1108 Jun Young Cheong et al.


(Sigma Aldrich) and 1.25 g ofN,N-dimethylformamide (DMF; Sigma Aldrich) were added and stirred for another 4h.When the electrospinning solution became transparent, electrospin- ning was carried out with the following conditions: applied voltage of 15 kV, flow rate of 20 µL/min, the distance between the current collector and the tip of the needle of 12 cm.When Sn precursor/PVP composite NFs were fabricated, they were put inside the box furnace which initiates the calcination process. NFs were calcined at 600°C for 1h and 700°C for 1h.


GLC Preparation


Multilayer graphene used for GLC was synthesized by chemical vapor deposition (CVD) on Cu foil (99.8%; Alfa Aesar, Haverhill, MA, USA). Cu foil was treated with 20% phosphoric acid (85%; Jinsei) to remove surface impurities and oxides and placed inside the CVD chamber. The tempe- rature was elevated to 1,050°C for 30min while 200 sccm ofH2 gaswas turned on.Then, the chamberwas stabilized for 60min. For the next 60 min, 20 sccm of CH4 gas was turned on as a carbon source for the graphene. Similar to our previous work (Yuk et al., 2016), 3–15 layers of graphene were synthesized. The synthesized graphene was transferred to holey carbon Au grid (Quantifoil, 300 mesh, 2-μm hole size) by dissolving Cu foil in 0.2M Ammonium persulfate [(NH4)2S2O8,Sigma Aldrich] for 6 h. Next, GLC was fabricated by dropping the mixture of SnO2 NTs and electrolyte [1.3M of lithium hexafluorophosphate (LiPF6) in a solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volumetric ration of 3:7 with 10wt% of fluoroethylene carbonate (FEC)] onto one graphene-transferred grid and placing another grid onto the top. Spontaneously, liquid was encapsulated between two graphene layers as it was dried in Ar atmosphere.


In situ TEM Analysis


A JEOL JEM 3010 TEM (JEOL, Tokyo, Japan) at 300kV accelerating voltage was used for real time observation. Energy dispersive–X-ray spectroscopy (EDS) elemental mapping analysis was performed using a JEOL JEM 2100F (JEOL) at 200 kV. Li electron energy loss spectroscopy (EELS) mapping was performed with a Titan ETEM G2 microscope (FEI) and GIF Quantum 966 at 300 kV.


Electrochemical Cell Testing and Ex Situ TEM Analysis Ex situ TEM images were taken on the electrode material


that was composed of SnO2 NTs, super P carbon black and poly(acrylic acid)/sodium carboxymethyl cellulose (50/50wt%/wt%; Aldrich) binder at a weight ratio of 80:10:10. They were slurry cast and dried in vacuum for 150°C for 2 h. The loading amount of active material was about 2 mg/cm2. It was assembled into the 2032 coin-type half cells, which consisted of lithium metal as a counter electrode, a separator (Celgard 2325), and an electrolyte consisting of 1.3M of LiPF6 in EC/DEC (3/7 v/v) with 10wt% of FEC, with the same electrolyte as used for GLC. The cell was cycled at a current density of 50mA/g between 0.01 and 3V using battery testing device (WBCS4000, Wonatech). During the discharge process, cycling was stop- ped at about 0.95V and the electrode was taken out from the half cell. It was washed with dimethyl carbonate and dried for 10 min in the glove box and analyzed later with TEM.


RESULTS AND DISCUSSION


GLC was employed as an in situ TEM observation platform to observe the dynamics ofNPs onNT in high magnification. For the preparation of GLCs, graphene was synthesized by CVD on a Cu foil and subsequently transferred on to TEM grids (Fig. 1). The electrolytes and materials used inside the GLC are identical to those used in our previous studies (Cheong et al., 2016). The role of graphene in the GLCs is mainly to effectively encapsulate the liquid inside the GLCs, as using graphene results in minimal damage from e-beam irradiation along with decreased interaction between the substrate and the sample due to its inert properties (Yuk et al., 2012; Chen et al., 2013). The SnO2 NTs were synthe- sized by electrospinning and subsequent calcination, during which Ostwald ripening took place to form hollow SnO2 NTs (Jang et al., 2015). Key advantages of SnO2NTs include short ion diffusion length and minimal volume expansion (Jang et al., 2015; Cheong et al., 2016), making them ideal to observe different interfacial reaction dynamics of NPs that form a NT under TEM. Structural integrity is maintained during the lithiation-induced phase transitions, so the growth of Sn crystals nucleated from SnO2 as a result of the


Figure 1. a: Digital micrograph images of graphene synthesized by chemical vapor deposition on Cu foil. b: Scanning electron microscopy image of transferred graphene on transmission electron microscopy grid, and (c) corresponding selected area diffraction patterns confirming graphene.


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