Thin, Flexible Secondary Li-Ion Paper Batteries

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Thin, Flexible Secondary Li-Ion Paper Batteries - page 1
ARTICLE Thin, Flexible Secondary Li-Ion Paper Batteries Liangbing Hu, Hui Wu, Fabio La Mantia, Yuan Yang, and Yi Cui* Department of Materials Science and Engineering, Stanford University, Stanford, California 94305. These authors contributed equally to this work. ntegration of electronics onto existing, widely used paper could bring unprec- edented opportunities for consumer electronics. 1 3 These devices can be paper- thin, flexible, lightweight and manufactured by a low cost, roll-to-roll printing process. Power sources are needed for the operation of the paper electronics, and ideally, a power source directly integrated onto pa- per would be preferred for easy system in- tegrations. On the other hand, secondary Li-ion batteries are key components in por- table electronics due to their high power and energy density and long cycle life. 4 In these devices, metal strips, mainly copper ( 10 mg/cm 2 ) and aluminum (5 mg/cm 2 ), are used as current collectors. Recently, solution-processed carbon nanotube (CNT) thin films have been widely studied and ap- plied as electrodes for optoelectronics due to their high conductivity and flexibility. 3,5 CNT thin films on plastic substrates have been explored as current collectors for su- percapacitors. 6 We recently demonstrated that paper coated with CNTs or silver nanowires can be used to replace heavy metals in supercapacitors and Li-ion batter- ies. 7 The CNT films on substrate function ef- fectively as current collectors and enable some new properties for devices. In this paper, we integrated all of the components of a Li-ion battery into a single sheet of paper with a simple lamination process. Free-standing, lightweight CNT thin films ( 0.2 mg/cm 2 ) were used as cur- rent collectors for both the anode and cath- ode and were integrated with battery elec- trode materials through a simple coating and peeling process. The double layer films were laminated onto commercial paper, and the paper functions as both the me- chanical support and Li-ion battery mem- I ABSTRACT There is a strong interest in thin, flexible energy storage devices to meet modern society needs for applications such as interactive packaging, radio frequency sensing, and consumer products. In this article, we report a new structure of thin, flexible Li-ion batteries using paper as separators and free-standing carbon nanotube thin films as both current collectors. The current collectors and Li-ion battery materials are integrated onto a single sheet of paper through a lamination process. The paper functions as both a mechanical substrate and separator membrane with lower impedance than commercial separators. The CNT film functions as a current collector for both the anode and the cathode with a low sheet resistance ( 5 Ohm/sq), lightweight ( 0.2 mg/ cm 2 ), and excellent flexibility. After packaging, the rechargeable Li-ion paper battery, despite being thin ( 300 m), exhibits robust mechanical flexibility (capable of bending down to <6 mm) and a high energy density (108 mWh/g). KEYWORDS: paper batteries · lamination · free-standing thin film · flexible brane. Due to the intrinsic porous structure of the paper, it functions effectively as both a separator with lower impedance than commercial separators and has good cycla- bility (no degradation of Li-ion battery after 300 cycles of recharging). After poly- mer sealing, the secondary Li-ion battery is thin ( 300 m), mechanically flexible, and has a high energy density. Such flexible sec- ondary batteries will meet many applica- tion needs in applications such as interac- tive packaging, radio frequency sensing, and electronic paper. CNT thin films were coated onto stain- less steel (SS) substrates with a solution- based process. Aqueous CNT ink was pre- pared with 10% by weight sodium dodecylbenzenesulfonate (SDBS) as the sur- factant. 8 The concentration of CNT is 1.7 mg/mL. The CNT ink was applied to the SS substrate with a doctor blade method. 9 A dried film with a thickness of 2.0 m was formed after drying the CNT ink on the SS substrate at 80 °C for 5 min. Slurries of bat- tery materials, Li 4 Ti 5 O 12 (LTO) (Sud Chemie) ¨ and LiCoO 2 (LCO) (Predmaterials & LICO), were prepared by mixing 70 wt % active *Address correspondence to Received for review July 29, 2010 and accepted September 03, 2010. Published online September 13, 2010. 10.1021/nn1018158 © 2010 American Chemical Society VOL. 4 NO. 10 5843–5848 2010 5843
