously, the electrochemical impedance is measured using a Schlumberger SI 1255 Frequency Response Analyzer and a SI 1286 Electrochemical Interface, both controlled by a Macintosh microcomputer. Impedance spectroscopy experiments were also performed in ‘‘sandwich’’ cells having a geometry close to the geometry of actual batteries: these cells consist of two lithium foils sandwiching a polymer electrolyte layer 30 – 100 Am thick. 3. Impedance spectroscopy A typical spectrum obtained on a ‘‘sandwich cell’’ is shown in Fig. 1. As reported elsewhere [4], we are able to identify and quantify the major contributions to the impedance spectrum shown in Fig. 1: i) the electrolyte resistance R b (I in Fig. 1), ii) the interface resistance R i (resitance of the surface layers and/or charge– transfer resistance, semi-circle II in Fig. 1) and iii) the
142
A. Teyssot et al. / Solid State Ionics 177 (2006) 141 – 143
Im (Z) (Ω)
-20
103 Hz 10 Hz 105 Hz 10-1 Hz
-10
10-3 Hz
IV
II
0
From the evolution of the impedance spectra of symmetric cells Li/PEO –LiTFSI/Li with time we can conclude that only the interface resistance R i is affected by aging: it increases, typically by a factor of two, over a period of 3 – 5 days. After this period it stabilizes.
I
4. Optical absorption and concentration measurements
III
10
10
0
20
30
Re (Z) (Ω) Fig. 1. Impedance spectrum of a Li/PEO – LiTFSI/Li cell.
diffusion resistance in the electrolyte R d (Warburg element IV in Fig. 1). There is also a medium frequency feature (small loop III in Fig. 1), that we could not identify.
transmitted light (relative intensity)
a) electrodes 1 0.9
1h 40min
0.8 0.7
5h 25min
0.6
Both the effects of aging and cycling were investigated. After assembling a Li/PEO – Li(CF3 – SO2N – SO2 – C6H4 – NO2)/Li cell, we put it in the optical setup and raise its temperature to 90 -C. We then observe that its optical absorption increases by a factor of 5 – 10 over a period of a few days (Fig. 2), corresponding to the period when the interface resistance R i increases. This darkening effect obviously originates from the electrode/electrolyte interfaces, as evidenced from the optical absorption profiles shown in Fig. 2(a). We attribute this effect to the release of chemical impurities as a result of chemical reactions at the electrodes. We note however that the electrolyte resistance R b is not affected, suggesting that the concentration of these impurities is small. As for the interface resistance increase, this effect saturates, and the optical properties of the electrolyte become almost constant. Then we perform successive polarizations of our cells at constant current: during these polarizations we are able to obtain light absorption maps (Fig. 3(a)) from which we can measure the evolution of the concentration in the electrolyte
a)
Li
0.5 30h 55min
0.4 0.3 0.2
electrolyte 0
0.05
0.1
0.15
0.2
0.25
0.3
x (cm) 1
Li
b)
0.8
optical absorption (relative intensity)
transmitted light (relative intensity)
b)
0.6
0.4
0.2
0
1.4 1.3 1.2 1.1 1 0.9 0.8 0.7
0
50
100
150
200
Time (h) Fig. 2. (a) Relative intensity of the light transmitted through the electrolyte measured in the direction perpendicular to the electrodes (the value of one refers to the intensity measured at the beginning of the experiment). A darkening effect is seen to proceed from both electrodes. (b) Evolution of the average light transmitted through the cell as a function of time.
0
0.05
0.1
0.15
0.2
0.25
0.3
x (cm) Fig. 3. (a) Image of a cell polarized under constant current (current density: 0.15 mA cm 2, top: negative electrode, bottom: positive electrode), after 15 mn, (b) Optical absorption along the vertical direction, averaged over the white box shown in Fig. 2(a) Here, the negative electrode is on the left, and the intensity is calculated in reference to an image taken before cycling the cell.
A. Teyssot et al. / Solid State Ionics 177 (2006) 141 – 143
(Fig. 3(b)), which is in agreement with theoretical predictions. This shows that our method allows for quantitative measurements of the concentration in a polymer electrolyte [3,5]. However, a parasitic effect, that we also attribute to the release of chemical impurities at the electrodes, appears after a time depending on the current density. This parasitic effect strongly changes the optical absorption profiles, making the concentration measurements almost impossible.
