Double-Network Nanostructured Hydrogel-Derived. Ultrafine Sn Fe Alloy in 3D Carbon Framework for

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1 Supporting Information Double-Network Nanostructured Hydrogel-Derived Ultrafine Sn Fe Alloy in 3D Carbon Framework for Enhanced Lithium Storage Hongxia Shi,, Zhiwei Fang,, Xiao Zhang, Feng Li, Yawen Tang, Yiming Zhou, Ping Wu,*,, and Guihua Yu*, Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing , China Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States H.S. and Z.F. contributed equally to this work. * * 1

2 Experimental Methods Synthesis of the Sn Fe/C G double-network hydrogel: Solution A was aqueous solution containing 0.4 M SnCl 4 and 2% glutaraldehyde. Solution B was aqueous solution containing 0.2 M K 4 Fe(CN) 6 and 10 mg ml -1 carboxymethyl chitosan. The Sn Fe/C G double-network hydrogel was synthesized by mixing freshly-made solutions A and B with a volume ratio of 1:1 at 20 o C. For comparison, Sn Fe cyanogel was obtained by mixing SnCl 4 aqueous solution with K 4 Fe(CN) 6 aqueous solution. And C G hydrogel was obtained by mixing glutaraldehyde aqueous solution with carboxymethyl chitosan aqueous solution. Synthesis of the Sn Fe@C framework electrode: The Sn Fe/C G double-network hydrogel was freeze dried, and the obtained double-network aerogel was pyrolyzed at 600 C under flowing 5%H 2 -Ar gas mixture for 3 h. The product was washed and dried in a vacuum oven, yielding the final Sn Fe@C framework electrode. For comparison, Sn Fe C composite and carbon-only control samples were also prepared by pyrolyzing single-network Sn Fe cyanogel and C G aerogel, respectively, instead of double-network aerogel by keeping the other conditions unchanged. Characterization: The morphology, composition, and structure of these products were examined by X-ray powder diffraction (XRD, Rigaku D/max 2500/PC), scanning electron microscopy (SEM, Hitachi S-5500), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010F, 200 kv) coupled with an energy-dispersive X-ray spectrometer (EDX, Thermo Fisher Scientific). The Fourier transform infrared (FTIR) spectra were performed on a Bruker Tensor 27 spectrometer. Nitrogen adsorption/desorption tests were examined at 77 K using a Micromeritics ASAP 2050 analyzer, and the surface area and pore size were calculated using 2

3 Brunauer Emmett Teller (BET) and Barrett Joyner Halenda (BJH) methods, respectively. Thermogravimetric analysis (TGA) was carried out using a NETZSCH STA thermal analyzer with a heating rate of 10 o C min -1 in air. Electrochemical measurement: The Sn Fe@C framework was dispersed in N-methyl-2- pyrrolidene (NMP) with conductive material (Super P carbon black) and binder (polyvinyldifluoride, PVDF) in a weight ratio of 80:10:10. The slurry was coated on a copper foil current collector and dried at 120 C for 12 h in a vacuum oven. Electrochemical tests were carried out using 2025-type coin cells (can size: 20 mm in diameter and 2.5 mm in thickness) assembled in an Ar-filled glove box (IL-2GB, Innovative Technology). The counter electrode was lithium foil, and the electrolyte was 1 M LiPF 6 in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 in volume). Cycling tests were measured on a LANHE CT2001A battery tester in the voltage range of V, and cyclic voltammetry (CV) measurements were conducted on a CHI 660B electrochemical workstation in the potential window of 0-2 V at a scan rate of 0.2 mv s 1. Electrochemical impedance spectroscopy (EIS) tests were carried out on a CHI 660B electrochemical workstation over the frequency range from 100 khz to 0.01 Hz. 3

4 Supporting Figures Figure S1. (a) Photograph of the Sn Fe cyanogel. (b) FTIR spectrum of the Sn Fe cyanogel (curve b) in comparison with K 4 Fe(CN) 6 (curve a). 4

5 Figure S2. (a) Photograph of the C G hydrogel. (b) FTIR spectrum of the C G aerogel (curve b) in comparison with chitosan (curve a). 5

6 Figure S3. SEM image (a) and its magnified version (b) of the Sn Fe/C G double-network aerogel. Inset in (a) is the corresponding SEM image with a lower magnification. 6

7 Figure S4. (a) TEM image and (b) HRTEM image of the Sn Fe cyanogel. Inset in (a) is the corresponding TEM image with a lower magnification. 7

8 Figure S5. (a) TEM image and (b) HRTEM image of the C G aerogel. Inset in (a) is the corresponding TEM image with a lower magnification. 8

9 Figure S6. XRD pattern of the Sn Fe/C G double-network aerogel. 9

10 Figure S7. TEM and HRTEM images of the Sn Fe/C G double-network aerogel. (b-d) are the HRTEM images of regions I, II, and III shown in (a), respectively. 10

11 Figure S8. EDX spectrum of the Sn Fe/C G double-network aerogel after removing KCl byproduct through immersing the corresponding hydrogel in water. 11

