Modeling Overcharge at Lithiated-Graphite Porous Electrodes: Plating and Dissolution of Lithium

Abstract

In recent work, we developed a set of equations that can be used to treat Li plating and subsequent electro-dissolution, and we analyzed how the equation system behaved for a particle of graphite, a fundamental unit of the negative electrode in lithium ion cells. In this work, we apply the same governing equations to the more realistic setting of a porous electrode consisting of many graphite particles. We propose a Li activity model wherein the activity of plated lithium differs from the activity of bulk Li in a thin layer (on the order of monolayers) at the interface with the graphite substrate and transitions smoothly to the activity of bulk Li at greater distances from the graphite interface. Since it is unclear at this point exactly how large this interfacial layer should be, we also show how to treat the limiting case, in which the interfacial layer thickness goes to zero and the activity of any deposited Li is that of bulk Li, which significantly complicates the porous-electrode analysis. We find the two approaches (i.e., finite thickness on the order of monlayers and zero thickness for the interfacial layer) yield very similar results.