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Thin, Flexible Secondary Li-Ion Paper Batteries - page 2
ARTICLE 5844 after bending down to 6 mm (Mandrel). Due to the excellent mechanical integrity of the double layer film and the loose in- teraction between the CNT film and SS, peeling off the double layer film from the SS is highly reproducible. After integrating the battery elec- trode materials on the lightweight CNT current collectors, a lamination process was used to fabricate the Li-ion paper bat- teries on paper. A solution of polyvi- nylidene fluoride (PVDF) polymer was Mayer-rod-coated on the paper substrate with an effective thickness of 10 m. The wet PVDF functions as a glue to stick the double layer films on paper. The concen- tration of PVDF in N-methyl-2-pyrrolidone (NMP) was 10% by weight. As shown in Figure 1c, the double layer films were laminated on the paper while the PVDF/ NMP was still wet. During this process, a Figure 1. (a) Schematic of fabrication process for free-standing LCO/CNT or LTO/CNT double layer thin films. The CNT film is doctor-bladed onto the SS substrate and dried. metal rod rolls over the films to remove An LTO or LTO slurry is then doctor-blade-coated on top of CNT film and dried. The air bubbles trapped between films and whole substrate is immersed into DI water, and the double layer of LTO/CNT or LCO/ the paper separator. After laminating CNT can be easily peeled off due to the poor adhesion of CNTs to the SS substrate. LTO/CNT on one side of the paper, the (b) (Left) 5 in. 5 in. LTO/CNT double layer film coated on SS substrate; (middle) the double layer film can be easily separated from the SS substrate in DI water; (right) the same process was used to put LCO/CNT final free-standing film after drying. (c) Schematic of the lamination process: the free- standing film is laminated on paper with a rod and a thin layer of wet PVDF on paper. on the opposite side of the paper to com- plete the Li-ion battery fabrication. Fig- (d) Schematic of the final paper Li-ion battery device structure, with both LCO/CNT and LTO/CNT laminated on both sides of the paper substrate. The paper is used as ure 1d,e shows the scheme and a final de- both the separator and the substrate. (e) Picture of the Li-ion paper battery before en- vice of the Li-ion paper battery prior to capsulation for measurement. encapsulation and cell testing. Although materials, 20 wt % Super P carbon, and 10 wt % poly- a paper-like membrane has been used as the separa- vinylidene fluoride (PVDF) binder (Kynar 2801) in tor for other energy storage systems including superca- N-methyl-2-pyrrolidone (NMP). The battery slurries pacitors, it is the first demonstration of the use of com- were applied to CNT/SS with the same doctor blade mercial paper in Li-ion batteries, 12 where paper is used method. The slurries were dried at 100 °C for 0.5 h. The as both separator and mechanical support. battery electrode material on the CNT film forms a The cross section of the laminated Li-ion paper bat- double layer film, where CNT films function as the cur- tery, with the CNT/LTO/paper/LCO/CNT structure, was rent collectors. As shown in Figure 1a, the double layer examined with SEM. Figure 2a reveals the surface mor- LCO/CNT or LTO/CNT film was lifted off by immersing phology of Xerox paper, with large fibers ( 20 m di- the SS in DI water followed by peeling with tweezers. ameter) and surface roughness (peak to valley is 10 Figure 1b shows a LTO/CNT film with a size of 7.5 cm m). Xerox paper lacks microsize holes, which makes it 12.5 cm on a SS substrate (left) being peeled off in wa- an excellent separator for Li-ion batteries with the lami- ter (middle) and in a free-standing form (right). Previ- nated electrode films. We tried coating battery elec- ously, CNT