Real-time observation of phase transition from layered to spinel phase under electron beam irradiation

The irreversible phase transition from a layered to a spinel structure, typically observed in an intercalation-type electrode, has been recognized as one of the main causes of capacitance fading, structural instability, and thermal instability in Li-ion batteries. However, observing the phase transition in a real environment in real-time is still challenging. Here, cobalt hydroxide, Co(OH) 2 was irradiated with an electron beam in a transmission electron microscope, and the phase transition was intensively investigated using in-situ high resolution transition electron microscopy. Both the alpha and beta-Co(OH) 2 phases changed into a spinel Co 3 O 4 phase, and formed nanograins. However, the alpha Co(OH) 2 showed a faster phase transition and dramatic volume shrinkage during the phase transition, which led to layer bending/discontinuity and the development of cracks. Our results provide a detailed explanation of the mechanism behind the deterioration of the layered structure and the emergence of defects during the phase transition, providing crucial information for designing the next generation battery.