Supplementary Information
Paintable Battery
Neelam Singh1, Charudatta Galande1, Andrea Miranda1, Akshay Mathkar1, Wei Gao2, Arava Leela Mohana Reddy1, Alexandru Vlad1,3 and Pulickel M. Ajayan1,2,*
1. Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX-77005.
2. Department of Chemistry, Rice University, Houston, TX-77005.
3. Institute of Information and communication Technologies, Electronics and Applied Mathematics, Electrical Engineering, Université catholique de Louvain, Louvain la Neuve, B-1348 Belgium.
*Corresponding author:
This file includes:
Materials and Methods SI-1 to SI-3
Supplementary Figures S1 to S5
Other Supplementary Materials for this manuscript includes the following:
Fabrication process of a spray painted Li-ion battery- Supplementary Movie S1
Materials and Methods
SI-1: Preparation of paints
Supplementary Figure S1: Recipes describing paint formulations developed for various components of the paint battery
The optimized recipes for preparing paints used in the present work are summarized in supplementary Fig. S1. Details of preparation of each paint are given below.
a. SWNT paint: HiPco SWNTs were obtained from Rice University. The SWNTs had to be first purified to remove catalyst content by treating them with a mixture of conc. HCl and H2O2 at 70°C for 8h followed by filtration and overnight drying1. The resulting mass of SWNTs was first ground to a fine paste with NMP in a mortar pestle. More NMP was then added to achieve a final concentration of ~5mg/ml of SWNTs in NMP and ultrasonicated in a bath sonicator for 2-3h. Finally, the solution was tip sonicated for 2 min at 2W of power and used for spraying the positive current collector films. These SWNTs remain suspended for up to 1h, after which a brief ultrasonication for 10-20 min is sufficient to redisperse any small aggregates that may have been formed. SWNT films spray-painted using this paint had poor conductivity for even very thick films, likely due to poor electrical contact between nanotubes2. To enhance the electrical contacts between the SWNTs, Super PTM (SP) conductive carbon (Timcal Ltd. Switzerland) powder was added to the SWNT suspension. A 20% w/w content of SP carbon lowered the sheet resistance to ~10 W/□ for a mass density of 2mg/cm2. We hypothesize that the SWNT films have a large number of microscopic air pockets, which the SP carbon particles help fill, improving contacts between nanotubes and yielding highly conductive films. During charge-discharge cycling of SWNT/LCO/MGE/LTO/Copper full cells, no significant voltage polarization was observed, showing that current collectors have very low resistance and had excellent electrical contact with the electrodes.
b. LCO (cathode or positive electrode) paint: Insulating Lithium Cobalt Oxide (LCO) electrodes are generally rendered conductive by addition of conductive agents such as carbon blacks and metal fibers3. First, LCO (99.8%, Sigma Aldrich), SP carbon and Ultra-fine graphite (UFG-5, Showa Denko America, Inc.) were ground together in a mortar in 85:5:3 weight ratio. A binder solution was prepared by dissolving 4% w/v Polyvinylidinefluoride (PVDF) (Sigma Aldrich) in NMP. The powdered mixture was then dispersed in the binder solution (56% w/v) by vortex mixing until a consistent ink (~60% w/v total solid content) was formed. Spray painted electrodes with only SP carbon as the conductive additive did not cycle well, possibly due to inhomogeneous distribution of the small SP carbon particles (50 nm) with far larger particles of LCO (7-10mm) in the painted electrode. Addition of a second conductive agent having a size comparable (5µm) to that of LCO particles provides more homogeneous distribution of the conductive additives and LCO particles in the final electrode. The uniform distribution of particles provides better conductive electrodes, enhancing the electrode utilization and capacity retention upon cycling.
c. LTO (anode, negative electrode) paint: The LTO paint was formulated similar to the LCO paint. An 80:10 mixture of LTO (Pred Materials International Inc.) and UFG conductive carbon was dispersed (60% w/v) in a 7% w/v PVDF solution to give ~67% solid content. The concentration was kept higher as compared to LCO paint to reduce the amount of NMP coming into contact with the polymer separator while spraying the LTO electrode.
d. Polymer paint: The polymer separator paint was formulated by mixing the three following solutions- 1) 9% w/v Kynarflex®-2801 (Kynarflex) (Arkema Inc.) in Acetone; 2) 9% w/v PMMA (MW 120,000; Sigma Aldrich) in Acetone; 3) 8% w/v fumed SiO2 (Cabot Inc.) in DMF – in a 6:2:1 volume ratio resulting in a paint with approximately 3:1 ratio of Kynarflex:PMMA and 10 wt.% SiO2 content.
e. Copper paint: Commercially available copper conductive paint (Casewell Inc.) was used as a negative current collector (anode) paint by diluting it with ethanol.
