Supporting Information for

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1 Supporting Information for Improved Sodium-Ion Storage Performance of Ultrasmall Iron Selenide Nanoparticles Feipeng Zhao, 1 Sida Shen, 1 Liang Cheng, 1 Lu Ma, 2 Junhua Zhou, 1 Hualin Ye, 1 Na Han, 1 Tianpin Wu, 2 Yanguang Li, 1 * and Jun Lu 3 * 1. Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon- Based Functional Materials and Devices, Soochow University, Suzhou , China 2. Advanced Photon Sources, X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439, USA 3. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA Corresponding authors yanguang@suda.edu.cn; junlu@anl.gov Experimental Methods Preparation of ultrasmall FeSe 2 NPs. In a typical synthesis, 15 ml of oleylamine (OM) and 10 ml 1-octadecene (ODE) were first added into a 50 ml three-necked flask, and heated to 120 C to remove moisture under the N 2 protection. 1 mmol of FeCl 2 4H 2 O was then added into the solution, and formed a clear solution after being magnetically stirred for 30 min. Subsequently, 2 mmol of Se powder dissolved in 4 ml of OM was injected into the solution, followed by stirring for another 10 min. The temperature of the reaction solution was rapidly raised to 150 C and maintained at this temperature for 30 min. After the reaction was concluded and solution cooled down to room temperature, the colloidal product was precipitated out by adding excess ethanol. It was collected by centrifugation at 8000 rpm and stored in hexane for later use. To prepare FeSe 2 NPs adsorbed on carbon black (FeSe 2 NPs/CB) for the battery application, a calculated amount of Super P carbon 1

2 black powder was added to the hexane solution of FeSe 2 NPs. The solid product was then precipitated out by adding excess ethanol, washed repeatedly with ethanol and water, and lyophilized. Finally, the product was annealed at 300 o C under Ar to remove organic residue. The weight ratio of FeSe 2 NPs to carbon black in the final product was controlled to be 7:2. Synthesis of bm-fese 2 : 0.26 g of Fe powder and 0.74 g of Se powder were mixed, and ball-milled together with ~20 g of stainless steel balls under the protection of Ar at 400 rpm for 24 h. Material Characterizations. X-ray diffraction (XRD) was performed on PAN alytical X-ray diffractometer at a scan rate of 0.05 o /s. Scanning electron microscopy (SEM) images were taken from Zeiss scanning electron microscope. Transmission electron microscopy (TEM) images were recorded on a FEI Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kv. X-ray photoelectron spectroscopy (XPS) was carried out using ULTRA DLD spectrometer. Thermogravimetric analysis (TGA) was conducted on Mettler Toledo TGA/DSC1 Thermal Analyzer in air. The temperature was programmed from 25 to 900 C at a rate of 10 C/min. NP Size Analysis. The average NP size was estimated using the Debye-Scherrer formula: d = γ θ where K was the dimensionless shape factor and taken to be 0.9, γ (= nm) was the X-ray wavelength, B (in radian) was the width at half the maximum intensity (FWHM); θ was the Bragg angle. Taking the (111) diffraction peak at 2θ~34.8 o from Figure 1b as an example, its FWHM was measured to be 0.52 o or rad, and d = size of FeSe 2 NPs was ~16 nm. γ =.. =~16 nm. So the average θ. (. ) Electrochemical Measurements. To prepare FeSe 2 NPs working electrode, FeSe 2 NPs/C was blended with 10 wt% carboxymethyl cellulose (CMC) binder with a 9:1 weight ratio and dispersed in water. The slurry was then uniformly cast onto a Cu foil and vacuum dried at 70 o C overnight. The 2

3 areal loading of FeSe 2 NPs was ~1 mg/cm 2. To prepare bm-fese 2 working electrodes, bm-fese 2 was blended with Super P carbon black and carboxymethyl cellulose (CMC) in 7:2:1 weight ratio, dispersed in H 2 O solution and magnetically stirred for 12 h. Thus formed slurry was then blade cast onto Cu foil to achieve an active material loading density of ~1 mg/cm 2, and dried at 70 C in a vacuum oven overnight. To prepare Na 3 V 2 (PO 4 ) 3 (NVP) working electrodes, NVP powder was blended with Super P carbon black and polyvinylidene fluoride (PVDF) binder in a weight ratio of 7:2:1 and dispersed in N-methylpyrrolidone (NMP). The slurry was cast onto an Al foil and vacuum dried at 120 C overnight. The areal loading of NVP material was ~7 mg/cm 2. Half-cell measurements were conducted in 2032-type coin cells, which were assembled in an Ar-filled glove box by pairing the working electrode with a piece of Na disk, separated by a glass fiber membrane. The electrolyte was 1 M NaPF 6 in diethylene glycol dimethyl ether (DEGDME). Galvanostatic charge/discharge tests were conducted in a voltage range of 0.01~2.4 V for FeSe 2 NPs and 2.5~4.0 V for NVP at different current rates on a MTI Battery Testing System. Cyclic voltammetry (CV) curves were collected on CHI 660E potentiostat at a scan rate of 0.05 mv/s. To evaluate the NVP//FeSe 2 full cell performance, the NVP cathode and the FeSe 2 anode were paired up in a weight ratio of 7:1 in 2032 type coin cells with the DEGDME electrolyte. Galvanostatic charge/discharge experiments were carried out between 0.15 and 3.0 V. 3

4 Figure S1. (a) Fe 2p and (b) Se 3d XPS spectra of FeSe 2 NPs. Figure S2. TEM images of FeSe 2 NPs/CB at different magnifications. 4

5 Figure S3. TGA curve of FeSe 2 NPs/CB. Figure S4. Electrochemical performance of FeSe 2 NPs/CB in the 1 M NaClO 4 /EC/DEC electrolyte. (a) CV curves for the first three cycles at a scan rate of 0.05 mv/s. (b) Cycling stability and corresponding Coulombic efficiency of FeSe 2 NPs/CB at 100 ma/g and 800 ma/g. 5

6 Figure S5. Structural characterizations of bm-fese 2. (a) SEM image of Fe powder as the starting precursor. (b) SEM image of Se powder as the starting precursor. (c) SEM image of bm-fese 2 prepared from ball-milling Fe and Se powders. (d) XRD pattern of bm-fese 2 in comparison with those of the starting precursors. 6

7 Figure S6. Electrochemical impedance spectroscopy (EIS) analysis of (a) FeSe 2 NPs/CB and (b) bm-fese 2 before and after the cycling. Figure S7. Structural characterizations and electrochemical measurements of NVP. (a) XRD and (b) SEM image of NVP. (c) Charge and discharge curves of NVP at 30 ma/g. (d) Cycling stability and corresponding Coulombic efficiency of NVP at 30 ma/g. 7