LiC6 Phase Mobility in Highly Oriented Pyrolytic Graphite (HOPG)

Abstract

Graphite, serving as the lithium host structure, is still the main component of the anode active material in today's commercial lithium-ion batteries [1]. Yet, the determination of solid diffusion coefficient of lithium in graphite has so far proved difficult, which is reflected by the observation that the values published in the literature vary over several orders of magnitude, which is likely due to the structural and phase-related complexity of the lithium intercalation kinetics in graphite, as well as the heterogeneity of the samples studied [2, 3].To address the latter point, our study aims to examine the lithiation of a highly oriented pyrolytic carbon (HOPG) disk, in which the basal planes are oriented normal to the disk height. In a first approach, we followed the lithiation of an HOPG disk by in-situ optical monitoring of the radial movement of the golden LiC6 phase in top-view images of the HOPG crystal (see left panel of Figure 1a), as was done previously by Guo et al. [3]. This was compared to a subsequent post-mortem analysis by splitting the HOPG disk along its height (see right panel of Figure 1a), clearly illustrating that the apparent LiC6 phase movement suggested by top-view images is mostly an artefact caused by HOPG crystal imperfections and that the LiC6 phase front position can only be quantified from the post-mortem analysis of split crystals. Based on the latter analysis approach, we show that the LiC6 phase ring thickness for different intercalation times and temperatures can be well described using a model based on Fickian diffusion in cylindrical geometry by defining a concentration ratio c/c0 for the appearance of the golden phase between 0.7-0.8, as shown in Fig. 1b [4]. Within this model framework, an apparent diffusion coefficient of the LiC6 phase of D0=0.7-1.4×10-13 [m2/s] at 25°C can be determined, with an activation energy of Ea=36.5-37.1 [kJ/mol].