SI-2: Optimization of polymer separator
Kynarflex, a copolymer of PVDF and HFP was chosen due to its good solubility in low-boiling solvents (such as acetone and THF) and its electrochemical stability in a wide potential window. Separators painted from Kynarflex paints in acetone were fibrous and highly porous (Supplementary Fig. S2a), and became mechanically unstable upon addition of liquid electrolyte due to large volume change by swelling. On the other hand, those made using Kynarflex inks in DMF had virtually no porosity (Supplementary Fig. S2b).
Mechanical robustness of the battery rests on good adhesion of the separator to substrates. Pure Kynarflex separators had poor adhesion to almost all substrates tested. Miscible thermoplastics such as PMMA are often used to promote adhesion in industrial PVDF coatings4,5. We found that 25% w/w of PMMA could be added to Kynarflex without compromising the mechanical properties of the separator (3:1 Kynarflex:PMMA by wt.). Separators made by using this PMMA:Kynarflex blend in acetone resulted in highly porous, well adhered separator films (Supplementary Fig. S2c). However, their electrolyte uptake was still large and caused instantaneous detachment from the substrate. Thus controlling the porosity to tailor the electrolyte uptake was deemed necessary.
As observed previously, separators made from Kynarflex/acetone paint were highly porous due to fast drying of polymer solution into fibrous strands during spraying, while Kynarflex/DMF inks dried slowly and resulted in non-porous films. Tailoring of porosity therefore, is tied to the solvents used. Since choice of solvents is limited, the porosity of the sprayed polymer separator was tailored by dissolving the Kynarflex/PMMA blend in a mixture of acetone and DMF in various ratios until the electrolyte uptake was sufficiently reduced to allow adhesion. It is evident from supplementary Fig. S2c-e that increasing proportion of DMF reduces the porosity of the final sprayed polymer film but on the other hand, the films adhered well even on addition of electrolyte. As a result, polymer separator films sprayed from 3:1 Kynarflex:PMMA in 1:8 DMF:Acetone were chosen for further studies. This reduced porosity, however, caused a four-fold increase in the electrolyte resistance (Fig. 2e). Inorganic oxide additives have been previously used to enhance electrolyte absorption by increasing porosity while increasing the mechanical stability of the microporous gel polymer electrolytes6,7(MGE). Thus, varying percentages of fumed SiO2 were added to the polymer separator paint. SEM micrographs show that the film containing no SiO2 has the lowest porosity and addition of SiO2 causes an increase in porosity (Supplementary Fig. S3) and hence increases the ionic conductivity (Fig. 2f). The ionic conductivity of polymer separator at 10% w/w SiO2 content was 1.24 x10-3S/cm, which is sufficiently high for Li-ion battery purposes.
Impedance spectra of polymer films sprayed from paints containing different solvent ratios are shown in supplementary Fig. S4a. Separators fabricated from paints with no DMF have very high ionic conductivity (, where R is the intercept of the spectrum with the real Z’ axis in high frequency region, l is the thickness and A is the area of the separator), while addition of DMF results in significant increase in electrolyte resistance (Fig. 2e, Supplementary Fig. S4a), in league with porosity of the separators (Supplementary Fig. S2). It is evident from impedance spectra that addition of SiO2 reduces electrolyte resistance, and that 20% w/w of SiO2 has no significant reduction as compared to 10% w/w content (Supplementary Fig. S4b & Fig. S3).
Supplementary Figure S2: Effect of DMF content in paint on separator morphology. Cross sectional SEM micrographs of spray painted polymer separators fabricated from: a, Pure Kynarflex in Acetone showing highly porous layered film; b, Pure Kynarflex in DMF with almost no porosity; (….contd)
Supplementary Figure S2: Effect of DMF content on separator porosity. (..contd) c, 3:1 Kynarflex:PMMA in Acetone having layered structure with more porosity than b; d, 3:1 Kynarflex:PMMA in 1:8 DMF:Acetone with lesser porosity than c; e, 3:1 Kynarflex:PMMA in 1:4 DMF:Acetone even lower porosity than d. SEM micrographs were taken after fracturing the polymer films dipped in liquid nitrogen for 5 min.