thin films have been coated mainly on plas- trode materials with the same slurries directly onto ei- tic substrate for use as transparent electrodes in various ther side of Xerox paper, and we found occasional device applications, including solar cells and light- shorting of the device due to the leakage of battery 3,5,10,11 emitting diodes. In this study, we found that CNTs electrode materials through paper. The lamination pro- have weaker interaction with metal substrates when cess provides an efficient approach for solving the leak- compared with plastic or paper substrates, which al- age problem by using Xerox paper as a separator be- lows us to fabricate free-standing films with integrated cause the battery electrode forms a solid film and is current collector and battery electrodes. The double integrated with the CNT film. An SEM image at low layer films obtained with this method are lightweight, magnification reveals that LTO/CNT and LCO/CNT form 2 2 with 0.2 mg/cm CNT and 2 10 mg/cm electrode a continuous, solid film (see Supporting Information). material. The free-standing double layer film shows a Figure 2b shows the cross section of the LTO/CNT low sheet resistance ( 5 Ohm/sq) and excellent flexibil- double layer on top of Xerox paper separator. The SEM ity, without any change in morphology or conductivity reveals the continuous morphology of CNT thin films VOL. 4 NO. 10 HU ET AL.
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ARTICLE Figure 2. (a) SEM of porous, rough Xerox paper as Li-ion battery separator. Inset shows the zoomed-in image of the surface morphology of the paper substrate. (b) SEM image of the cross section for the laminated paper Li-ion battery. The layers thicknesses are CNT 2 m, LTO 30 m, paper 100 m, LCO 30 m, and CNT 2 m. (c) Zoomed-in image of the cross section of the CNT/LTO double layer, as in the red box in (b). (d) SEM image of the CNT film as a current collector, as in the blue box in (b). with thicknesses of 2 m. The composite LTO elec- trode film is densely packed with a thickness of 30 m. The thickness of Xerox paper used in this study is 100 m. The porous morphology of paper allows the electrolyte to diffuse efficiently into it, which allows the paper to be used effectively as a separator. Figure 2c reveals the zoomed-in image of the interface be- tween LTO and CNT as in Figure 2b, which shows no CNT penetration into the LTO layer. CNT thin films form continuous mechanical supports and serve as electrical current collectors for the electrodes. The sheet resis- tance of the CNT thin film is measured with a four-point probe and is 5 Ohm/sq, and it can be further de- creased with acid doping such as with HNO 3 or SOCl 2 . 13 A similar double layer resulting from the integration of the cathode material, LCO, on top of CNT film was ob- served, as well (see Supporting Information). Figure 2d shows the surface of a highly conductive CNT film as a current collector. To evaluate the performance of paper as an effec- tive separator membrane for Li-ion batteries, its stabil- ity in the electrolyte and the effect of the impurities, mainly OH groups, in a large voltage range with respect to Li metal were tested. Pouch cells were fabricated with CNT films as cathodes, Li-metal as anodes, and Xerox paper as the separators (see Supporting Informa- tion). The cells were cycled with 50 A/cm 2 current den- sities between 1 and 4.3 V (Figure 3a). The charge and discharge capacities are minimal, 0.01 mAh/cm 2 , which shows that the irreversible capacities from both the paper separator and the CNT film are negligible ( 0.001 mAh/cm 2 ). It has been reported that CNT thin films have been used as anodes for Li-ion batteries due to their large surface areas, but they show large irrevers- ible capacities and low coulombic efficiencies for the first cycle when cycled below 1 V vs Li/Li . 14 Due to the small mass loading of the CNTs, 0.2 mg/cm 2 , and the operating voltages of LTO and LCO (above 1 V), the irreversible capacities from CNTs are negligible. Furthermore, paper shows low resistivity in the electro- lyte. Impedance spectroscopy was used to obtain infor- mation on the resistivity of the solution in the paper. Coin cells with LTO versus Li metal were made, and the Nyquist plot at open circuit conditions is reported (Fig- ure 3b). The high frequency intercept of the impedance spectrum with the x-axis represents the resistance of the solution in the pores of the separator, R SL , as evi- denced in the plot. In the inset of Figure 3b, the value of R SL for different thicknesses of the separator is re- ported. The value of R SL is given by the following expression: R SL ) F S L τ A f () (1) 5845 VOL. 4 NO. 10 5843–5848 2010
