A phosphorene graphene hybrid material as a high-capacity anode for sodium-ion batteries

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1 SUPPLEMENTARY INFORMATION DOI: /NNANO A phosphorene graphene hybrid material as a high-capacity anode for sodium-ion batteries Jie Sun, Hyun-Wook Lee, Mauro Pasta, Hongtao Yuan, Guangyuan Zheng, Yongming Sun, Yuzhang Li, Yi Cui* Part I. Experimental procedures 1.1 Materials 1.2 Equipment and techniques 1.3 In situ TEM Supplementary Movie 1 Sodiation of a black phosphorous along x-axis. Supplementary Figure 1 Multiple measures of expansion rate along x- and y-axis, respectively, by in-situ TEM. Part II. Supplementary Tables and Figures 2.1 Summaries of anode materials for sodium ion batteries. Supplementary Table 1. Specific capacities and average charge/discharge potentials of SIB anode materials. 2.2 Black phosphorus Calculation of volume and direction expansion taking place from black phosphorus to Na 3 P. Supplementary Figure 2 Ex-situ XRD patterns of black phosphorus taken before charging and after charging down to different voltages. Supplementary Figure 3 Electrochemical performance of black phosphorus in different voltage ranges. Supplementary Figure 4 The XRD spectra of black phosphorus anode after cycling. NATURE NANOTECHNOLOGY 1

2 2.3 Phosphorene Supplementary Figure 5 Raman of mono-layer and few-layer phosphorene. Supplementary Figure 6 SEM image of phosphorene nanobelt. Supplementary Figure 7 TEM image of phosphorene nanobelt. 2.4 Phosphorene/graphene hybrid Optimization of the phosphorus content of graphene/phosphorene. Supplementary Table 2. Calculation of various phosphorus contents. Supplementary Table 3. The specific surface area of graphene, phosphorene and phosphorene/graphene hybrid. Supplementary Figure 8 TEM and elemental dot-mapping images of phosphorene/graphene hybrid. Supplementary Figure 9 Nyquist plots of the phosphorene/graphene anode. Supplementary Figure 10 The cyclic voltammograms of the phosphorene/graphene anode. Supplementary Figure 11 TEM image of the phosphorene/graphene hybrid after one cycle Supplementary Figure 12 Electrochemical characterization of phosphorene/graphene anode with high mass loading. 2.5 Graphene Supplementary Figure 13 Electrochemical characterization of graphene anode. 2

3 Part I. Experimental procedures 1.1 Materials The bulk black phosphorus used in the majority of experiments was purchased from Smart Elements. It was ground down to a powder and sieved through a 0.5 mm mesh sieve to remove the larger particles. The graphite powder was purchased from Sigma-Aldrich (Product Number ). N-Methyl-2-pyrrolidone (NMP) and anhydrous ethanol were obtained from Sigma-Aldrich and used as received. 1.2 Equipment and techniques Raman spectroscopy was performed on a Renishaw RM2000 confocal Raman spectrometer with a 514 nm excitation laser (laser spot size of 0.5 µm) operated at a low power level (~2 mw) in order to avoid any heating effects. Samples were sealed in transparent glass for Raman measurements. SEM images were taken using a FEI XL30 Sirion SEM (accelerating voltage 5 kv). Elemental analysis was performed using energy-dispersive X-ray spectroscopy equipped in the SEM. High-resolution TEM (HRTEM) images and energy-filtered TEM maps were performed on a FEI Tecnai G2 F20 TEM installed with a CCD camera operating at an acceleration voltage of 200 kv. Energy-dispersive X-ray spectroscopy (EDS) measurements were collected on a FEI Tecnai G2 F20 TEM operating at an acceleration voltage of 200 kv and equipped with a DX-4 analyzer (EDAX). The sample was diluted and prepared by pipetting a few mls of the dispersion onto the holey carbon grid (400 mesh) purchased 3

4 from Agar Scientific. HRTEM measurement was performed on phosphorene and phosphorene/graphene sandwich structures using FEI Titan environmental TEM operated at 80 kv, which was equipped with image Cs aberration corrector. The HRTEM image in Figure 2c (Main text) was taken at the over-focus 10 nm, which allows an optimal imaging condition with the bright atom contrast. XPS was performed on PHI 5000 VersaProbe, using an Al Kα (λ = 0.83 nm, hυ = ev) X-ray source operated at 2 kv and 20 ma. Atomic force microscopy (AFM) images were recorded on a Digital Instruments NanoScope IIIa atomic force microscope by tapping mode. For AFM measurement, the phosphorene suspension was delivered dropwise onto a 90 nm SiO 2 -capped Si substrate surface which was rinsed briefly with water and blow-dried with air. The Brunner Emmet Teller (BET) specific surface area (SSA) was measured with an ASAP2020M (Micromeritics Instrument Corp.) 1.3. In situ TEM In situ TEM experiments. The in situ electrochemical characterization was carried out in an FEI Titan environmental TEM at an acceleration voltage of 80 kv. Nanofactory Instruments Dual-Probe STM-TEM in situ sample holder, as the schematic shown below, was used to demonstrate the sodiation reaction of black phosphorus material. The experimental set-up of the holder was assembled inside Ar glove box to protect unexpected oxidation reaction of the phosphorous and Na electrodes. During insertion of the holder into the TEM, the counter electrode was exposed to air for about 2 s generating a thin Na 2 O layer acting as a solid electrolyte. A relative bias of -3.5 V was induced between the two electrodes, which caused Na ions to be transferred to phosphorous electrode through the solid electrolyte. 4

