1110 Jun Young Cheong et al. The growth and agglomeration dynamics of the Sn NPs


on the surface of SnO2 NTs are presented in Figure 7. At the initial stage at 70 s, Sn NPs were seen as-formed by the initial conversion reaction at the surface, in addition to the SEI layer formation by chemical lithiation, determined by EELS mapping (Fig. 8). The electron beam dosage was calculated as 8,987.52 e−/Å2


S from the equation (electron beam


Figure 5. a: Transmission electron microscopy (TEM) image and (b) corresponding selected area diffraction patterns (SAED) of SnO2 nanotube (NT). c: TEM image and (d) corresponding SAED of SnO2 NT after the conversion reaction.


Figure 6. a: Transmission electron microscopy (TEM) image and (b) selected area diffraction patterns (SAED) of SnO2 nanotubes (NTs) after the 1st cycle, and (c) TEM image and (d) correspond- ing SAED of SnO2 NTs after the 50th cycle.


completely transformed into that of Sn after the conversion reaction. TEM images and diffraction patterns of SnO2 NT after the 1st cycle and 50th cycle are shown in Figure 6. Except for the presence of some super P carbon black that was originally mixed together with SnO2, it can be suggested that agglomeration of SnNPs leads to structural degradation, where larger Sn NPs are formed after the 50th cycle. Nevertheless, the initial stage of the conversion reaction, where the conversion reaction takes place on the surface of SnO2, is subject to further investigation.


dosage=pixel intensity per count/(area ×exposure time)) based on the previous literature (Chen et al., 2013). The value is slightly smaller than that in the previous literature on SEI layer formation (Cheong et al., 2016), which allows sufficient decomposition of electrolytes and lithiation, but is safe from e-beam damage that can hinder the in situ observation. Through further analysis using EELS spectra (Fig. 8c), it has been confirmed that the SEI layer was formed, having a peak at 61 and 69 eV, in accordance with the previous literature (Cheong et al., 2016; Chang et al., 2017). Formation of Li2O was also confirmed by EELS spectra (Fig. 9), in accordance with equation (1). With time lapse (at 85 s), more nucleated Sn NPs were seen hopping around randomly at the surface. By the time most of the Sn NPs were nucleated at the inter- face, agglomeration of Sn NPs started to take place. Initially, the agglomerated Sn NPs maintained a spherical shape during this process. Images from 85 s to 200 s clearly suggest that the agglomeration started at the surface of the SnO2 NTs. In previously reported literature (Sellers et al., 2010), β-Sn has the lowest surface energy on {100} plane. Agglom- eration by coalescence is observed on the plane that has the lowest surface energy in metal NPs such as Pt and Au (Yuk et al., 2012, 2013). It is likely that the coalescence of Sn NPs also occurred in a same manner. From 200 to 443 s, approaching the completion of the agglomeration process, larger Sn NPs were formed in direct contact with the elec- trolyte on the surface of the SnO2 NTs. Relatively large Sn NPs with some overlapping regions were present at 443 s. The width and length of the agglomerated Sn NPs were ~28 and 13.5 nm, respectively, on average. Based on this obser- vation, this agglomeration process can be schematically illustrated as shown in Figure 7b. During the final stabiliza- tion process, further agglomeration and merging took place while the NP size increased. This led to the formation of larger Sn NPs on the SnO2 surface. High resolution (HR)- TEM image of these Sn NPs are shown in Figure 7c. The structure maintained a β-Sn phase as confirmed by fast Fourier transform (FFT) patterns. In order to quantify agglomeration dynamics of the Sn NPs, both their numbers and sizes with respect to time were further calculated (Fig. 7d). The number increased until 80 s and steadily decreased thereafter, while the size of Sn NPs steadily increased. The decrease in the overall number of Sn NPs and increase of the size of NPs can be attributed to the agglomeration of NPs, as many particles merge together and become larger particles. At the same time, the size of Sn NPs steadily increased right from the beginning. To confirm whether the in situ TEM observation that we have taken is a common phenomenon, repeated in situ TEM analysis was also performed, showing again the formation of SEI layer by chemical lithiation along


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