Supplementary Figure S3: Effect of SiO2 content on separator porosity. Cross sectional SEM micrographs of fractured spray painted polymer separators fabricated from 3:1 Kynarflex:PMMA in 1:8 DMF:Acetone doped with a, No SiO2; b, 10% SiO2; c, 20% SiO2. Polymer film containing no SiO2 had lowest porosity. Layered and fibrous structure can be achieved by increasing the SiO2 content. SEM micrographs were taken after fracturing the polymer films dipped in liquid nitrogen for 5 min.
Supplementary Figure S4: Electrochemical impedance spectra (EIS) of spray painted separators a, Comparison of EIS spectra of Kynarflex:PMMA MGE painted with varying DMF:Acetone ratios up to 1:4; b, comparison of EIS spectra of MGE with varying SiO2 content up to 20 wt.%. The insets in (a) and (b) show the data in high frequency region the linear behavior and intercepts with real Z’ axis can be seen. Two Stainless steel electrodes were used as blocking electrodes (no charge-transfer at the interface) for recording EIS spectra in the 100 KHz-1Hz frequency range. MGE was formed by soaking the separator in liquid electrolyte (1 M LiPF6 solution in 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate) for at least 2h.
SI-3: Fabrication of tile cells by spray painting
A spray painted Li-ion battery on glazed ceramic tile at various stages of fabrication is shown in supplementary Fig. S5. The cell area was 5x5 cm2 and had a capacity of ~30 mAh. Nine such cells were used in the demonstration described in the main text.
Cell activation: Efficient operation of the cell is critically dependent on addition of sufficient electrolyte and efficient soaking by the microporous separator. The liquid electrolyte was slowly added to the cell from the exposed sides of the separator as well as the top (electrode) to allow the cell to soak it up completely. The cells were allowed to soak up the electrolyte for more than 2 h before starting any measurements.
Supplementary Figure S5: Tile cell fabrication. a, Glazed ceramic tile; b, 1st layer- SWNT current collector painted on the tile; c, 2nd layer- LCO cathode onto the SWNT current collector (+ve electrode); d, 3rd layer- Kynarflex-PMMA porous polymer separator covering the LCO electrode ; e, 4th layer - LTO anode (-ve electrode) onto polymer separator as a replica of LCO electrode; f, 5th layer - Copper current collector onto the anode layer; g, Cell framed with 3M 615 thermo-bonding film and laminated PE-Aluminum-PET sheet; h, top packaging sheet; i, Completed cell, sealed by hot press at ~200oC after adding electrolyte in glove box.
References
1. Wang, Shan, H., Hauge, R. H., Pasquali, M. & Smalley, R. E. A Highly Selective, One-Pot Purification Method for Single-Walled Carbon Nanotubes. J. Phys. Chem. B 111, 1249–1252 (2007).
2. Hecht, D., Hu, L. & Grüner, G. Conductivity scaling with bundle length and diameter in single walled carbon nanotube networks. Applied Physics Letters 89, 133112–133112–3 (2006).
3. Ahn, S. et al. Development of high capacity, high rate lithium ion batteries utilizing metal fiber conductive additives. J. Power Sources 81–82, 896–901 (1999).
4. Iezzi, R. A., Gaboury, S. & Wood, K. Acrylic-fluoropolymer mixtures and their use in coatings. Progress in Organic Coatings 40, 55–60 (2000).
5. Tracton, A. A., Coating technology handbook 3rd edn (CRC Press, Taylor & Francis Group, Boca Raton, 2006).
6. Caillon-Caravanier, M., Claude-Montigny, B., Lemordant, D. & Bosser, G. Absorption ability and kinetics of a liquid electrolyte in PVDF–HFP copolymer containing or not SiO2. J. Power Sources 107, 125–132 (2002).
7. Li, Z., Su, G., Wang, X. & Gao, D. Micro-porous P(VDF-HFP)-based polymer electrolyte filled with Al2O3 nanoparticles. Solid State Ionics 176, 1903–1908 (2005).
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