5 During the observation, the electron beam was spread out to avoid beam effects. Supplementary Movie 1 Sodiation of a black phosphorous along x-axis. When the sodium oxide acting as a solid electrolyte touched the y-edge, sodium can transport into the interlayers along x-axis. A 92% expansion along the y-axis was observed, while the dimension along the x-axis remained nearly unchanged. The anisotropic expansion along the y-axes is the result of the weaker P-P bonding along the y-axis, as well as the preferentially easier Na ion diffusion along the x-axis. The movie is played at 8x speed. Supplementary Figure 1 Multiple measures of expansion rate along x- and y-axis, respectively, by in-situ TEM. 3% and 92% expansion along x- and y-axis, respectively. 5

6 Part II. Supplementary Tables and Figures 2.1 Summaries of anode materials for sodium ion batteries. Supplementary Table 1. Specific capacities and average charge/discharge potentials of sodium ion battery anode materials. Materials Theoretical capacity (ma h g -1 ) Average charge potential (V vs. Na/Na + ) Average discharge potential (V vs. Na/Na + ) C [11] < Ge [13] 369 (NaGe) Sn [15] 846 (Na 15 Sn 4 ) Pb [17] 485 (Na 15 Pb 4 ) Sb [18] 660 (Na 3 Sb) Na [6] P [19] 2596 (Na 3 P) Capacities calculated based on the mass of active material before sodiation. 6

7 2.2 Black phosphorus Calculation of volume and direction expansion taking place from black phosphorus to Na 3 P. volume expansion=m mol (Na 3 P)*ρ(black phosphorus)/(m mol (black phosphorus)* ρ(na 3 P))=499% 7

8 Supplementary Figure 2 Ex-situ XRD patterns of black phosphorus taken before charging and after charging down to different voltages. a, In wide range. b, Amplification of (002) diffraction peaks. c, Amplification of (004) diffraction peaks. 8

9 Supplementary Figure 3 Electrochemical performance of black phosphorus in different voltage ranges. a-c, The charge discharge profiles of black phosphorus electrodes at the first two cycles with a current density of 0.05 A g -1, between V, V and V, respectively. All the electrodes were prepared by coating a slurry of active material (black phosphorus), conductive additive (graphene) and polyvinylidene fluoride (PVDF) binder (43.5:46.5:10 wt.%) on a copper foil collector. d, Reversible desodiation capacity for the first 30 galvanostatic cycles of the black phosphorus electrodes between different voltage ranges at a current density of 0.05 A g -1. 9

10 Supplementary Figure 4 The XRD spectra of black P anode after cycling. a, Before cycling. b, Discharged state after 30 cycles between V. c, Discharged state after 1 cycle between V. When it was cycled between V, the layered structure can be preserved with the (002) peak shifting to lower angle, due to the exfoliation of black P by repeatedly intercalating Na ions. It indicates that sodiation can be used as a new method to prepare phosphorene. When it was cycled between V, crystalline black P transformed to amorphous after one cycle, because that it undergoes a alloy reaction of P+3Na=Na 3 P. Although the black P crystal was not preserved after 1 cycle, the advantages of black phosphorus as a starting material are following: 1) Among the vairious phosphorus allotropes (e.g. white, red, black), only black phosphorus can be easily tailored to nanosheet (phosphorene); 2) Phosphorene assembled with graphene to fabricate the sandwiched structure, which can overcome the anisotropic expansion in the first charging process; 3) During the following cycles, the expansion was buffered and confined between the two-dimensional galleries of graphene layers. 10

11 2.3 Phosphorene Supplementary Figure 5 Raman of mono-layer and few-layer phosphorene. a, Raman spectra of phosphorene with different number of layers. b, Atomic displacements of Raman active modes. 11

12 Supplementary Figure 6 SEM image of phosphorene nanobelt. Supplementary Figure 7 TEM image of phosphorene nanobelt. 12

