Dealloying at high homologous temperature: Morphology diagrams

Dealloying under conditions of high homologous temperature, Th, (or high intrinsic diffusivity of the more electrochemically reactive component) is considerably more complicated than at low Th since solid-state mass transport is available to support this process. At low Th the only mechanism available for dealloying a solid is percolation dissolution, which results in a bicontinuous solid-void morphology for which nanoporous gold serves as the prototypical example. At high Th, there is a rich set of morphologies that can evolve depending on alloy composition and the imposed electrochemical conditions, including negative or void dendrites, Kirkendall voids and bi-continuous porous structures. We report on a study of morphology evolution upon delithiation of Li-Sn alloys, produced by the electrochemical lithiation of Sn sheets. Electrochemical titration and time of flight measurements were performed in order to determine the intrinsic diffusivity of Li, D_Li, as a function of alloy composition, which ranged from ∼5 x 10^-8 - 4 x 10^-12 cm²s^-1. The activation energy for D_Li was measured in the temperature range 30-140°C and found to be 37.4, 37.9 and 22.5 kJ/mole, respectively for the phases Li₂Sn₅, LiSn and Li₇Sn₃. Morphology evolution was studied under conditions of fixed dealloying potential and fixed current density and our results are summarized by the introduction of dealloying morphology diagrams that reveal the electrochemical conditions for the evolution of the various morphologies.