Article pubs.acs.org/JPCC
Rapid Atomic Li Surface Diffusion and Intercalation on Graphite: A Surface Science Study Lynn Mandeltort and John T. Yates, Jr.* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States ABSTRACT: The diffusion of dilute metallic lithium across the surface and into the bulk of atomically clean highly oriented pyrolytic graphite in ultrahigh vacuum is reported. Auger electron spectroscopy and a surface-dependent chemical reaction were utilized to monitor the coverage, oxidation state, and diffusion of Li. A very small diffusion activation energy (0.16 ± 0.02 eV; 15.4 ± 1.8 kJ/mol) is found for Li surface diffusion across the graphite basal plane. A model involving diffusion-rate-limited Li atom transport across the basal plane of graphite through step edge sites into the interior is proposed. These measurements indicate that the diffusion coefficient, D, for Li is very large (D(300 K) ≈ 5 × 10−6 cm2 s−1).
I. INTRODUCTION The thermally activated diffusion of metallic Li into graphite is of current fundamental importance in relation to the behavior of Li in lithium-ion batteries. 1,2 From a fundamental perspective, the role of step edges and the mechanism for diffusion into layered structures such as graphite have been only superficially explored. Experimental studies of Li diffusion are missing for well-defined graphite substrates under atomically clean conditions, where ultrahigh vacuum measurement techniques are employed. The experiments reported here involved vacuum-deposited Li atoms on atomically clean low defect density HOPG (highly oriented pyrolytic graphite). The Li atoms remain in a reduced condition during diffusion. There are many investigations describing the diffusion of Li and other alkali metals in graphite; for brevity, we have referenced only a fraction of the body of literature on the subject.1−11 These studies involve factors that obscure the fundamental issues related to understanding pure Li diffusion across the basal plane of atomically clean graphite, which is the goal of this experiment. The limitations of much of the literature are summarized below: 1. Poor maintenance of surface purity and poor control of the oxidation state of Li. 2. Measurements made under electrochemical conditions in the presence of electrical fields and electrolytes. 3. Measurements made with poor or uncharacterized structure quality graphite. 4. Measurements made at high Li coverage where Li−Li interactions influence energetics. 5. Focus on diffusion into graphite yielding values of the diffusion coefficient that are strongly influenced by the structure, defect density, and crystallite size of the graphite. These diffusion coefficients, while likely involving a contribution from lateral diffusion processes © XXXX American Chemical Society
along the graphite basal plane, are difficult to compare, and a very wide range of Li diffusion coefficients is reported from D = ∼10−6 to 10−10 cm2 s−1.2,6,8 Li intercalates into graphite via diffusion through graphene layer edge sites and occurs easily due to the small size of the Li atom compared with the separation of the graphene layers in graphite crystals.12 It is known that Li will also diffuse into the bulk through two C atom vacancy sites with high activation energies.1,11 It is also known from theoretical studies that Li diffusion across a graphene sheet is slightly dependent on diffusion direction and the electronic influence of different graphene edge structural motifs; Li surface diffusion activation energies in the range 0.2 to 0.4 eV have been calculated recently.10,13,14 Li does not form ordered structures upon adsorption on graphite15,16 and has been reported to form clusters on graphene at room temperature and high coverage.17 It is likely that clustering of Li atoms at graphene sheet edge sites occurs at low temperature, as has been seen for Cs on graphite at 300 K,18 where Cs clusters have been observed by STM to be localized at graphite edge defect sites as well as at vacancy defect sites (made by ion bombardment). To our knowledge, the experimentally determined energy barrier for the elementary step in metallic Li surface diffusion across the graphite basal plane has not been reported. We find a surface-diffusion activation energy of 0.16 ± 0.02 eV corresponding at 300 K to a high surface diffusion coefficient D (300 K) ≈ 5 × 10−6 cm2 s−1. Received: August 14, 2012 Revised: October 31, 2012
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dx.doi.org/10.1021/jp308101c | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
II. EXPERIMENTAL SECTION Experiments were conducted in an ultrahigh vacuum system pumped with an ion pump, a turbo pump, and a Ti sublimation pump to obtain an ultimate pressure of ∼3 × 10−11 Torr. The system contained a PHI CMA Auger spectrometer and a shielded, differentially pumped, line-of-sight quadrupole mass spectrometer (UTI-100C) with a 3 mm aperture located ∼3 mm from the sample surface during temperature-programmed desorption (TPD) measurements. High-quality HOPG (SPI Supplies, grade SPI-1) was cleaved in air using the Scotch tape method and mounted on a Ta support plate in which an indentation was carefully milled to match the HOPG sample dimensions (0.5 × 10 × 10 mm). The graphite was affixed to the plate across the corners with thin Ta strips and ceramic cement (Aremco 669) on the backside. The HOPG was reported to have an average crystallite size of >1 mm, corresponding to an edge site defect density of ∼109 C atoms cm−2 or
