Visualizing the electrochemical reaction of ZnO nanoparticles with lithium by in situ TEM: two reaction modes are revealed.

The lithiation reaction of ZnO as an anode in a lithium-ion battery (LIB) is unclear. The electrochemical behavior of ZnO was investigated inside a transmission electron microscope (TEM) by constructing a nano-LIB using an individual ZnO/graphene sheet as the electrode. The lithiation reaction of ZnO/graphene was monitored by simultaneous determination of the structure with high-resolution TEM, electron diffraction pattern and electron energy-loss spectroscopy. Two kinds of reaction modes were revealed in terms of different reaction rates. One was the violent reaction mode, in which one particle can evolve into an aggregate of many nanoparticles within the Li2O matrix in 1-2 min. The other was the peaceful evolution mode, in which each ZnO nanoparticle evolves into a core-shell particle with multi-domains constituted of Zn and LiZn nanograins. Abnormally large Zn nanocrystals grow quickly in the violent reaction mode, which can suppress the formation of LiZn and impair the reversible capacity. Our observations give direct evidence and important insights for the lithiation mechanism of metal oxide anodes in LIBs.