143
the electrolyte, first revealing concentration profiles in accordance with theoretical predictions. However, a parasitic signal attributed to chemical reactions at the electrode/electrolyte interface is also observed, that prevents concentration measurements over a long period of time. Further experiments are now in progress to investigate the origin of this parasitic effect. References
5. Conclusion We are able to identify and quantify the major contributions to the impedance of a Li/polymer electrolyte/Li cell, namely the electrolyte resistance, the interface resistance and the diffusion resistance. The interface resistance increases with aging during a period of 3– 5 days. During the same period, in situ optical measurements reveal that light absorption of the cell increases. From light absorption experiments performed while cycling cells after this initial period we obtain concentration maps in
[1] M. Armand, J.M. Chabagno, M.J. Duclot, in P. Vashishta, J.N. Mundy, G.K. Shenoy (Eds.), Fast Transport in Solids, Elsevier, North-Holland, 1979, p. 131. [2] M. Armand, W. Gorecki, R. Andreani, in: B. Scrosati (Ed.), Second Int. Symp. on Polymer Electrolytes, Elsevier Applied Science, London, 1990, p. 91. [3] A. Teyssot, C. Belhomme, R. Bouchet, M. Rosso, S. Lascaud, M. Armand, J. Electroanal. Chem. 584 (2005) 70. [4] R. Bouchet, S. Lascaud, M. Rosso, J. Electrochem. Soc. 150 (2003) A1385. [5] C. Brissot, M. Rosso, J.-N. Chazalviel, S. Lascaud, J. Electrochem. Soc. 146 (1999) 4393.
Evolution of the electrode–electrolyte interface in a lithium–polymer battery Anna Teyssot a, Michel Rosso a,*, Renaud Bouchet b, Stephane Lascaud c a
Laboratoire de Physique de la Matiere Condensee, CNRS-Ecole Polytechnique, 91128 Palaiseau Cedex, France b MADIREL, CNRS-Universite de Provence, Centre St Jerome, 13397 Marseille, France c EDF/R&D division, BP 1, F77250, Moret sur Loing, France Received 27 September 2005; accepted 14 October 2005
Abstract Using impedance spectroscopy and in situ optical absorption experiments, we have studied the evolution of a lithium electrode/polymer electrolyte interface during aging and cycling. During a period of 3 – 5 days after assembling a cell, aging has a detrimental effect on the interface: this effect is observed both on the interface impedance and on the optical properties of the electrolyte. After this initial period, optical absorption measured during cycling a cell reveals a concentration evolution in the electrolyte in agreement with theoretical predictions. D 2005 Elsevier B.V. All rights reserved. PACS: 82.45.Qr; 82.45.Wx Keywords: Lithium polymer battery; Lithium dendrite; Impedance spectroscopy; In situ microscopy
1. Introduction The cyclability of lithium batteries with a metallic lithium negative electrode is very sensitive to the state of the lithium/ electrolyte interface: in particular it depends on the composition and morphology of the passivation layer formed at the surface of lithium, and it is severely affected by the formation of dendrites. In order to study the evolution of this interface upon aging and upon cycling, we have performed impedance spectroscopy (IS) and optical absorption (OA) measurements in symmetrical Li/polymer –electrolyte/Li cells. In particular, the OA experiments permit in situ measurements of concentration profiles in the vicinity of the electrode/electrolyte interface. 2. Experiments We have performed experiments in symmetrical Li/ polymer – electrolyte/Li cells cycled under galvanostatic conditions, at 90 -C. The polymer electrolyte consisted of poly(ethylene oxide) (PEO, Mw = 3.105). Two salts were used: LiN(CF3SO2)2 (abbreviated in LiTFSI) [1,2], and Li(CF3 – SO2N – SO2 – C6H4 – NO2) with the nitro group in
* Corresponding author. Tel.: +33 1 69 33 46 67; fax: +33 1 69 33 30 04. E-mail address: [email protected] (M. Rosso). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.10.026
the para position. The latter salt, synthesized by Michel Armand, has good electrochemical properties (similar to LiTFSI) and has an optical absorption band in the range 310– 430 nm. A detailed description of our experimental setup is given in Ref. [3]: this setup allows to capture a map of the light absorbed through a lithium/polymer cell operated at 90 -C. In the present experiments, the light source is an array of diodes emitting at 430 nm. The light absorption is directly related to the salt concentration in the electrolyte. Simultaneously, the electrochemical impedance is measured using a Schlumberger SI 1255 Frequency Response Analyzer and a SI 1286 Electrochemical Interface, both controlled by a Macintosh microcomputer. Impedance spectroscopy experiments were also performed in ‘‘sandwich’’ cells having a geometry close to the geometry of actual batteries: these cells consist of two lithium foils sandwiching a polymer electrolyte layer 30 – 100 Am thick. 3. Impedance spectroscopy A typical spectrum obtained on a ‘‘sandwich cell’’ is shown in Fig. 1. As reported elsewhere [4], we are able to identify and quantify the major contributions to the impedance spectrum shown in Fig. 1: i) the electrolyte resistance R b (I in Fig. 1), ii) the interface resistance R i (resitance of the surface layers and/or charge– transfer resistance, semi-circle II in Fig. 1) and iii) the
142
A. Teyssot et al. / Solid State Ionics 177 (2006) 141 – 143
Im (Z) (Ω)
-20
103 Hz 10 Hz 105 Hz 10-1 Hz
-10
10-3 Hz
IV
II
0
From the evolution of the impedance spectra of symmetric cells Li/PEO –LiTFSI/Li with time we can conclude that only the interface resistance R i is affected by aging: it increases, typically by a factor of two, over a period of 3 – 5 days. After this period it stabilizes.
