3542
Langmuir 1997, 13, 3542-3549
Study of the Surface Composition of Highly Smooth Lithium Deposited in Various Carbonate Electrolytes Containing HF Soshi Shiraishi, Kiyoshi Kanamura,* and Zen-ichiro Takehara Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-01, Japan Received September 9, 1996X We investigated the physical state of the surface film on lithium electrodeposited in various carbonate electrolytes containing HF in order to clarify the interfacial reaction and the electrodeposition process of lithium in nonaqueous electrolyte under a presence of a small amount of HF. The morphology and surface composition of lithium were analyzed with a scanning electron microscope and X-ray photoelectron spectroscopy (XPS). Propylene carbonate, ethylene carbonate, dimethyl carbonate, and diethyl carbonate were selected as solvents, and LiCF3SO3, LiBF4, and LiClO4 were selected as electrolyte salts. A small amount of aqueous hydrogen fluoride solution was used as a source of HF. The lithium electrodeposited in all electrolytes without HF was in a dendritic shape. On the other hand, the morphology of the lithium deposited in all electrolytes containing HF was highly smooth with a hemispherical shape. The surface composition and morphology of electrodeposited-lithium were strongly affected by the presence of HF in carbonate electrolytes rather than by the solvents or salts. Various basic lithium compounds in the surface film were converted to LiF through acid-base reactions with HF. These results suggested that the surface film on lithium and morphology of lithium seemed to be more sensitive to a minor component, HF, in carbonate electrolytes. Moreover, the XPS analysis revealed that lithium electrodeposited in carbonate electrolytes containing HF was covered with a highly stable and ultrathin (20-50 Å) film consisting of a LiF/Li2O bilayer. From these results, it can be concluded that such a stable thin surface film promotes the highly smooth deposition of lithium.
1. Introduction Recently, rechargeable lithium batteries have been expected to be a new energy storage device for electric vehicles and load-leveling systems. A rechargeable lithium metal battery is very attractive for its much higher energy density compared with others. However, various problems arise when lithium metal is used as an anode in a nonaqueous electrolyte. One of the most serious problems is a low coulomb efficiency for the lithium metal anode. Lithium is electrodeposited with a dendritic shape under some deposition conditions, the dendritic lithium can be easily separated from a current collector, resulting in the low efficiency of the lithium metal anode.1-3 According to a SEI (solid electrolyte interface) model,1 lithium is covered with an ionically conductive surface film consisting of various lithium compounds which strongly affect the morphology of lithium electrodeposited in nonaqueous electrolytes. Many researchers have investigated the surface film formed on lithium with many electrochemical analyses,4-13 X
Abstract published in Advance ACS Abstracts, June 1, 1997.
(1) Pled, E. J. Electrochem. Soc. 1979, 126, 2047. (2) Rauch, R. D.; Brummer, S. B. Electrochim. Acta 1977, 22, 75. (3) Tobishima, S.; Arakawa, M.; Hirai, H.; Yamaki, J. J. Power Sources 1989, 26, 449. (4) Geronov, Y.; Schwager, F.; Muller, R. H. J. Electrochem. Soc. 1982, 129, 1422. (5) Verbrugge, M. W.; Koch, B. J. Electroanal. Chem. 1994, 367, 123. (6) Pletcher, D.; Rohan, J. F.; Ritchie, A. G. Electrochim. Acta 1994, 39, 2015. (7) Wang, X.; Nishina, T.; Uchida, I. J. Surf. Finish. Soc. Jpn. 1995, 46, 941. (8) Thevenin, J. G.; Muller, R. H. J. Electrochem. Soc. 1987, 134, 273. (9) Takami, N.; Ohsaki, T.; Inada, K. J. Electrochem. Soc. 1992, 139, 1849. (10) Aurbach, D.; Zaban, A. J. Electroanal. Chem. 1993, 348, 155. (11) Fringant, C.; Tranchant, A.; Messina, R. Electrochim. Acta 1995, 40, 513. (12) Osaka, T.; Momma, T.; Nishimura, K.; Tajima, T. J. Electrochem. Soc. 1993, 140, 2745.
