Designed construction of Graphene and Iron Oxide Freestanding Electrode with Enhanced Flexible Energy Storage Performance

Transcription

1 Supporting Information Designed construction of Graphene and Iron Oxide Freestanding Electrode with Enhanced Flexible Energy Storage Performance Meng Li 1, Feng Pan 1, Eugene Shi Guang Choo 2, Yunbo Lv 1, Yu Chen 1 and Junmin Xue 1 * 1 Department of Materials Science and Engineering, National University of Singapore, Singapore, Carl Zeiss Pte. Ltd., Microscopy Business Group, Singapore, * Corresponding author: Tel: address: msexuejm@nus.edu.sg (Dr J.M. Xue) S-1

2 Experimental Chemicals Graphite flakes (2-15µm) were obtained from Alfa-Aesar. Sodium nitrate (NaNO 3 ), concentrated sulfuric acid (H 2 SO 4, 98%), potassium permanganate (KMnO 4 ) and hydrogen peroxide (H 2 O 2 ) were used as received. Iron chloride hexahydrate (FeCl 3 6H 2 O), ammonium phosphate monobasic (NH 4 H 2 PO 4 ), and N-methyl-2-pyrrolidone were purchased from Sigma-Aldrich. Porous graphene nano-powders were purchased from Graphene Supermarket. All chemicals were used as received without any further purification. Preparation of Graphene Oxide (GO) Graphene oxide was prepared by oxidation of natural flake graphite powder through modified Hummer s method 1. The typical synthesis can be found in our previous report 2. Preparation of graphene@iron oxide freestanding hybrid films To prepare graphene@iron oxide hybrid films (GFeF), a homogeneous suspension of graphene oxide and iron oxide nanoparticles should be carefully prepared. 25 ml of GO aqueous (1 mg/ml) was sonicated for 30 min to form a stable solution. Subsequently, 25 ml of iron oxide nanoparticle aqueous (1 mg/ml) was mixed with GO aqueous and sonicated for another half hour to form homogenous dispersion. This GO@iron oxide suspension was vacuum filtered through a mixed cellulose ester filter membrane (200 nm pore size) to obtain layered hybrid films. In the filtration assisted self-assembly process, mixed cellulose ester filter membrane S-2

3 (pore size of 200 nm) was used the primary filter layer and normal filter paper (pore size around 2 um) was placed in the bottom as secondary protective layer. Additionally, it is noted that the diameter of filter papers should be larger than that of suspensions container on the upper layer (details can be found in our previous work 3 ) in order to facilitate peeling off the films from the filter paper. In the first step, little amount of pure GO suspensions were dropped into suspension container to just cover filter membrane. The filtration process is further retarded due to the capillary pressure between GO sheets on the filter membrane. After few minutes, little amount of GO@iron oxide suspension was subsequently dropped into the container in the flowing steps, and the slow filtration process provide enough duration for iron oxide nanoparticles to be mono-dispersed and self-assemble in between GO layers. The whole process normally takes several hours. In order to peel off the hybrid film from filter membrane, another few hours standing should be kept after filtration. Finally, the hybrid films were cut into certain size and annealed to 300 o C for 1 hour under nitrogen protection in order to reduce graphene oxide. The thickness of the films can be controlled by adjusting the concentration or the volume of the aqueous suspension. All the samples mass were determined by weighting the films vacuum dried at 80 o C for 12 hours. The thickness of the films was measured with five independence slices and the average values were used to calculate the volume. S-3

