Surface-oxidized, freeze-cast cobalt foams: Microstructure, mechanical properties and electrochemical performance

Abstract

Cobalt with anisotropic open porosity is fabricated by directional solidification of aqueous slurries of nanometric Co3O4 powder where ice dendrites push powders into aligned interdendritic spaces, followed by ice sublimation, reduction of the oxide to metallic Co powders, and sintering of these Co powders into parallel lamellae. As the Co3O4 powder slurry fraction decreases (from 10 to 4 vol%), Co lamellae width in the final foam also decreases (from 93 to 8 μm) while foam porosity increases (from 66 to 85%). A drop in solidification temperature (from −10 to −50 °C) decreases porosity (from 77 to 63%) and lamellae width (from 11 to 5 μm) at a constant 8 vol% slurry fraction. Finally, with increasing sintering time (for −10 °C solidification temperature and 8% slurry fraction), Co foam porosity decreases (from 77 to 68%) and lamella width strongly increases (from 10 to 59 μm), consistent with sintering-induced coalescence of lamellae. The Co foams exhibit high strength but relatively low stiffness as compared to simple theoretical models, consistent with internal Co lamella buckling. A uniform Co oxide layer is grown by oxidation to create an active coating on the Co lamellae useful for lithium-ion storage. A coin-cell test carried out on the oxidized Co foam demonstrates a capacity (1283 mAhg−1) almost twice that of a control oxidized Co foil anode, owing to its considerably larger surface area. Finite-element analysis is used to compute stresses and plastic strain evolutions during the lithiation process to understand the effect of oxide layer thickness and roughness, and micropores within the Co lamellae.