In situ transmission electron microscopy (TEM) as a fast-growing technique has attracted tremendous attention in diverse scientific research because it can acquire dynamic information and allow for mechanistic understanding of various physical and chemical processes and materials systems . Specifically, the advancement of in situ TEM in chemically reactive environments has enabled the direct real-time observation of electrochemical reactions in electrode materials for lithium ion batteries . Previous studies have found different types of electrochemical reactions with lithium via various mechanisms such as intercalation, alloying, and conversion, which confirmed the lithiation processes following the thermodynamic reaction pathways. In addition to that, we would like to utilize in situ TEM imaging and spectroscopy approaches to build direct correlations between microstructure and electrochemistry on atomic to nanoscale and reveal the kinetics of phase transformations during the electrochemical reactions.
We primarily focus on transmission metal oxides and sulfides with a large number of openings in their crystal framework, such as the spinel (Fe3O4) and 2D layered (CuS) structures [3, 4], to accommodate the uptake of guest Li ions. Although it is generally believed that these materials should follow the conversion reaction, we found the intermediate lithiated phases resulted from the intercalation reactions at the beginning; and more importantly, such phase transformations are sensitive to the reaction kinetics that are common in realistic battery coin cells. With further performance tests of coin-cells at various C-rates, we are able to explicitly establish the relationship that crosslinks the structure evolution, electrochemical properties, and the reaction pathways on the atomic level, and address the importance of the effects of kinetics and dimensionality on the phase transformation. Our findings provide insights into understanding phase transformation mechanisms in spinel and layered structures, and also show implications for improving performance in future design of battery electrodes.
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