Passivating lithium electrodes with trimethylsilylacetylene

Transcription

1 Ž. Solid State Ionics Passivating lithium electrodes with trimethylsilylacetylene J.S. Sakamoto a, F. Wudl b,c,1, B. Dunn a,c,) a Department of Materials Science and Engineering, UniÕersity of California-Los Angeles, Los Angeles, CA 90095, USA b Department of Chemistry, UniÕersity of California-Los Angeles, Los Angeles, CA 90095, USA c Exotic Materials Institute, UniÕersity of California-Los Angeles, Los Angeles, CA 90095, USA Accepted 18 June 2001 Abstract In this work, it was determined that the increase in electrode resistance as a function of immersion time was suppressed by coating lithium electrodes with trimethylsilylacetylene Ž TMSA.. The increase in resistance as a function of time was determined to be and V cm 2 rday for the uncoated and trimethylsilylacetylene-coated lithium electrodes, respectively. Additionally, in an experiment that simulated galvanostatic cycling conditions, the polarization during lithium deposition was 100 mv less for the trimethylsilylacetylene-coated electrode. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Trimethylsilylacetylene; Lithium; Batteries; Passivation layer 1. Introduction The attractive advantages of using metallic lithium as a high-energy density electrode in lithium secondary batteries are limited by the formation of a passivation layer that forms when lithium is imw1 4 x. It has been mersed in an organic electrolyte shown that the passivation layer that forms on the surface of lithium, when immersed in organic solw1 4 x. Typically, the passivation layer consists of lithium vent-based electrolytes, can limit performance compounds such as lithium carbonate w5,6x which impede lithium-ion diffusion and act as an electronic ) Corresponding author. Tel.: q ; fax: q addresses: bdunn@ucla.edu Ž B. Dunn., wudl@chem.ucla.edu Ž F. Wudl.. 1 Tel.: q ; fax: q insulator, thus retarding the net charge transfer prowx 7. Moreover, the longer the immersion time, cess the greater the thickness of the passivation layer and the slower the overall charge transfer process. There are several other performance limitations that result from the formation of a passivation layer. This includes the irreversible consumption of lithium, which results in decreased cycle life, and dendriticrcolumnar plating of lithium, which can lead to cell failure by short-circuiting. If the surface of lithium could be stabilized appropriately, the performance and safety of secondary lithium batteries could be enhanced significantly. A number of attempts have been made to improve the performance of lithium anodes by modifying the surface before or during cycling. One approach has involved the application of polymer coatings to protect the lithium and prevent the formation of a passivating layer. For example, poly-ž2-vinylpyri r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž. PII: S X

2 296 ( ) J.S. Sakamoto et al.rsolid State Ionics dene. and poly-ž ethylene oxide. Ž PEO. have been used, as lithium pre-coatings, to modify the passivawx 8. Additionally, self-assembled monolay- tion layer ers with poly-ž ethylene oxide. -like head groups have been used in a similar manner wx 9. This surface modification approach demonstrated that the increase in passivating layer resistance as a function of time could, indeed, be retarded. Another approach has involved the incorporation of additives such as CO 2, N O, and SO in the electrolyte to alter the lithium 2 2 morphology during deposition w10 x. This approach was shown to enhance cycling performance by forming thin inorganic films on the lithium electrode. The present paper considers the application of a thin film of trimethylsilylacetylene Ž TMSA. on lithium electrodes as a means of controlling the passivation layer that develops when lithium is immersed in an electrolyte consisting of 1.0 M LiClO 4 in propylene carbonate Ž PC.. The approach here is different from that investigated with other polymers in that there is an attempt to deliberately form a stable lithium polyacetylene complex w11x which effectively limits the continued growth of the passivation layer without adversely influencing ion transport. By minimizing the growth rate of the passivation layer, the overall lithium electrode resistance will not increase significantly with time and should lead to less polarization at the lithiumrelectrolyte interface. 2. Experimental The impedance of TMSA-coated lithium electrodes was compared to uncoated lithium electrodes using a three-electrode cell configuration. A nickel electrode was used as the counter electrode, lithium was used as a reference electrode, and TMSA-coated and uncoated lithium electrodes were used as the working electrodes. All lithium electrodes were cut into 1=2-cm strips and then scraped with a sharp blade to produce a shiny metallic surface. The TMSA-coated electrode was prepared by immersing a freshly scraped lithium electrode Ž Alfa. into the TMSA Ž Alfa. monomer and removed. Two different TMSA-coated electrodes were made, one with four layers Ž 4-TMSA. and another with six layers Ž6- TMSA.. The electrolyte was a 1 M solution of LiClO Ž Aldrich. in propylene carbonate Ž PC. 4. The LiClO4 was dried in a vacuum oven at 1408C for 10 h prior to being added to the PC. The uncoated and TMSA-coated electrodes were immersed in the electrolyte, and AC impedance measurements ŽPrinceton Applied Research were made periodically over a period of 40 days. The frequency range for the measurements was 0.01 Hz to 65 khz. Exchange current densities Ž i. o were determined using Tafel measurements using the same three-electrode configuration. The voltage Žvs. LirLi q. was swept bey1 tween y250 and q250 mv at 1 mv s ŽPAR 273A.. Galvanostatic charging and discharging Ž lithium deposition and dissolution, respectively. over a limited number of cycles was used to determine polarization effects. Two different types of cells were constructed. One cell contained a lithium counter electrode and an uncoated lithium-working electrode, while the other cell contained a lithium counter electrode and a working electrode with five coats of TMSA. As before, the electrolyte was 1 M LiClO4 in propylene carbonate and the nominal surface areas of all the electrodes were the same Ž4 2. Ž. 2 cm. A current density of " 1.5 marcm was applied for 3 h intervals with a rest period of 1 h between lithium deposition and dissolution. These cells were assembled in an argon glove box, sealed in a stainless steel container and removed from the glove box for cycling. The TMSA coating, AC impedance and Tafel experiments were conducted in the glove box. 3. Results and discussion The complex impedance diagrams for both the coated and uncoated lithium electrodes consist of a single semicircle Ž Fig. 1.. The behavior exhibited by the uncoated electrode in Fig. 1 is similar to that reported by Thevinin and Muller wx 7, and is attributed to the typical resistorrcapacitor coupling used to describe the effects of surface layers on lithium electrodes. The TMSA-coated electrode is also characterized by a single semicircle. The significant difference between the two electrodes is the diameter of the respective semicircles; the resistance associated with the surface layer on the lithium electrode is substantially less when the electrode is coated with

