J. Phys. Chem. B 2001, 105, 123-134
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Imaging for Uniformity of Lithium Metal Surface Using Tapping Mode-Atomic Force and Surface Potential Microscopy Soshi Shiraishi,*,† Kiyoshi Kanamura,‡ and Zen-ichiro Takehara§ Department of Chemistry, Faculty of Engineering, Gunma UniVersity, Kiryu, Gunma 376-8515, Japan, Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan UniVersity, Hachioji, Tokyo 192-0397, Japan, and Department of Chemistry, Faculty of Engineering, Kansai UniVersity, Soita, Osaka 564-8680, Japan ReceiVed: June 2, 2000; In Final Form: September 7, 2000
Tapping mode-atomic force microscopy (TMAFM) and surface potential microscopy (SPoM) were conducted to investigate the surface condition of the electrodeposited lithium on nickel substrate. Distribution of surface potential corresponds to that of thickness or chemical composition of the surface film on lithium metal. The fine lithium particles deposited in the nonaqueous electrolyte containing HF showed no significant distribution of surface potential. This means that the highly uniform surface film was formed on these lithium particles. Therefore, it can be said the surface modification of lithium surface with HF leads to the formation of uniform surface films on lithium deposits. On the other hand, prominent distribution of surface potential was observed for the lithium metal after dissolution and deposition cycles in the electrolyte containing HF and the dendritic lithium at the initial stage of growth. Consequently, these results confirm that the inhomogeneity of the surface film on lithium metal promotes the formation of dendritic lithium.
Introduction Rechargeable lithium metal battery is very attractive for an energy storage because of the highest energy density of lithium metal in theory.1 However, the reversibility of the lithium metal anode is very low for a practical use. The low reversibility is caused by both high reactivity of lithium metal with electrolytes and a morphological change of lithium.2-4 From many electrochemical and spectroscopic analyses, it has been found that the morphological problem (a dendrite formation) is caused by a concentrated current that is produced by a low uniformity of a surface film on lithium metal.5-10 However, the relationship between the uniformity of the surface film and the morphology of lithium deposit has not been clear, yet. Several years ago, we found that an addition of a small amount of HF into nonaqueous electrolytes was effective in suppressing the dendrite formation of lithium. This effect is realized by the thin and compact surface film consisting of LiF/Li2O bi-layer which is formed by the addition of HF.11-13 Such a surface film is uniform enough to avoid the concentrated current.12 By the way, this discussion is supported by indirect proofs obtained from X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electrochemical analyses. However, any direct proofs for the high uniformity of the surface film modified by HF have not been obtained, yet. So, the confirmation of the HF effect has to be obtained by direct analyses such as special microscopic observation for topography and physical/chemical condition. Scanning probe microscopes (SPMs) have got much attention as a new tool for surface analyses since the scanning tunnel electron microscope (STM) was developed in 1982.14 SPMs provide two-dimensional distribution (image) for various physi* Corresponding author: Tel: -81-277-30-1352. FAX: +81-277-301353. E-mail:
[email protected]. † Gunma University. ‡ Tokyo Metropolitan University. § Kansai University.
cal/chemical conditions of sample surfaces by a microprobe scanning on a sample surface.15,16 An atomic force microscope (AFM), which is one of the famous SPMs, gives a topographical image by a pull or repulsion force between atoms in a microprobe and sample surface. Moreover, it does not need electronic conductivity of the sample,17 so that the AFM became one of the most popular tools in a wide surface scientific field. In fact, there are already some AFM studies for lithium depositiondissolution.18-21 On the other hand, since in general AFM sometimes causes serious damage to soft samples by a scanning microprobe, a new type AFM called “Tapping Mode AFM (TMAFM)” was recently developed by Digital Instruments Co. in order to avoid such a problem. In the TMAFM, the microprobe is oscillated mechanically by a vibration of a bi-morph piezo device to attain an intermittent contact of the microprobe with a sample surface. The intermittent contact extremely minimizes such damages caused by a friction (the force of tip to the sample surface in the TMAFM is lower than 1 nN, while contact AFM is 50 nN in air or 1 nN in liquid.). So, TMAFM will be effective in the observation of a soft sample (for example, rough surface of lithium electrode after many dissolution-deposition cycles, which is accumulated by soft and fragile residual films) under ex-situ. In recent years, a new type SPM, which can measure surface potential (volta potential or work function), has been focused in a surface analysis for an electrically functional thin film. This SPM is called “Kelvin Probe Microscope”,22 “Scanning Maxwell Stress Microscope (SMM)”,23 or “Scanning Surface Potential Microscope (SSPM)”.24 In these SPMs, two-dimensional distribution of surface potential on a sample is displayed as an image. The surface potential distribution is not only derived from the contact potential difference between the probe and the sample but also is affected by dielectric moment of adsorbent or thin film on sample surface. This means that the surface potential is very sensitive to the surface chemical composition of the sample. So, the surface potential measurement can be
10.1021/jp002014x CCC: $20.00 © 2001 American Chemical Society Published on Web 12/12/2000
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expected as a very useful tool for a microscopic analysis in a surface chemistry. As discussed above,, the TMAFM and the surface potential microscopy (SPoM) seem to be also promising methods for microscopic analysis of the surface film on lithium metal. On the other hand, there are also some defects in the TMAFM/ SPoM as follows. (1) It takes a longer time to measure the TMAFM/SPoM images than a normal AFM image (one image in TMAFM/SPoM needs about 15 min while in contact AFM only about several minutes) because of the limitation for the time constant in the system. (2) The horizontal resolution in TMAFM/SPoM is lower (≈1 nm) than contact AFM (
