High-Resolution TEM Observation of Sn Nanoparticles on SnO2 Nanotubes 1109


conversion reaction can be visualized well, compared with SnO2 nanowires which may exhibit larger volume changes. It has been demonstrated in the recent literature (Chang et al., 2017) that the conversion reaction of SnO2 inside the GLCs is similar to the conversion reaction pathway in an electro- chemical cell, and the conversion reaction of various metal oxides can be visualized inside the GLC. As demonstrated in a scanning electron microscopy image in Figure 2a, a hollow SnO2 NT consists of many SnO2 nanograins. The X-ray


diffraction (XRD) pattern (Fig. 2b) of the SnO2 NTs con- firms the typical cassiterite structure (JCPDS 41-1445). TEM-EDSmapping (Fig. 3) and EELS mapping (Fig. 4) were carried out on SnO2 within the GLC to confirm the presence of C, O, F, P, Sn, and Li both in and around the NT. Upon electron beam irradiation, chemical lithiation was initiated inside the GLC by the decomposition of LiPF6 into PF5 and LiF, which further dissociates into Li (Ghatak et al., 2012; Yuk et al., 2014). Electrons involved in the reaction were supplied from the electron beam so that the lithiation process can proceed inside the active material.


A series of conversion reactions of SnO2 to Sn at the


interface of SnO2 NTs are shown in Figure 5. Initially, before electrochemical lithiation, the crystal structure of a NT is tetragonal, as evidenced by the diffraction pattern (Fig. 5b). After the conversion reaction, both the morphological and phase transitions of the NTs occurred. The nanograins of the cNTs became larger and surficial layer was formed around theNTs, which are suggested to be the formation of a SEI layer arising from the decomposition of electrolytes, and the crystal structure of NTs was also transformed into β-Sn, following the conversion reaction path as shown in the following equation (Li, et al., 2010; Cheong et al., 2016):


SnO2 + 4Li+ + 4e- !2Li2O+ Sn: (1) As seen from equation (1), the conversion reaction


involves the reduction of SnO2 into Sn and the formation of a Li2O matrix. Based on the TEM image (Fig. 5c) and diffraction pattern in Figure 5d, the crystal structure of SnO2


Figure 2. a: Scanning electron microscopy image and (b) X-ray diffraction patterns of SnO2 nanotubes.


Figure 4. a: High angle annular dark field scanning transmission electron microscopy image of SnO2 nanotube and (b) electron energy loss spectroscopy mapping of Li in the red box in a.


Figure 3. High angle annular dark field scanning transmission electron microscopy image of SnO2 nanotube inside graphene liquid cell and transmission electron microscopy energy dispersive–X-ray spectroscopy mapping of C, O, F, P, and Sn.


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