3 ( ) J.S. Sakamoto et al.rsolid State Ionics Fig. 1. Nyquist plot for uncoated and 6-TMSA electrodes. The uncoated electrode was immersed in electrolyte for 672 h. The TMSA-coated electrode was immersed for 960 h. TMSA. The TMSA-coated electrode has a resistance that is more than 10 times lower than the uncoated sample Ž5 vs. 60 V cm 2. despite the fact that the TMSA-coated electrode was immersed in the electrolyte nearly 300 h longer. The effect of electrolyte exposure on the resistance of the various electrodes is shown in greater detail in Fig. 2. For the uncoated electrode, the increase in resistance with immersion time compares well with that reported by Liebenow and Luhder wx 8. The results indicate rather dramatically that the TMSA-coated electrodes exhibit relatively little increase in impedance with immersion time. Moreover, the initial resistance for the TMSA samples is appreciably lower than that of the uncoated electrodes. The rate of increase in resistance was determined to be and V cm 2 rday for the uncoated and 6-TMSA electrodes, respectively. This indicates that the TMSA most likely forms a protective coating on lithium, which effectively limits further reaction between lithium and the electrolyte. The stability of the layer over time suggests that it is relatively insoluble in the electrolyte. In contrast, coatings of poly-ž ethylene oxide. Ž PEO., which have been inveswx 8, are soluble in organic tigated in a similar manner electrolytes. This behavior is shown in Fig. 3; the initially high resistance of PEO-coated lithium decreases with time due to its gradual dissolution. It is important to note that although PEO is dissolved, it modifies the lithium surface and does not act as a passivating layer itself. Unlike PEO, the resistance measured for TMSA-coated electrodes is initially low and remains relatively constant with time Ž Fig. 3.. Another feature of interest concerns the slightly different behavior for the four-layer and six-layer TMSA coatings. The resistance of the 4-TMSA lithium electrode increased faster than the 6-TMSA Fig. 2. Electrode resistance plotted as a function of time for 4-TMSA, 6-TMSA and uncoated electrodes. The data are comwx 8 pared to that for an uncoated electrode reported in Ref.. Fig. 3. Electrode resistance plotted as a function of time for 6-TMSA and PEO-coated electrodes. Values for PEO electrodes were determined by combining Rbulk and Rcharge transfer values in Ref. wx 8.