Thin, Flexible Secondary Li-Ion Paper Batteries - page 4
ARTICLE Figure 3. (a) Galvanostatic charging/discharging curves of CNT/paper vs a Li metal anode to show the negligible capacitance between 1 and 4.3 V. (b) Impedance of Xerox paper as a separator in a Li-ion battery test where LTO is the anode and Li metal is the cathode. The inset shows the imped- ance with different layers of paper. (c) Galvanostatic charging/discharging curves of the LTO anode (1.3 1.7 V) half cells with conductive paper cur- rent collectors. The mass of the LTO electrode is 1.8 mg. The current rate is C/5. (d) Cycling performance of LTO nanopowder (C/5, 0.063 mA) half cells. where S is the resistivity of the electrolyte ( 100 · cm for the standard EC/DEC solution), 15 L is the thickness of the separator, A is transversal area (the area perpen- dicular to the axis of the electrode), is tortuosity (the ratio between the path length of the ions and the thick- ness of the electrode), and f is pore fraction (the ratio between the pore volume and the total geometrical volume of the electrode). The ratio /f is important for the separator, which indicates how easy it is for the electrolyte to penetrate through. The value of the ratio between the tortuosity and the pore fraction for the pa- per is /f 9.1, while it is /f 28.8 for the standard separator. This fact is significant because it demon- strates that the paper will show a better conductivity than the standard separator at the same thickness. The cheap, commercial paper functions as an effective re- placement for a standard separator membrane and can serve as well as a mechanical support with similar im- pedance and a smaller ratio between the tortuosity and the pore fraction. To test the feasibility of using Xerox paper as the separator in Li-ion batteries with the lamination pro- cess, half cells were made with CNT/LTO or CNT/LCO with lithium foil as a counter electrode (see Support- ing Information). Voltage profiles closely match those with metal current collectors according to previous work, and no apparent voltage drop was observed. 16 18 Figure 3c shows the voltage profile for a half cell of CNT/ LTO, and no apparent voltage drop was observed when the voltage profiles for first, 30th, and 300th cycles were compared. The cycling performance of these conduc- tive paper-supported electrodes is shown in Figure 3d. 5846 VOL. 4 NO. 10 HU ET AL. The CNT/LTO electrodes achieved initial discharge ca- pacities of 147 mAh/g and exhibited a capacity reten- tion of 95% after 300 cycles at C/5. These values are close to those obtained for metal collector-based Li- ion batteries. 16 18 The coulombic efficiencies for the CNT/LTO half cells are generally over 99.0%. We also ob- served an increase in the coulombic efficiencies and dis- charge capacities over the first few cycles. Our recent work also shows that paper is stable in the electrolyte solution for eight months in Li-ion battery test, where the same electrolyte was used as in this study. Full cells with integrated current collectors and bat- tery electrodes onto a single sheet or paper are fabri- cated with the same lamination process. Previously, Friend et al. reported two-layer polymer diodes fabri- cated by lamination followed by annealing. 19 Yang et al. has demonstrated stacked plastic solar cells with an electronic glue-based lamination process with inter- face modification. 