Supporting Information for

Supporting Information for A Few-Layer SnS 2 /Reduced Graphene Oxide Sandwich Hybrid for Efficient Sodium Storage Fengzhang Tu,, Xin Xu, Pengzi Wang, Ling Si, Xiaosi Zhou,*, and Jianchun Bao*, Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China College of Chemistry and Materials Science, Longyan University, Longyan 364012, P. R. China *(X.Z.) E-mail: zhouxiaosi@njnu.edu.cn. Tel./Fax: +86-25-85891027 *(J.B.) E-mail: baojianchun@njnu.edu.cn. Tel./Fax: +86-25-85891936 S1

Table S1. Theoretical capacities and average discharge/charge voltages of sodium-ion battery anode materials. Materials Theoretical capacity (mah g 1 ) Average discharge voltage (V vs. Na/Na + ) Average charge voltage (V vs. Na/Na + ) C 1 <360 0.05 0.5 0.05 0.5 Ge 2 369 (NaGe) 0.05 0.2 0.55 0.75 Sn 3 847 (Na 15 Sn 4 ) 0.1 0.4 0.2 0.6 Pb 4 485 (Na 15 Pb 4 ) 0.05 0.15 0.1 0.25 Sb 5 660 (Na 3 Sb) 0.3 0.6 0.6 0.9 P 6 2596 (Na 3 P) 0.2 0.4 0.5 0.7 Na 7 1166 0 0 8 SnS 2 843 (Na 2 S 2 + Na 15 Sn 4 ) 0.5 0.8 1.2 1.4 S2

Figure S1. SEM image of SnS 2 particles. S3

Figure S2. Electrochemical activities of SnS 2 particles within different voltage ranges. (a, c, e) The discharge charge curves of the first three cycles for the SnS 2 particle electrodes at a current density of 50 ma g 1, between 1.7 2.5 V, 0.8 2.5 V, and 0.01 2.5 V, respectively. All the electrodes were produced by coating a slurry of SnS 2 particles, conductive additive (RGO), and carboxymethyl cellulose sodium (CMC) binder (63:27:10 wt %) on a copper current collector. (b, d, f) Discharge capacities for the first 100 galvanostatic cycles of the SnS 2 particle electrodes in various voltage windows at the current density of 50 ma g 1. S4

Table S2. Calculation of volume expansion occurring from SnS 2 to Na 2 S 2 and Na 15 Sn 4. SnS 2 particles Na 2 S 2 Na 15 Sn 4 Lattice type Hexagonal Hexagonal Cubic a (Å) 3.65 7.64 13.1 b (Å) 3.65 7.64 13.1 c (Å) 5.90 5.40 13.1 α (degrees) 90 90 90 β (degrees) 90 90 90 γ (degrees) 120 120 90 Cell volume (Å 3 ) 68.07 272.97 2248.09 Density (g cm 3 ) 4.46 2.01 2.38 Molar mass (g mol 1 ) 182.81 110.11 819.69 Volume expansion = [M mol (Na 2 S 2 )/ρ(na 2 S 2 ) + 1/4*M mol (Na 15 Sn 4 )/ρ(na 15 Sn 4 )]*ρ(sns 2 )/M mol (SnS 2 ) = 344% S5

Figure S3. Ex situ TEM, SAED and HRTEM images of SnS 2 particles collected before sodiation and after discharging to various voltages. (a c) Before cycling. (d f) Fully charged state after 100 cycles between 1.7 2.5 V. (g i) Fully charged state after 1 cycle between 0.01 2.5 V. When the SnS 2 particle electrode is operated between 1.7 2.5 V, the layered structure can be maintained with the d-spacing of (001) plane expanding owing to some sodium ions trapped in the SnS 2 particles. This suggests that sodiation can be applied as a novel approach to synthesize SnS 2 nanosheets. When it is run between 0.01 2.5 V, crystalline SnS 2 particles transform into new structures after one cycle, because SnS 2 experiences the conversion reaction of SnS 2 + 2Na = Na 2 S 2 + Sn and the alloy reaction of 4Sn + 15Na = Na 15 Sn 4. S6

