Nanostructure-Mediated Phase Evolution in Lithiation/Delithiation of Co₃O₄

Nanostructured transition-metal oxides have been under intensive investigation for their tantalizing potential as anodes of next-generation lithium-ion batteries (LIBs). However, the exact mechanism for nanostructures to influence the LIB performance remains largely elusive. In this work, we discover the nanostructure-mediated lithiation mechanism in Co3O4 anodes using ex situ transmission electron microscopy (TEM) and X-ray diffractometry: while Co3O4 nanosheets exhibit a typical two-step conversion reaction (from Co3O4 to CoO and then to Co0), Co3O4 nanoarrays can go through a direct conversion from Co3O4 to Co0 at a high discharge rate. Such nanostructure-dependent lithiation can be rationalized by the slow lithiation kinetics intrinsic to Co3O4 nanoarrays, which at a high discharge rate may cause local accumulation of lithium to initiate a one-step Co3O4-to-Co0 conversion. Combined with the larger volume change observed in Co3O4 nanoarrays, the slow lithiation kinetics can lead to inhomogeneous expansion with large stress developed at the reaction front, which can eventually cause structure failure and irreversible capacity loss, as explicitly observed by in situ TEM as well as galvanostatic discharge-charge measurement. Our observation resolves the nanostructure-dependent lithiation mechanism of Co3O4 and provides important insights into the interplay among lithiation kinetics, phase evolution, and lithium-storage performance, which can be translated into electrode design strategies for next-generation LIBs.