13 2.4 Phosphorene/graphene hybrid Optimization of the P content of graphene/phosphorene. The atom-thick phosphorene was assembled with graphene to form a layered composite as an anode material for sodium batteries. Considering that the elastic buffer space between the phosphorene should be reserved for the y axial directional expansion to double size, the carbon/phosphorus molar ratio (named as C/P) was calculated assuming monolayer graphene and phosphorene, as following, C / P = n1(graphene) S(graphene) / S1(graphene) n1( phosphorene) S( phosphorene) / S1( phosphorene) Where n 1 (graphene) of 2 and n 1 (phosphorene) of 4 indicate the atomic number of single layer in one cell of graphite and black phosphorus, respectively. S(graphene) and S(phosphorene) indicate the area of monolayer graphene and phosphorene, respectively. S 1 (graphene) and S 1 (phosphorene) are Å 2 and 14.5 Å 2, which indicate the area of single layer in one cell of graphite and black phosphorus, respectively. Considering that the real sample is made of nanosheets of 1 to 5 layers, there may be a discrepancy between experiments and calculation, so that we have prepared a series of layered phosphorene/graphene hybrids with various C/P ratios to compare their cycling performance. Supplementary Table 2. Calculation of various phosphorus contents. S(graphene) /S(phosphoren) C/P ratio m(graphene) /m(phosphorene) phosphorus content (wt%)

14 Supplementary Table 3. Specific surface areas (SSA) of graphene, phosphorene and phosphorene/graphene hybrid. graphene phosphorene phosphorene/graphene SSA (m 2 /g) As shown in Supplementary Table 3, the specific surface area (SSA) of graphene or phosphorene is much higher than graphite and black P, respectively. The graphene alone has a large SSA, thus explaining the low first cycle Coulombic efficiency. In the case of phospohorene, the massive volume expansion results in a poor solid-electrolyte interphase (SEI) formation thus explaining the limited first cycle Coulombic efficiency. Hypothetically, there s no interaction between phosphorene and graphene for assembling, the SSA of the phosphorene/graphene composite would be around the average value of phosphorene and graphene (650 m 2 /g) because of the presence of 48.3%wt P. In fact, the phosphorene/graphene composite has a lower SSA of 508 m 2 /g. It indicates that the phosphorene samdwiched between graphene layers is helpful to decrease the surface area, contributing to a better high first-cycle Coulombic efficiency. 14

15 Supplementary Figure 8 TEM and elemental dot-mapping images of phosphorene/graphene hybrid containing 48.3 wt% P. a, Bright-field TEM image of phosphorene/graphene composite (scale bar: 5 µm). b, Dark-field TEM image of phosphorene/graphene composite (scale bar: 5 µm). c, The corresponding selected area EELS elemental dot-mapping images of C and P elements. 15

16 Supplementary Figure 9 Nyquist plots of the phosphorene/graphene anode at 1.5 V vs Na/Na + in the discharged state for different cycles. For the electrochemical impedance spectroscopy (EIS), the same cell was run for 0 3 cycles at a current density of 100mA/g, respectively. The impedance curves for the 1st, 2nd and 3th cycles are almost coincident. It indicates that the SEI is stable, attributing to FEC additive [1]. 16

17 Supplementary Figure 10 The cyclic voltammograms of the phosphorene/graphene and red P/graphene anodes at a scanning rate of 0.1 mv s 1. For comparison of the electrochemical performance with phosphorene/graphene hybrid, the red phosphorus/graphene mixture was prepared by grinding red P and graphene powder with the same C/P ratio of 2.78/1. For the first chargeing in both of red phosphorus and phosphorene, the wide peak at around 0.8 V is generated from the formation of SEI in EC/DEC/FEC electrolyte. The reduction and oxidation peaks of red phosphorus are ~0.2 V and ~0.65 V, respectively. Whereas, the pair of redox peaks of phosphorene are ~0.45 V and ~0.58 V, respectively. The hysteresis (ΔEp) of phosphorene is much lower than that of red P. Furthermore, the redox peaks in phosphorene are reversible, whereas the redox peaks of red phosphorus reduced for the 2nd cycle. 17

18 Supplementary Figure 11 TEM image of the phosphorene/graphene hybrid in desodiated state after one cycle (scale bar: 5 µm). 18

19 Supplementary Figure 12 Electrochemical characterization of phosphorene/graphene anode with a mass loading of 2 mg cm -2. a, Charging-discharging curves at different current densities. b, Specific capacities as a function of cycle numbers and its Coulombic efficiency. 19

20 2.5 Graphene Supplementary Figure 13 Electrochemical characterization of graphene anode for sodium batteries. a, The galvanostatic discharge charge curves of graphene electrode at the first two cycles between V with a current density of 0.05 A g -1. b, Reversible charge-discharge capacities for the first 30 galvanostatic cycles of the graphene electrode between V at a current density of 0.05 A g -1. Reference [1] Yabuuchi, N., Matsuura, Y., Ishikawa, T., Kuze, S., Son, J. Y., Cui, Y. T., Oji, H., Komaba, S. Phosphorus Electrodes in Sodium Cells: Small Volume Expansion by Sodiation and the Surface-Stabilization Mechanism in Aprotic Solvent. ChemElectroChem 1, (2014). 20