I
4. Optical absorption and concentration measurements
III
10
10
0
20
30
Re (Z) (Ω) Fig. 1. Impedance spectrum of a Li/PEO – LiTFSI/Li cell.
diffusion resistance in the electrolyte R d (Warburg element IV in Fig. 1). There is also a medium frequency feature (small loop III in Fig. 1), that we could not identify.
transmitted light (relative intensity)
a) electrodes 1 0.9
1h 40min
0.8 0.7
5h 25min
0.6
Both the effects of aging and cycling were investigated. After assembling a Li/PEO – Li(CF3 – SO2N – SO2 – C6H4 – NO2)/Li cell, we put it in the optical setup and raise its temperature to 90 -C. We then observe that its optical absorption increases by a factor of 5 – 10 over a period of a few days (Fig. 2), corresponding to the period when the interface resistance R i increases. This darkening effect obviously originates from the electrode/electrolyte interfaces, as evidenced from the optical absorption profiles shown in Fig. 2(a). We attribute this effect to the release of chemical impurities as a result of chemical reactions at the electrodes. We note however that the electrolyte resistance R b is not affected, suggesting that the concentration of these impurities is small. As for the interface resistance increase, this effect saturates, and the optical properties of the electrolyte become almost constant. Then we perform successive polarizations of our cells at constant current: during these polarizations we are able to obtain light absorption maps (Fig. 3(a)) from which we can measure the evolution of the concentration in the electrolyte
a)
Li
0.5 30h 55min
0.4 0.3 0.2
electrolyte 0
0.05
0.1
0.15
0.2
0.25
0.3
x (cm) 1
Li
b)
0.8
optical absorption (relative intensity)
transmitted light (relative intensity)
b)
0.6
0.4
0.2
0
1.4 1.3 1.2 1.1 1 0.9 0.8 0.7
0
50
100
150
200
Time (h) Fig. 2. (a) Relative intensity of the light transmitted through the electrolyte measured in the direction perpendicular to the electrodes (the value of one refers to the intensity measured at the beginning of the experiment). A darkening effect is seen to proceed from both electrodes. (b) Evolution of the average light transmitted through the cell as a function of time.
0
0.05
0.1
0.15
0.2
0.25
0.3
x (cm) Fig. 3. (a) Image of a cell polarized under constant current (current density: 0.15 mA cm 2, top: negative electrode, bottom: positive electrode), after 15 mn, (b) Optical absorption along the vertical direction, averaged over the white box shown in Fig. 2(a) Here, the negative electrode is on the left, and the intensity is calculated in reference to an image taken before cycling the cell.
A. Teyssot et al. / Solid State Ionics 177 (2006) 141 – 143
(Fig. 3(b)), which is in agreement with theoretical predictions. This shows that our method allows for quantitative measurements of the concentration in a polymer electrolyte [3,5]. However, a parasitic effect, that we also attribute to the release of chemical impurities at the electrodes, appears after a time depending on the current density. This parasitic effect strongly changes the optical absorption profiles, making the concentration measurements almost impossible.
143
the electrolyte, first revealing concentration profiles in accordance with theoretical predictions. However, a parasitic signal attributed to chemical reactions at the electrode/electrolyte interface is also observed, that prevents concentration measurements over a long period of time. Further experiments are now in progress to investigate the origin of this parasitic effect. References
5. Conclusion We are able to identify and quantify the major contributions to the impedance of a Li/polymer electrolyte/Li cell, namely the electrolyte resistance, the interface resistance and the diffusion resistance. The interface resistance increases with aging during a period of 3– 5 days. During the same period, in situ optical measurements reveal that light absorption of the cell increases. From light absorption experiments performed while cycling cells after this initial period we obtain concentration maps in
[1] M. Armand, J.M. Chabagno, M.J. Duclot, in P. Vashishta, J.N. Mundy, G.K. Shenoy (Eds.), Fast Transport in Solids, Elsevier, North-Holland, 1979, p. 131. [2] M. Armand, W. Gorecki, R. Andreani, in: B. Scrosati (Ed.), Second Int. Symp. on Polymer Electrolytes, Elsevier Applied Science, London, 1990, p. 91. [3] A. Teyssot, C. Belhomme, R. Bouchet, M. Rosso, S. Lascaud, M. Armand, J. Electroanal. Chem. 584 (2005) 70. [4] R. Bouchet, S. Lascaud, M. Rosso, J. Electrochem. Soc. 150 (2003) A1385. [5] C. Brissot, M. Rosso, J.-N. Chazalviel, S. Lascaud, J. Electrochem. Soc. 146 (1999) 4393.