20 The laminated Li-ion paper battery has the structure illustrated in Figure 1d (see Support- ing Information, as well). After the CNT/LCO and CNT/ LTO films were laminated onto the two sides of Xerox paper, the whole device was sealed with 10 m PDMS (see Supporting Information) in an Ar-filled glovebox using LiPF 6 in EC/DEC electrolyte. The Li-ion paper bat- tery is thin, 300 m in total. The anode and cathode mass loadings are 7.2 and 7.4 mg/cm 2 , respectively. The assembled paper battery was taken outside of the glovebox for battery testing. As shown in Figure 4a, the paper battery is able to light up a red LED continu- ously for 10 min without fading. Due to the small thick- ness and the great flexibilities of current collectors us- ing CNT thin films, the whole device shows excellent flexibility (Figure 4b). No failure was observed for the paper battery after manually bending the device down to 6 mm for 50 times (see Supporting Information also). Figure 4c shows the first cycle voltage profile of the Li- ion paper battery sealed with a transparent bag, where the thickness of the plastic is 10 m. The cycling per- formance of the stacked cells is shown in the inset of Figure 4d. The first coulombic efficiency is 85%, slightly lower than that of a typical Li-ion battery with LCO and LTO electrodes. After the first cycle, the coulombic efficiency is 94 97%. The discharge retention is 93% af- ter 20 cycles. For practical applications, especially for large-scale energy storage, good self-discharge perfor- mance is crucial. The voltage was monitored after the battery was charged to 2.7 V for 5 min at a C/10 rate and disconnected. As shown in the inset in Figure 4d, the voltage drops about 2% instantly, which is due to the IR drop after switching off the current. After that, a 5.4 mV voltage drop was observed for the full cell after 350 h. This is equivalent to 0.04% self-discharge if the Li-ion paper battery is fully charged after a month. The self-discharge performance could be further im- proved through device fabrication process modifica- tions such as better sealing, longer vacuum baking
Thin, Flexible Secondary Li-Ion Paper Batteries - page 5
ARTICLE times, and lower moisture levels by using standard dry rooms. There is a great need for development of light- weight, thin, and flexible batteries for portable elec- tronic applications with low power consumption, 1.0 mW. Ajayan et al. developed flexible batteries and su- percapacitors based on nanocomposite paper in 2007; Mihranyan et al. developed ultrafast all-polymer paper- based batteries in 2009; and we explored conductive paper for energy storage recently. 7,21,22 Enfucell Inc. and Blue Spark Inc. have recently developed a flexible and soft battery by using a printing method on plastic sub- strates. The Li-ion paper battery developed in this ar- ticle has advantages in various aspects. In Ajayan’s nanocomposite-based battery, Li metal was used as one electrode and is neither thin nor flexible. The poly- mer battery developed by Mihranyan et al. performs as a mixed battery and capacitor, which shows a nonflat discharge curve and has a large thickness ( 2 mm). The soft batteries from Enfucell and Blue Spark are made on plastic substrates, not paper, and are not recharge- able. Figure 4e and Table 1 in Supporting Information show the comparison of our flexible, thin paper battery with theirs. The blue arrow indicates the improvement direction for flexible storage devices. Our paper battery is rechargeable and has a higher energy density, 108 mWh/g, based on the total mass of the device, and it is much thinner ( 300 m). Currently, we are using car- bon CNTs with a price of $200/g. The CNT weight in our device is less than 0.2 mg/cm 2 , which is $0.02/ cm 2 . Therefore, the CNT cost is negligible. Due to the porous structure of CNT thin film as current collector, the sealing of the paper thin film battery will be more challenging. One method for increasing the total en- ergy for the Li-ion paper battery is through stacking layer upon layer, as in Figure 4f, where conductive CNT films function as current collectors, and extended metal strips at the edge serve as connections to the external circuit. To demonstrate the feasibility of the stacking of the paper battery, we have fabricated a cell with 9 lay- ers stacked in parallel (see Supporting Information). The individual cells are separated by 10 m plastic. The stacked cells in parallel are sealed