4 Characterization The morphologies of the obtained films were imaged using a SUPRA 40 field-emission scanning electron microscopy (ZEISS, Germany). Axis Ultra DLD X-ray photoelectron spectrophotometer equipped with an Al K α X-ray source ( ev) was used to perform the X-ray photoelectron spectroscopy (XPS) spectra. Powder X-ray diffraction (XRD) pattern was measured by a powder diffractometer (Bruker D8 Advanced Diffractometer System) with Cu Kα ( A) source. Fourier Transform Infrared (FT-IR) spectra were recorded on a Varian 3100 FT-IR (Excalibur series) spectrophotometer. Samples were prepared by casting pellets from sample/kbr mixtures 64 scans were signal-averaged with a resolution of 4 cm -1 at room temperature. Conductivity measurements were carried out on a Signatone S Conductivity Meter using four-point probe head. FIB imaging and 3D re-construction: The notch cross-section was prepared by focused ion beam (FIB) milling using AURIGA 60 FIB-SEM Crossbeam (Carl Zeiss Microscopy GmbH, Germany), and the procedures were same with our previous report 4. 3D composite data was obtained by automated serial section imaging technique in the ZEISS SmartSEM controller. After every acquired image, a 7 nm slice was removed using the FIB probe configured with a milling current of 30 Kv: 50 pa. This provided about 7 to 9 slices cross-section slices per particle for accurate reconstruction. A total of 274 slices were collected with cycle time of 1.7 minutes each. The image stack was aligned and reconstructed in 3D with ImageJ 3DViewer plugin without additional image processing. S-4

5 Electrochemical Measurements The as-obtained hybrid films were directly used as working electrode without any other polymer binders or conductive additives. The sizes of electrode films were cut to 1.2 cm diameter (for coin cell) and 4 cm 4cm (for pouch cell), and the areal mass loading of dried hybrid film was ~1.6 mg/cm 2. The cyclic voltammetry (CV) and galvanostatic charge-discharge were performed on the electrochemical analyzer (Biologic VMP3 system) and Neware battery testing equipment, respectively. These electrochemical measurements were carried out using both half cells and asymmetric full cells which were lab-assembled in argon-filled glove box. Half cells were first assembled for measuring the performance of each GFeF film negative electrode and graphene positive electrode separately. Both negative and positive electrodes were measured using standard CR2025 coin cells, where lithium metal foil was used as the counter and reference electrode, 1 M LiPF 6 in (1:1, Vol/Vol) ethylene carbonate (EC) and diethyl carbonate (DEC) was employed as the electrolyte, and Celgard 2400 as separator. For the GFeF film electrode, the as-prepared coin cells should be placed at a standstill overnight in order to allow the electrolyte to penetrate into the film electrode. For the preparation of graphene positive electrode, the graphene powder was well milled with 10 wt.% polyvinylidene fluoride in N-methyl-2-pyrrolidone to form homogeneous slurry which was subsequently coated onto the aluminum foil. After drying at 100 o C overnight, the sheet was pressed and punched into 12 mm diameter electrodes. The loading amount can be adjusted by controlling the thickness of slurry. In order to obtain stable and reliable performance for the hybrid Li-ion S-5

6 capacitor, the full cell was firstly assembled into coin cells with pre-lithiated GFeF anode (this was carried out by directly contacting the electrode with a Li foil wet by the 1 M LiPF 6 electrolyte 5 ). A flexible pouch cell was assembled with the above positive electrode and negative electrode (pre-lithiation treatment) with the area of 4 cm 4cm. Nickel strips were jointed to the side of the positive and negative electrodes as the electrode tabs. The same electrolyte and separator as the coin cells were used for the pouch cells. The whole assembly was packaged with a flexible aluminum/polyethylene foil by vacuum sealing machine 6-7. The total gravimetric capacitance of a supercapacitor in Farads per gram was calculated from the galvanostatic discharge process according to the following equation: C total = I t/( V m), where I is the discharge current (A), t is the discharge time (S), V is the voltage change (V) excluding the IR drop during the discharge process and m (g) is the total mass of the active material 8 (including the positive, negative and binder mass in the asymmetric supercapacitor). The specific volumetric capacitance in Farads per cubic centimeter was calculated based on the gravimetric capacitance according the equation of C vol = C total m/v, where m (g) is the total mass the film electrode, V (cm 3 ) is the volume of film electrode. Energy density in Watt-hour per kilogram and power density in Watt per kilogram were calculated according the following equations 9 : E(Wh/kg)=0.5 C total V 2 /3.6 P(W/kg)=3600E/t (where V in volt is the cell operating voltage, t in second is the discharge time) S-6

