Nanovoid formation and annihilation in gallium nanodroplets under lithiation-delithiation cycling.

The irreversible chemomechanical degradation is a critical issue in the development of high-capacity electrode materials for the next-generation lithium (Li)-ion batteries. Here we report the self-healing behavior of gallium nanodroplets (GaNDs) under electrochemical cycling at room temperature, observed with in situ transmission electron microscopy (TEM). During lithiation, the GaNDs underwent a liquid-to-solid phase transition, forming a crystalline phase (LixGa) with ~160% volume expansion. Owing to the uneven Li flow during lithiation, the fully lithiated GaNDs exhibited highly distorted morphologies. Upon delithiation, the reverse phase transition occurred, accompanied with the nucleation and growth of a nanosized void. After the GaNDs were fully delithiated, the nanovoid gradually annihilated. Our analysis, along with phase field modeling and experimental measurements of the void growth and annihilation, provides mechanistic insights into the void formation and annihilation mechanism. The GaNDs may function as an effective healing agent in durable composite electrodes for high-performance Li-ion batteries, wherein active components, such as Si, are susceptible to fracture.