Figure S4. Digital photographs of aqueous suspension of few-layer SnS 2 nanosheets, graphene oxide (GO) and a mixture of SnS 2 nanosheets and GO. Digital photographs of aqueous dispersions of SnS 2 nanosheets (1 mg ml 1 ), GO (1 mg ml 1 ), and a mixture gained by blending the dispersions of SnS 2 nanosheets and GO at a volume ratio of 5:4, which results in the FL-SnS 2 /RGO hybrid with a SnS 2 /RGO mass ratio of 2.42:1. Red light beams incident from the side to illustrate the Tyndall effect. S7

Figure S5. XPS survey scan of FL-SnS 2 /RGO. S8

Optimized SnS 2 content in the FL-SnS 2 /RGO hybrid. The few-layer SnS 2 nanosheets were assembled with GO to produce a sandwiched hybrid precursor. The SnS 2 /GO molar ratio can be computed based on monolayer SnS 2 nanosheets and GO nanosheets as well as the following equation: where n 1 (SnS 2 ) of 1 and n 1 (GO) of 2 show the molecule number of SnS 2 and CO 0.44 (Figure S6) in one cell of single-layer SnS 2 and GO, respectively. S(SnS 2 ) and S(GO) represent the area of monolayer SnS 2 and GO, respectively. S 1 (SnS 2 ) and S 1 (GO) are 11.54 Å 2 and 5.22 Å 2, which imply the area of one cell in single-layer SnS 2 and GO, respectively. Taking into account that the real samples are composed of nanosheets less than or equal to 8 layers, there may be a difference between calculation and experiment, thus we have synthesized three sandwiched SnS 2 /GO hybrid precursors with different area ratios to compare the cycling performances of the final product of FL-SnS 2 /RGO. Note that we can calculate the contents of SnS 2 in various FL-SnS 2 /RGO hybrids via the molar ratios of SnS 2 /GO (Table S3) and the mass retentions of SnS 2 nanosheets and GO after annealing (Table S4). Table S3. Calculation of various SnS 2 contents. S(SnS 2 )/S(GO) n(sns 2 )/n(go) m(sns 2 )/m(go) m(sns 2 )/m(rgo) SnS 2 (wt %) 0.34 0.078 0.75 1.42 58.6 0.57 0.13 1.28 2.42 70.8 1.41 0.32 3.07 5.80 85.3 Table S4. Mass retentions of few-layer SnS 2 nanosheets and graphene oxide after annealing at 300 o C for 2 h with a heating rate of 5 o C min 1 under argon atmosphere. SnS 2 nanosheets GO Mass retention (wt %) a 94.9 50.3 a Average of the three test results. S9

Figure S6. XPS analysis of graphene oxide. From the XPS results, the molar ratio of O/C is calculated to be approximately 0.44. Thus, graphene oxide can be abbreviated as CO 0.44. S10

Figure S7. TGA curves of the FL-SnS 2 /RGO hybrids under air flow with a temperature ramp of 10 o C min 1 from room temperature to 800 o C. The weight percentages of SnS 2 in the FL-SnS 2 /RGO hybrids can also be determined by TGA, based on the thorough weight loss of RGO combustion and the partial weight loss from the transformation of SnS 2 into SnO 2. According to the following equation, the contents of SnS 2 in the hybrids are calculated to be about 59.1, 71.5, and 84.7 wt %, respectively, which are close to the results demonstrated in Table S3. S11

Figure S8. Nyquist plots of the FL-SnS 2 /RGO anode. Electrochemical impedance spectroscopy (EIS) was measured by applying a sine wave with an amplitude of 10.0 mv over the frequency range of 10 khz to 100 mhz. The same cell was cycled for 1 3 cycles at a current density of 100 ma g 1. The impedance profiles for the 1st, 2nd and 3rd cycles are nearly overlapped. This suggests that the solid electrolyte interphase (SEI) layer is stable, which benefits from the FEC additive. S12

