Lithium Diffusion Pathways in Graphitic Carbon Anodes
Graphitic carbon is widely used as an anode material in lithium-ion batteries. For high-power applications such as hybrid electric vehicles, however, prolonged cycling at high rates can damage a graphite anode and lead to plating of lithium metal, thereby decreasing the lifetime and capacity of the battery. Elucidating lithium diffusion pathways in graphite provides insight into why these anodes are limited by modest charge/discharge rates. The Persson and Kostecki Groups, in collaboration with other BATT investigators, have quantified lithium-ion diffusivity as a function of transport direction in graphite anodes.[1] Electrochemical experiments combined with first-principles calculations indicate that lithium diffusion in graphite is several orders of magnitude faster in the direction parallel, as opposed to perpendicular, to the graphene plane. These results provide guidelines for designing graphite anodes with preferential orientation for higher rate capability, which translates to faster charging batteries.
Highly oriented pyrolytic graphite (HOPG) was used in a carefully controlled experimental setup to determine lithium diffusivity as a function of transport direction. The HOPG consisted of single-crystalline graphene domains that were virtually parallel (angular spread of less than 1°). By using two configurations of this HOPG membrane, lithium-ion diffusion coefficients in the directions perpendicular and parallel to the graphene planes were estimated to be 8.7 × 10−12 cm2 s−1 and 4.4 × 10−6 cm2 s−1, respectively (Figure 1). Computational results using first-principles calculations (without assuming a diffusion mechanism) verified the high intralayer lithium diffusivity in bulk graphite (∼10−7 cm2 s−1).
While interlayer transport is expected to be slower due to the more obstructed pathway for lithium ions along grain boundaries, the difference in diffusivities had not been quantified until now. Literature reports on lithium-ion diffusivities for composite graphite electrodes have varied from 10−6 to 10−16 cm2 s−1. This work reconciles the wide range of lithium diffusivities by deconvoluting the variables associated with composite electrode studies and showing a dependence on the size of graphene domains and their orientation relative to the movement of lithium ions.
[1] K. Persson, V.A. Sethuraman, L.J. Hardwick, Y. Hinuma, Y.S. Meng, A. van der Ven, V. Srinivasan, R. Kostecki, G. Ceder. J. Phys. Chem. Lett. 2010, 1, 1176-1180.