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spectroscopic analyses,14-32 and other analytical methods.33-35 Some additives (CO2,36-38 2-Me-furan39) were suggested in order to modify the surface film on lithium, in a practical sense. However, the relationship between the morphology and the surface film composition (13) Odziemkowski, M.; Irish, D. E. J. Electrochem. Soc. 1992, 139, 3063. (14) Nazri, G.; Muller, R. H. J. Electrochem. Soc. 1985, 132, 2050. (15) Schwager, F.; Geronov, Y.; Muller, R. H. J. Electrochem. Soc. 1985, 132, 285. (16) Odziemkowski, M.; Krell, M.; Irish, D. E. J. Electrochem. Soc. 1992, 139, 3052. (17) Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E. J. Electrochem. Soc. 1987, 134, 1611. (18) Aurbach, D.; Chusid, O. J. Electrochem. Soc. 1993, 140, L1. (19) Aurbach, D.; Zaban, A.; Schechter, A.; Ein-Eli, Y.; Zinigrad, E.; Markovsky, B. J. Electrochem. Soc. 1995, 142, 2873. (20) Wang, K.; Ross, P. N., Jr.; Kong, F.; McLarnon, F. J. Electrochem. Soc. 1996, 143, 422. (21) Wang, K.; Chottiner, G. S.; Scherson, D. A. J. Phys. Chem. 1993, 97, 11075. (22) Zavadil, K. R.; Armstrong, N. R. Surf. Sci. 1990, 230, 47. (23) Zavadil, K. R.; Armstrong, N. R. J. Electrochem. Soc. 1990, 137, 2371. (24) Aurbach, D.; Daroux, M.; McDougall, G.; Yeager, E. B. J. Electroanal. Chem. 1993, 358, 63. (25) Kanamura, K.; Tamura, H.; Takehara, Z. J. Electroanal. Chem. 1992, 333, 127. (26) Kanamura, K.; Shiraishi, S.; Tamura, H.; Takehara, Z. J. Electrochem. Soc. 1994, 141, 2380. (27) Kanamura, K.; Tamura, H.; Shiraishi, S.; Takehara, Z. J. Electrochem. Soc. 1995, 142, 340. (28) Shiraishi, S.; Kanamura, K.; Takehara, Z. J. Appl. Electrochem. 1995, 25, 584. (29) Kanamura, K.; Tamura, H.; Shiraishi, S.; Takehara, Z. J. Electroanal. Chem. 1995, 394, 49. (30) Kanamura, K.; Shiraishi, S.; Takehara, Z. J. Electrochem. Soc. 1994, 141, L108. (31) Kanamura, K.; Shiraishi, S.; Takehara, Z. J. Electrochem. Soc. 1996, 143, 2187. (32) Deng, Z.; Spear, J. D.; Rudnicki, J. D.; McLarnon, F. R.; Cairns, E. J. J. Electrochem. Soc. 1996, 143, 1514. (33) Ishikawa, M.; Yoshitake, S.; Morita, M.; Matsuda, Y. J. Electrochem. Soc. 1994, 141, L159. (34) Aurbach, D.; Zaban, A. J. Electrochem. Soc. 1995, 142, L108. (35) Mori, M.; Shinagawa, Y.; Suzuki, T.; Naoi, K. 36th Battery Symp. Japan, 2B18, P.169. The Electrochem. Soc., Japan (1995).
© 1997 American Chemical Society
Morphology of Electrodeposited Lithium
is still unclear in spite of the many efforts. In our previous works, we found that the surface film was affected by HF as one of the impurities in the nonaqueous electrolyte and that the addition of a small amount of HF into nonaqueous electrolytes (e.g., propylene carbonate containing 1.0 M LiClO4) was effective in suppressing the dendrite lithium deposition.30,31 We suggested that the effect of HF is related to the surface film consisting of a LiF/Li2O layer which may be formed by chemical reactions of HF with lithium metal and basic lithium compounds. In our previous paper31 relating to nonaqueous electrolytes containing an HF additive, we first discussed effects of deposition current density and HF concentration, added into propylene carbonate containing 1.0 mol dm-3 LiClO4, on the morphology of lithium deposits, in order to understand the deposition process. However, the surface composition of lithium deposited in electrolytes containing HF was not quantitatively clear. Moreover, the HF effect in various kinds of nonaqueous electrolytes has not yet been reported. If the above interfacial reaction (LiF/Li2O formation) is superior to other reactions, the HF effect will be independent of the kind of electrolyte. Therefore, in this study, the dependence of the surface and bulk composition and morphology of lithium on components of electrolyte was investigated to confirm the above supposition. Moreover, a semiquantitative analysis for the surface film on lithium was performed in order to clarify an interfacial structure of lithium metal in nonaqueous electrolytes under the presence of a small amount of HF. 2. Experimental Section Electrodeposition. A nickel plate (Nilaco Corp., Japan) was used as a substrate electrode (6 mm × 20 mm). The nickel substrate was polished with fine alumina powders (0.05 µm) to obtain a mirror surface and then dipped in an ultrasonic bath containing pure water and acetone to remove residual alumina powders. Lithium metal foil (Honjoh Metal Co., Japan) was used as the reference and counter electrodes. Propylene carbonate containing 1.0 mol dm-3 LiBF4 (LiBF4/PC) or LiCF3SO3 (LiCF3SO3/PC), ethylene carbonate-dimethyl carbonate mixture (1:1 in volume ratio) containing 1.0 mol and dm-3 LiClO4 (LiClO4/EC + DMC), and ethylene carbonate-diethyl carbonate mixture (1:1 in volume ratio) containing 1.0 mol dm-3 LiClO4 (LiClO4/EC + DEC) were used as base electrolytes (Mitsubishi Chemical Co., Japan). The water contents in these electrolytes were less than 20 ppm (1 × 10-3 mol dm-3), determined by the Karl Fischer Moisture Titrator (MKC-210, Kyoto Denshi Kogyo Co., Japan). Electrolytes containing HF were prepared by an addition of an aqueous hydrofluoric acid solution (46 wt %, Wako Pure Chemical Industries, Ltd., Japan) into these base electrolytes. Concentrations of HF and H2O in the electrolytes containing HF were 10 × 10-3 and 14 × 10-3 mol dm-3, respectively. Therefore, the water content in these electrolytes increased after the addition of HF. An electrochemical deposition of lithium was performed under galvanostatic conditions at 1.0 mA cm-2. The charge density for all electrodepositions was 1.0 C cm-2. After the deposition of lithium, the electrode was washed with pure PC, DEC, or DMC (Mitsubishi Chemical Co.) to remove electrolyte salts. All procedures were conducted in an argon dry atmosphere (dew point