Table S5. Specific surface areas of SnS 2 particles, graphite, SnS 2 nanosheets, RGO and the FL-SnS 2 /RGO hybrid. SnS 2 particles graphite SnS 2 nanosheets RGO FL-SnS 2 /RGO Specific surface area (m 2 g 1 ) 7.3 0.7 99.0 63.7 75.3 As displayed in Table S5 and Figure S9, the specific surface areas of SnS 2 nanosheets and RGO are much larger than that of SnS 2 particles and graphite, respectively. The considerable volume variation of SnS 2 nanosheets during sodiation/desodiation causes an unstable SEI film, resulting in the poor first-cycle Coulombic efficiency. In the case of RGO, the high specific surface area is responsible for its limited first-cycle Coulombic efficiency. Hypothetically, there is no sandwich structure formed between SnS 2 nanosheets and RGO after the assembling process, the specific surface area of the FL-SnS 2 /RGO hybrid would be around 88.7 m 2 g 1 due to the existence of 70.8 wt % SnS 2 nanosheets. Actually, the FL-SnS 2 /RGO hybrid has a lower specific surface area of 75.3 m 2 g 1. It means that the SnS 2 nanosheets sandwiched between RGO nanosheets is helpful to diminish the specific surface area, giving rise to a higher first-cycle Coulombic efficiency. Figure S9. N 2 adsorption desorption isotherms of RGO, SnS 2 nanosheets, and the FL-SnS 2 /RGO hybrid. S13

Figure S10. Cyclic voltammetry curves of the FL-SnS 2 /RGO anode at a scanning rate of 0.1 mv s 1. In the first cathodic scan, two distinct peaks are observed at about 1.46 1.81 and 0.15 0.96 V, respectively. The former peak is associated with the intercalation of sodium ions into SnS 2 nanosheets. The latter is attributed to both the conversion and alloy reactions because it is difficult to distinguish the conversion and alloy peaks during the first discharging process. 9 It is worth mentioning that this peak also contains the contribution from SEI generation since its intensity considerably decreases in the ensuing cycles. In the anodic scan, two broad peaks below 1.49 V (~0.78 and 0.91 1.49 V) are ascribed to the multi-step dealloying reaction of Na x Sn, while the prominent peak at ~1.63 V corresponds to the reversible conversion reaction from Sn to SnS 2. S14

Figure S11. STEM and EDX elemental mapping images of the FL-SnS 2 /RGO electrode material after 5 cycles. S15

Figure S12. Electrochemical properties of the RGO anode for sodium-ion batteries. (a) The galvanostatic discharge charge profiles of the initial three cycles for the RGO electrode between 0.01 and 2.5 V at a current density of 100 ma g 1. (b) Disharge charge capacities for the first 100 cycles of the RGO electrode in the voltage range of 0.01 2.5 V at the current density of 100 ma g 1. S16

Table S6. A survey of electrochemical performances of SnS 2 -based anodes in sodium-ion batteries. Active material Ratio of active material (wt %) Mass loading (mg cm 2 ) Voltage range (V vs. Na/Na + ) Current density (ma g 1 ) FL-SnS 2 /RGO 90 ~0.8 0.01 2.5 100 GF SnS NH 100 1.0 1.2 0.01 3 100 C@SnS/SnO 2 @Gr 80 1.0 ± 0.2 0.01 2 30 SnS 2 NC/EDA RGO 80 1 ± 0.15 0.01 3 200 SnS 2 nanosheets 70 Not reported 0.005 3.0 100 SnS 2 /G-20. 75 1.0 1.5 0.01 2.5 200 SnS 2 /rgo 75 Not reported 0.005 3.0 200 SnS 2 -rgo 80 2 0.01 2.5 200 SnS 2 /C 80 Not reported 0.005 2.5 50 SnS 2 -RGO 80 0.8 1 0.01 2.5 200 SnS@graphene 80 1.0 ± 0.2 0.01 3 30 GO-SnS 2-300 60 Not reported 0 2 50 SnS 2 /graphene 70 ~1.0 0.01 3 20 Cycling stability (mah g 1 ) 826 (after 100 cycles) 1010 (after 200 cycles) 713 (after 70 cycles) 680 (after 100 cycles) 647 (after 50 cycles) 619 (after 100 cycles) 509 (after 300 cycles) 627 (after 100 cycles) 570 (after 100 cycles) 628 (after 100 cycles) 940 (after 50 cycles) ~600 (after 50 cycles) 670 (after 60 cycles) Reference This work 9 10 11 12 13 14 8 15 16 17 18 19 S17

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