Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2016. Supporting Information for Adv. Mater., DOI: 10.1002/adma.201604015 High Performance Graphene/Ni 2 P Hybrid Anodes for Lithium and Sodium Storage through 3D Yolk Shell-Like Nanostructural Design Chao Wu, Peter Kopold, Peter A. van Aken, Joachim Maier, and Yan Yu*
Supporting Information High Performance Graphene/Ni 2 P Hybrid Anodes for Lithium and Sodium Storage through 3D Yolk-shell-like Nanostructural Design Chao Wu, Shuhe Hu, Peter Kopold, Peter A. van Aken, Joachim Maier, and Yan Yu * Shuhe Hu, Prof. Dr. Y. Yu CAS Key Laboratory of Materials for Energy Conversion Department of Materials Science and Engineering University of Science and Technology of China Hefei 230026, China E-mail: yanyumse@ustc.edu.cn Dr. C. Wu, P. Kopold, Prof. Dr. P. A. van Aken, Prof. Dr. J. Maier, and Prof. Dr. Y. Yu Max Planck Institute for Solid State Research, Heisenbergstr.1, Stuttgart 70569, Germany Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, China. E-mail: yanyumse@ustc.edu.cn EXPERIMENTAL SECTION Synthesis of NiNH 4 PO 4 H 2 O nanorods. In a typical synthesis, 5 mmol of Ni(NO 3 ) 2 H 2 O and 7 mmol of NaH 2 PO 4 were dissolved into 15 ml of deionized water. Then, 10 ml of ethylene glycol, 7.5 ml of NH 3 H 2 O and 5 mmol of Na 2 CO 3 were added into the above solution. The mixed solution was transferred into a Teflon-lined stainless steel autoclave for solvent-thermal reaction. After reaction for 15 h at 180 o C, the NiNH 4 PO 4 H 2 O nanorod precursor was obtained by washing and separation. Synthesis of Ni 2 P pgn. 30 ml of NiNH 4 PO 4 H 2 O nanorod precursor solution containing 0.34 g solids was mixed with 30 ml of graphene oxide (GO) solution. After dropwise adding 10 ml of aqueous solution (containing 10 mg hexadecyltrimethylammonium bromide) into the above mixed solution, the coagulum consisting of NiNH 4 PO 4 H 2 O nanorods and GO was observed, collected, and freeze-dried. After annealing at 650 o C for 2 h in Ar/H 2 (5 vol% H 2 ) with a heating rate of 2 o C min -1, the desired Ni 2 P GN product was achieved. The content of graphene is about 21 wt% by element analysis. Synthesis of Ni 2 P@carbon. 0.34 g of NiNH 4 PO 4 H 2 O nanorod precursor was mixed with glucose which serve as carbon source to form amorphous carbon layer on the Ni2P particles. After the mixture was annealed at 650 o C for 2 h in Ar/H 2 (5 vol% H 2 ), the desired of Ni 2 P@carbon was collected. The content of carbon is about 18 wt%. 1
Materials characterization. The morphology of the as-prepared samples was investigated by field-emission scanning electron microscopy (FE-SEM, Zeiss Gemini DSM 982) and transmission electron microscopy (TEM, JEOL, 200 kv). Nitrogen adsorption and desorption isotherm measurement were performed with a Quantachrome Adsorption Instrument. The crystal structures of the as-prepared samples were recorded by X-ray diffraction (XRD) (Philips) using Cu Ka radiation. Electrochemical measurement. The electrochemical measurements were carried out in a 2032- type coin-cell. For lithium storage testing, a metallic lithium film works as the counter electrode with 1 M LiPF 6 in the ethylenecarbonate/diethyl carbonate (1:1 v/v). For sodium storage testing, a metallic sodium film serves as the counter electrode with 1.0 M NaCF 3 SO 3 in diethyleneglycol dimethylether. The working electrodes were made by casting slurry containing active material, PVDF binder, and carbon black (super P) with a mass ratio of 8:1:1 on a Cu foil. The diameter of working electrodes is 10 mm. Note that all the capacity values of Ni 2 P pgn are calculated based on the mass of Ni 2 P nanocrystals and graphene. The galvanostatic charge-discharge tests were performed on a battery system (Neware BTS). Electrochemical impedance spectrum measurements were performed on an electrochemical workstation (VoltaLab 80) in the frequency range from 100 KHz to 0.1 Hz. 2
Figure S1. SEM image of Ni 2 P pgn at high magnification. 3
Figure S2. SEM images of Ni 2 P@carbon at (a) low magnification and (b) high magnification. Figure S3. (a) The size distribution of N 2 P nanoparticles for Ni 2 P pgn. (b) TEM image of Ni 2 P pgn at high magnification. 4
Figure S4. (a) Nitrogen adsorption/desorption isotherms and (b) the pore size distribution plots of Ni 2 P@carbon. Figure S5. Initial charge/discharge profiles of graphene at 0.1 A g -1. 5
Figure S6. Initial charge/discharge profiles of the Ni 2 P@carbon electrode at 0.1 A g -1. 6
Table S1. The comparison of Li-storage performance between Ni 2 P pgn and other phosphide electrode materials Name Charge capacity after cycling Capacity retention References Ni 2 P pgn Ni 2 P@carbon Graphene-supported Ni 2 P Graphene-wrapped Ni 2 P Carbon-coated Ni 2 P nanoparticles CoP hollow spheres Mesoporous FeP nanoparticles 511 mah g -1 after 250 cycles at 0.1 A g -1 93% after 250 cycles This work 218 mah g -1 after 120 cycles at 0.1A g - 1 76% after 120 cycles This work 449.9 ma h g -1 after 50 cycles at 54.2 ma g -1 78% after 50 cycles Ref.5 399 mah g -1 after 30 cycles at 500 ma g -1 85% after 30 cycles Ref.25 About 50 mah g-1 after 40 cycles at 27 ma g -1 About 33% after 40 cycles Ref.13 630 mah g -1 after 100 cycles at 178 mah g -1 83% after 100 cycles Ref.26 355 ma h g -1 after 30 cycles at 144 mag -1 91% after 30 cycles Ref.27 7
Figure S7. Impedance spectra of the Ni 2 P pgn and Ni 2 P@carbon electrodes. The inset is the equivalent circuit. R e is the electrolyte resistance, and C dl and R ct are the double-layer capacitance and charge-transfer resistance, respectively. Z w is the Warburg impedance related to the diffusion of lithium ions into the bulk electrodes. 8
Figure S8. Schematic diagrams of 3D yolk-shell nanostructure and common nanostructure, showing that 3D yolk-shell nanostructure provides highly efficient ion/electron channels around the active nanoparticles and realize a short storage time (τ) because of shortening the diffusion length (L) of Li into active particles (τ = L 2 /2D where, D is chemical diffusion coefficient). 9
Figure S9. SEM images at low magnification (a) and high magnification (b) for the Ni 2 P pgn electrodes after cycling. (c) Schematic diagram of cross-section view of 3D yolk-shell-like Ni 2 P pgn electrodes before and after electrochemical cycling. 10