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.