LiC₆ Phase Mobility in Highly Oriented Pyrolytic Graphite

Determining the solid diffusion coefficient of lithium in graphite anode active materials for lithium-ion batteries is challenging due to the complex intercalation dynamics, the structural complexity, particle size/shape distribution, and the superposition of liquid electrolyte transport processes in the pores of an electrode. To minimize these influences, we examined the lithiation of highly oriented pyrolytic graphite (HOPG) disk electrodes, with the basal planes stacked normal to the disk thickness (0.5 mm). At first, the radially progressing lithiation of the HOPG disks was followed in situ by top-view optical monitoring of the golden LiC₆ phase. However, post-mortem analysis of split HOPG disks revealed that HOPG crystal surface imperfections lead to the artefact that the apparent LiC₆ phase progression determined from top-view images does not correspond to that in the bulk of the disks. Thus, the LiC₆ phase front position can only be quantified through post-mortem analysis of split HOPG disks. The intercalation time and temperature dependence of the LiC₆ phase progression can be reasonably well described by a Fickian diffusion model in cylindrical geometry, yielding an apparent diffusion coefficient of the LiC₆ phase front of D₀ = 0.6-1.0 × 10⁻¹³ m² s⁻¹ at 25 °C, with an activation energy of E_a = 35.4-39.0 kJ mol⁻¹ (between 10-55 °C).