7 Supporting Results Figure S1: SEM image of bare iron oxide nanoparticles. S-7

8 Figure S3: Morphology comparison of cross-sectional SEM images for GFeF electrode before lithiation at magnifications of 5K (a), 20K (b) and 60K (c); after cycling test at same magnifications of 5K (d), 20K (e) and 60K (f). Figure S4: Electrochemical characterizations of porous graphene based half cell (voltage limits 1.5~4.5 V, vs. Li/Li+). a) CV profiles measured at scan rate of 2 and 10 mv s-1; b) Galvanostatic charge-discharge curves at the current density of 0.05 A g-1; c) Cyclability of the porous graphene based half cell. Inset showing the optical image of porous graphene electrode. S-8

9 Figure S5: CV curve for GFeF//pG HEC at the scan rate of 2 mv s -1 divided into EDLC and pseudocapacitance portions. 10 Figure S6: Variation of gravimetric and volumetric capacitance of GFeF//pG HSC (based on the total mass and volume of anode and cathode) as a function of current density. S-9

10 Figure S7: Bending cycle stability of GFeF//pG HSC. References: 1. Hummers, W. S.; Offeman, R. E., Preparation of Graphitic Oxide. JACS 1958, 80 (6), Li, M.; Ding, J.; Xue, J., Mesoporous Carbon Decorated Graphene as an Efficient Electrode Material for Supercapacitors. J. Mater. Chem. A 2013, 1 (25), Li, M.; Tang, Z.; Leng, M.; Xue, J., Flexible Solid-State Supercapacitor Based on Graphene-Based Hybrid Films. Adv. Funct. Mater. 2014, 24 (47), Mao, L.; Li, M.; Xue, J.; Wang, J., Bendable Graphene/Conducting Polymer Hybrid Films for Freestanding Electrodes with High Volumetric Capacitances. RSC Adv. 2016, 6 (4), Hassoun, J.; Bonaccorso, F.; Agostini, M.; Angelucci, M.; Betti, M. G.; Cingolani, R.; Gemmi, M.; Mariani, C.; Panero, S.; Pellegrini, V.; Scrosati, B., An Advanced Lithium-Ion Battery Based on a Graphene Anode and a Lithium Iron Phosphate Cathode. Nano Lett. 2014, 14 (8), Liu, B.; Zhang, J.; Wang, X.; Chen, G.; Chen, D.; Zhou, C.; Shen, G., Hierarchical Three-Dimensional Znco2o4 Nanowire Arrays/Carbon Cloth Anodes for a Novel Class of High-Performance Flexible Lithium-Ion Batteries. Nano Lett. 2012, 12 (6), Xiong, P.; Peng, L.; Chen, D.; Zhao, Y.; Wang, X.; Yu, G., Two-Dimensional Nanosheets Based Li-Ion Full Batteries with High Rate Capability and Flexibility. Nano Energy 2015, 12 (0), Wang, H.; Zhi, L.; Liu, K.; Dang, L.; Liu, Z.; Lei, Z.; Yu, C.; Qiu, J., Thin-Sheet Carbon Nanomesh with an Excellent Electrocapacitive Performance. Adv. Funct. Mater. 2015, 25 (34), S-10

11 9. Zhang, J.; Zhao, X. S., On the Configuration of Supercapacitors for Maximizing Electrochemical Performance. ChemSusChem 2012, 5 (5), Lin, Z.; Liu, Y.; Yao, Y.; Hildreth, O. J.; Li, Z.; Moon, K.; Wong, C.-p., Superior Capacitance of Functionalized Graphene. J. Phys. Chem. C 2011, 115 (14), S-11