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Tunnel structure manganese dioxide (MnO2) has attracted much attention due to its potential applications in heterogeneous catalysis, chemical adsorption, molecular sieves and ion exchange. At the same time, because of its large tunnel structure, the material also has potential in the field of charge storage. However, the rich tunnel heterogeneity of Todorokite-type manganese oxide hinders the accurate understanding of the energy storage mechanism of this multi-type material. Recently, the research team of Xu Feng and Sun Litao of Southeast University, together with the research team of Peking University and Nanjing University of Science and Technology, initially obtained uniform tunnel structure MnO2 nanorods through repeated research on preparation methods, and with the help of PicoFemto®In situ sample rod in the electron mirror to build a battery model, combined with electron diffraction, high-resolution imaging, electron energy loss spectrum, DFT calculation and phase field simulation at the nanoscale to study the nanorod lithium and lithium behavior, and in-depth analysis of lithium storage mechanism. The results were published in Nano Energy under the title "Atomic-level tunnel engineering of todorokite MnO2 for precise evaluation of lithium storage mechanisms by in situ transmission electron microscopy. (https://doi.org/10.1016/j.nanoen. 2019.06.036,Nano Energy 63 (2019) 103840)

Figure 1 Structural identification and resolution of tunnel-structured MnO2 nanorods using HAADF-STEM.

Figure 2 The lithiation behavior of τ-MnO2 was studied in situ.

Using PicoFemto®STM-TEM Holder, the researchers fixed τ-MnO2 nanorods and Li @ Li2O on half copper mesh and tungsten tip respectively as the two poles of the battery and aligned them. After applying a voltage to the battery, combined with clear, high-resolution in situ photos, the researchers found two different Li + transport pathways. First, propagation along the longitudinal direction of the nanorods resulted in 28.78% radial expansion. Second, the expansion due to lithiation creates a close lateral contact point between the two nanorods, allowing them to create a new lateral transport path. This new lateral lithium transport path allows the second nanorod to also exhibit the same radial expansion as the first nanorod. At the same time, observation of Li + transport between the nanorods in the local contact region can clearly indicate that Li + diffuses along the a-c plane within the lithiated nanorods. In addition, the researchers also observed the structural evolution during the in-situ delithiation of τ-MnO2.

Figure 3 In-situ study of the structural evolution during in-situ lithiation.