12 Figure S9. (a) SEM and (b) TEM images of the Sn framework. 12

13 Figure S10. STEM image and its elemental maps of Sn (red), Fe (green), and C (yellow) of the Sn Fe@C framework in a low magnification. 13

14 Figure S11. (a) TGA curve of the Sn framework. (b) XRD pattern of the Sn framework after TGA. As seen from TGA curve (Figure S11a), the weight variation of the Sn framework can be mainly attributed to the oxidation of Sn Fe alloy and carbon components during TGA tests. The oxidation of Sn Fe alloy leads to a weight increase, whereas the removal of carbon component leads to a weight decrease of the product. Additionally, the observed crystalline phases from the oxidation product after TGA can be indexed to SnO 2 (JCPDS no ) and Fe 2 O 3 (JCPDS no ) (Figure S11b). Therefore, the carbon content in the Sn Fe@C framework can be calculated to be 45.4 wt%, according to the following equation based on the Sn/Fe ratio (2:1) and oxidation products (SnO 2 and Fe 2 O 3 ) after TGA. 14

15 Figure S12. TEM image (a) and its magnified version (b) of the Sn Fe C composite prepared by pyrolyzing single-network Sn Fe cyanogel. Inset in (a) is the corresponding TEM image with a lower magnification. 15

16 Figure S13. TEM image (a) and its magnified version (b) of the carbon-only control sample prepared by pyrolyzing single-network C G aerogel. Inset in (a) is the corresponding TEM image with a lower magnification. 16

17 Figure S14. The initial three CV curves (0.2 mv s -1 ) of the Sn Fe C composite (a) and carbononly sample (b). 17

18 Figure S15. The equivalent circuit model for the fitting of impedance plots. This equivalent circuit model includes an ohmic resistance (R Ω ), double-layer capacitance (C DL ), charge transfer resistance (R CT ), and Warburg impedance (Z W ), respectively. 18

19 Table S1 Comparison of the average particle sizes of Sn M alloy particles between the Sn Fe@C framework and previous Sn M@C ternary materials. Sn M@C ternary materials Alloy sizes (nm) Ref Sn Fe@C framework 2.7 This work G Sn Co composite D Cu 6 Sn composite 24 2 Ni 3 Sn nanosheets Fe Sn/C nanoparticles Cu 6 Sn nanospheres Co Sn pgn electrode Sn Co G composite Ni 3 Sn composite

20 Table S2 The fitting results of charge transfer resistance (R CT ) for the Sn Fe@C framework in comparison with Sn Fe C composite and carbon-only control samples from EIS tests. Sn Fe@C framework Sn Fe C composite Carbon-only sample R CT (Ω) Figure 4f shows the Nyquist plots of the Sn Fe@C framework in comparison with Sn Fe C composite and carbon-only control samples in fresh cells. As observed, the semicircle diameter for the Sn Fe@C framework in high frequency region is much smaller than those of Sn Fe C composite and carbon-only sample. Moreover, the impedance plots are fitted using an equivalent circuit model (Figure S15). The value of R CT for the Sn Fe@C framework is 27.7 Ω, much lower than those of Sn Fe C composite (227.6 Ω) and carbon-only sample (119.8 Ω). The low charge-transfer resistance of the Sn Fe@C framework ensures its high rate capability toward lithium storage. 20

21 Table S3 Comparison of the lithium storage performance between the Sn framework and previous Sn M alloy-based anodes. Anode materials Cycling stability (mah g -1 ) Rate capability (mah g -1 ) Ref Sn Fe@C framework G Sn Co composite 3D Cu 6 Sn composite Ni 3 Sn nanosheets 594 at 0.1 A g -1 (100 cycles) 516 at 0.1 A g -1 (500 cycles) 571 at 0.07 A g -1 (60 cycles) 470 at 0.7 A g -1 (60 cycles) 366 at 1 A g -1 (200 cycles) 585 at 0.11 A g -1 (100 cycles) 350 at 0.57 A g -1 (180 cycles) 491 at 1 A g at 10 A g -1 This work NA at 1 A g at 10 A g at 0.57 A g at 2.85 A g -1 3 Fe Sn/C nanoparticles 373 at 0.1 A g -1 (50 cycles) NA 4 Cu 6 Sn nanospheres 518 at 0.06 A g -1 (200 cycles) 275 at 1.2 A g at 3 A g -1 5 Co Sn pgn electrode 595 at 1 A g -1 (150 cycles) 430 at 10 A g -1 6 Sn Co G composite 560 at 0.5 A g -1 (60 cycles) 483 at 0.8 A g -1 7 Ni 3 Sn composite 554 at 0.1 A g -1 (200 cycles) at 0.8 A g at 1.6 A g -1 8 Sn Fe Co alloy composite 510 at 0.05 A g -1 (50 cycles) 443 at 1 A g -1 9 meso-co 0.3 Sn 0.7 material 530 at 0.07 A g -1 (50 cycles) ~400 at 1.3 A g Sn Fe C composite 444 at 0.06 A g -1 (170 cycles) 430 at 0.6 A g -1 (140 cycles) NA 11 21

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