4 298 ( ) J.S. Sakamoto et al.rsolid State Ionics Fig. 4. Tafel plots for uncoated and TMSA-coated lithium electrodes. The voltage between "0.25 V was swept at 1 mvrs. electrode, but was still significantly slower than the uncoated electrode Ž Fig. 2.. The expectation here is that the additional TMSA coatings in the 6-TMSA electrode provided better surface coverage Ži.e. fewer pinholes. than the 4-TMSA electrode. Altogether, the AC impedance results show that the TMSA coating suppresses the impedance increase associated with the growth of a passivating layer. Tafel plots for the uncoated and TMSA-coated electrodes, 10 min after immersion in electrolyte, are shown in Fig. 4. The curve profiles for both electrodes are virtually identical during both anodic and cathodic polarization. This suggests that the presence of the TMSA coating does not significantly affect the kinetic behavior of lithium electrodes. The exchange current densities Ž i. 0 for the uncoated and TMSA-coated electrodes were determined to be 0.95 and 0.69 marcm 2, respectively. In addition, the fact that the cathodic polarization Ž deposition. response for the TMSA-coated electrode is similar to the anodic polarization Ž dissolution. response indicates that the coating is not dissolved during polarization. The galvanostatic cycling experiment Ž Fig. 5. indicates that the polarization during dissolution Ž0.4 V. is approximately the same for both the uncoated and TMSA-coated Ž five layers. electrodes. During deposition, however, the TMSA-coated electrode exhibited far less polarization. As shown in Fig. 5, the TMSA-coated electrode was polarized to y0.35 V while the uncoated electrode was polarized to y0.45 V. This increased polarization was apparent for the 15 cycles tested. From this, it is interesting to note that the electrode polarization, both anodic and cathodic, is approximately the same Ž "0.35 V. for the TMSA-coated electrode. In comparison, the uncoated electrode polarized more during lithium deposition Ž y0.45 V. than during lithium removal Žq0.35 V.. One reason for this response can be associated Fig. 5. Lithium depositionrdissolution experiment comparing uncoated and TMSA-coated electrodes. A current density of "1.5 marcm 2 was applied for 3 h.

5 ( ) J.S. Sakamoto et al.rsolid State Ionics with the solid polymer layer Ž SPL. model proposed by Thevinin and Muller wx 7. The surface layer in this model consists of a lithium-containing phase ŽLi 2CO3 and other possible organic compounds. dispersed in a polymer electrolyte. If the lithium phase is isolated as islands, then higher polarization is expected on reducing lithium than on dissolving it. Because the TMSA-coated electrode suppresses the growth of a significantly thick passivating layer, it is believed that most of the deposition occurs close to or on the surface of the electrode. As a result, isolated islands of lithium do not form, and hence, there is less polarization during deposition as compared to uncoated electrodes. 4. Conclusion A combination of electrochemical measurements have been used to establish that trimethylsilylacetylene Ž TMSA. has beneficial effects on the electrochemical properties of lithium electrodes. It is expected that the deposition of this polymer leads to the formation of a stable lithium polyacetylene comw11x which effectively limits the reaction be- plex tween lithium and the 1.0 M LiClO4 propylene carbonate electrolyte. AC impedance measurements indicated that the rate of resistance increase for an electrode immersed in this electrolyte for several weeks reduced significantly when the lithium electrode was coated with TMSA. Moreover, under galvanostatic cycling, the TMSA-coated electrode polarized less than the uncoated lithium electrode during dissolution. While the mechanism for suppressing the growth of surface layers still needs to be verified, it would seem that the approach taken here, that of using the high reactivity of lithium to deliberately form a protective layer, represents an interesting direction for improving the long-term performance of lithium electrodes. Acknowledgements This work was funded by California Industry and by the State of California UC-SMART program under Contract References wx 1 J.O. Besenhard, G. Eichenger, J. Electroanal. Chem. 68 Ž. 1 Ž wx 2 E. Peled, J. Electrochem. Soc. 126 Ž wx 3 E. Peled, in: J.P. Gabano Ž Ed.., Lithium Batteries, Academic Press, London, 1983, p. 43, Chapter 3. wx 4 M. Garreau, J. Thevinin, D. Warin, Prog. Batteries Sol. Cells 2 Ž wx 5 A.N. Dey, B.P. Sullivan, J. Electrochem. Soc. 117 Ž wx 6 G. Eichenger, J. Electroanal. Chem. 74 Ž wx 7 J. Thevinin, R.H. Muller, J. Electrochem. Soc. 134 Ž wx 8 C. Liebenow, K. Luhder, J. Appl. Electrochem. 26 Ž wx 9 R.N. Mason, M. Smith, T. Andrews, D. Teeters, Solid State Ionics 118 Ž w10x S.B. Brummer, F.W. Dampier, V.R. Koch, R.D. Rauh, T.T. Reise, J.H. Young, EIC, Final Report on Grant AER , w11x The nature of the species responsible for the effects observed in this publication is being established by a combination of spectroscopic methods including FTIR, NMR, as well as elemental analysis and will be the subject of future publication. The passivating layer cannot be simply lithium trimethlysilyl acetylide because the latter would be expected to dissolve in the electrolyte during the stripping and depositing cycling.