Lithiation of ZnO nanowires studied by in-situ transmission electron microscopy and theoretical analysis

Transition-metal oxides constitute an important family of high-capacity anodes for Li-ion batteries. ZnO is a model material due to the high theoretical capacity and its representative reaction mechanism upon lithiation. We investigate the structural evolution, mechanical degradation, and stress-regulated electrochemical reactions of ZnO nanowires during the first lithiation through coordinated in-situ transmission electron microcopy experiments, continuum theories, and first-principles computation. Lithiation induces a field of stress in ZnO nanowires. The stress field mediates the electrochemical reaction and breaks the planar solid-state reaction front into a curved interface. The tensile stress in the lithiated shell causes surface fracture in the basal plane of nanowires. The compressive stress in the unlithiated core retards local reactions and results in an uneven lithiation on a given basal plane. We also observe that metallic Zn nanoparticles aggregate in the amorphous matrix of the reaction products. At a critical size, Zn nanoparticles impede the propagation of the reaction front due to the thermodynamically unfavorable lithiation reaction. The results provide fundamental perspectives on the chemomechanical behaviors of oxides for the next-generation Li-ion batteries. 2015 Elsevier Ltd. All rights reserved.