Ⅲ. JABUTICABA-INSPIRED HYBRID CARBON FILLER/POLYMER ELECTRODE
3.2 E XPERIMENTAL
Fabrication of stretchable electrode
Multiwalled carbon nanotubes (110 mg, MWCNTs, Sigma Aldrich) with diameter of 110~170 nm and length of 5~9 µm and super P carbon black (110 mg, Imerys S.A., France) with diameter of 50~60 nm was dispersed in hexane (5 mL, Sigma Aldrich) by ultra-sonication process for 2 h.
And then, the dispersed composite solution was homogeneously mixed with ecoflex base (1g, Ecoflex 00-10, Smooth-On) using a vortex mixer for 10 min. Subsequently, the ecoflex curing agent (1:10 ratio for curing agent to base) was added to the composite solution. Simultaneously, it poured into a glass petri-dish (diameter 60 mm) and was transferred in a humidity chamber (from 60% to 100% humidity at room temperature) to construct porous structure onto the surface via breath figure process. The composite was thermally cured in an oven at 80 oC for 2 h. Finally, the fabricated composite has a thickness of 200 μm.
Synthesis of active materials
We synthesized polyimide coated activated carbon (PI@AC) as anode materials for aqueous lithium-ion battery. Firstly, 1,4,5,8-naphthalenetetracarboxylic dianhydride (1 mmol, Sigma Aldrich) and AC (0.2 g, product) were mixed with 4-chlorophenol (20 g, Sigma Aldrich) at 60 °C.
After then, ethylenediamine (0.15 mL, Sigma Aldrich) was added to the mixture and followed by refluxing at 220 °C for 10 h. The mixture was cooled to room temperature and the product was rinsed with ethanol and deionized water, and followed by vacuum filtration. The obtained powder was annealed at 320 °C under argon atmosphere for 4 h. To synthesize of LMO@CNT as cathode material, we firstly synthesized MnO2@CNT. In a typical synthetic process, 100 mg of CNT (Hanwha Nanotech Corp, Korea) and a certain amount of KMnO4 (250 mg, Sigma Aldrich) were mixed together in an agate mortar. The mixed powder was poured into 100 mL of water and stirring for 10 min. 0.5 mL of concentrated H2SO4 (95%, Samchun) was added to the above solution with an additional stirring of 30 min. Then, the mixture was heated in an oil bath at 80 °C for 2 h. The precipitate was collected by filtration and washed repeatedly with deionized water after the mixture was cooled to room temperature. The product was dried in a vacuum oven at 60 °C for 12 h to obtain CNT@MnO2 composite. To make LiMn2O4@CNT nanocomposites, 0.25 g of the MnO2@CNT nanocomposites and 0.26 g of LiOH·H2O (Sigma Aldrich) were mixed with 60 mL of H2O. And then, the mixture was uniformly mixed for 1 h before transferring into 80 mL autoclave. After the hydrothermal treatment at 160 °C for 12 h, the resulting precipitates were filtered and washed with distilled water. After the autoclave was cooled to room temperature, the
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final product was dried in a ventilation oven at 60 °C for 12 h.
Characterization
Microstructure of LMO@CNT was investigated using X-ray diffractometer (XRD, Bruker D8- Advance), which was performed at 3 kW using Cu Kα radiation. Transmission electron microscopy (TEM) images of conductive fillers and active materials were obtained using JEM 2100 (JEOL) operated at an accelerated voltage of 200 kV. The morphologies of nanocomposite and active materials were investigated using a field-emission scanning electron microscopy (FE- SEM, Hitachi S-4800). The FT-IR spectra of PI@AC was investigated using a Varian 670 IR spectrometer. The TGA analysis of LMO@CNT was investigated using TA-Q500 under air atmosphere. The mechanical properties were measured using a tensile strength measurement (DA-01, Petrol, Korea). The tensile strength samples were cut with the dog bone shape. The sheet resistance was conducted using a four-point probe machine (FPP-RS8, Dasol Eng., Korea). To measure electrical resistance of carbon/composite under strain using home-made strain equipment, it was attached to conductive fabric using silver paste to reduce contact resistance. The change of R value at stretching state was measured using multi meter in the strain ranging from 0 to 200%
In-situ SAXS analysis
SAXS measurements were performed at the 6D UNIST-PAL beamline of the Pohang Accelerator Laboratory in Korea. The energy of the X-rays was 11.6 keV (wavelength, λ = 1.0688 Å) and the sample-to-detector distance was 3510 mm. Scattering patterns were collected using a 2D CCD detector (MX225-HS, Rayonix L.L.C., USA). The samples were left perpendicular to the beam direction and elongated in the vertical direction at a constant stretching rate of 20 um/s.
The scattered intensity was obtained in the strain range from 0 to 200%.
Electrochemical test
The stretchable electrode, which was composed of active materials (LMO@CNT and PI@AC), conductive carbon (super P), and polymeric binder (PVDF) in a weight ratio of 8:1:1, was fabricated by spray coating method on hot plate at 150 oC. To remove residual solvent, the electrode was placed in vacuum oven at 150 oC for 10 h. And then it was pressed by rolling press machine at 20 MPa. The active loading density was 0.5 mg cm-2. The electrodes were cut in the form rectangular shape (2 cm x 1 cm). The electrochemical properties of half and full cell were investigated using an electrochemical tester (Biologic science instrument, VSP) with a 1 M Li2SO4 aqueous electrolyte. Both three-electrode system and stretchable full cell were assembled in open environment. Pt electrode and Ag/AgCl electrode were used as the counter and reference electrode, respectively. The full cell was performed between 0.00 and 2.00 V at room temperature
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and measured using two-electrode system with LMO@CNT and PI@AC as working electrode and counter electrode, respectively. The mass ratio of cathode and anode materials was designed as 1:1.3.
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