within a transparent plastic bag. The cells were enclosed and sealed inside the transparent plastic bag in an Ar-filled glovebox with an Al strip on the cathode side and a Cu strip on the an- ode side extending out for outside electrical contact (Figure 4f, right). In this way, the multiple cells are con- nected in parallel. The stacked cells were tested and showed similar performance to individual cells, where the total current is equal to the sum of the individual cells. Figure 4. (a) Lighting a red LED with a Li-ion paper battery which is encap- sulated with 10 m PDMS. (b) Flexible Li-ion paper batteries light an LED device. (c) Galvanostatic charging/discharging curves of a laminated LTO LCO paper batteries, a structure as in Figure 1d. (d) Self-discharge be- havior of a full cell after being charged to 2.6 V. The initial drop is due to the IR drop after turning off the charging current. Inset: cycling performance of LTO LCO full cells. (e) Comparison of our paper Li-ion battery with a poly- mer paper battery. The green arrow indicates the target of the paper battery. (f) Schematic for stacked cells separated by 10 m plastic paper. An indi- vidual cell is made with laminated LTO/CNT and LCO/CNT on either side of a piece of Xerox paper. A small piece of Cu is connected on the LTO/CNT side and Al on the LCO/CNT side. Figure S8 (Supporting Information) shows the operation of the stacked Li-ion paper battery. Since the device scale is small and the sheet resistance of CNT film is 5 Ohm/ sq, the sheet resistance effect on voltage drop is small. This concept can be applied to a multiwalled CNT with enough film thickness for high surface conductance. In conclusion, we have demonstrated a Li-ion bat- tery integrated onto a single sheet of paper through a simple lamination process. The paper substrate func- tions as both the substrate and the separator, and highly conductive CNT films function as current collec- tors for both the anode and the cathode. Such re- chargeable energy storage devices are thin, flexible, and lightweight, which are excellent properties for vari- ous applications where embedded power devices are needed, such as RFID tags, functional packaging, and new disposable applications. METHODS Free Standing LTO/CNT Double Layer Films: Aqueous CNT ink was prepared with 10% by weight sodium dodecylbenzenesulfonate (SDBS) as surfactant based on commercial, arc-discharged CNTs (Carbon Solutions, Inc.). The CNT ink was then blade-coated onto a SS substrate. The SS substrate was used as received. Slur- ries of battery materials, Li 4 Ti 5 O 12 (LTO) (Sud Chemie) and LiCoO 2 ¨ (LCO) (Pred Materials & LICO), were prepared by mixing 70 wt VOL. 4 NO. 10 5843–5848 2010 5847
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ARTICLE % active materials, 20 wt % Super P carbon, and 10 wt % poly- vinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidone (NMP) as the solvent. The slurries were stirred overnight at room temperature. Afterward, the slurries with a thickness of 125 m were blade-coated on top of CNT films on SS substrates and dried at 100 °C for 1 h. The double layer LTO/CNT or LCO/CNT films were formed on SS substrates. To delaminate the double layer films, the SS was immersed into a beaker with DI water. After gentle shaking of the beaker, the double layer films easily delaminated from the SS substrate. Fabrication and Test of Li-Ion Batteries: For half cell tests of LTO/ CNT and LCO/CNT, coin cells were fabricated. Lithium metal foil (Alfa Aesar) was used as the counter electrode in each case. Xerox paper was used as the separator. Lithium metal and free- standing LTO/CNT or LCO/CNT films were punched into round shapes. The parts for coin cell assembly were purchased from MTI Corporation (Richmond, CA). A 1 M solution of LiPF 6 in EC/ DEC (1:1 vol/vol; Ferro) was used as the electrolyte. The charge/ discharge cycles were performed at different rates at room tem- perature. The devices were assembled in an argon-filled glovebox with oxygen and water contents below 1 and 0.1 ppm, respectively. The Li-ion battery tests were performed by either a Bio-Logic VMP3 battery tester or an MTI battery analyzer. Acknowledgment. Y.C., L.H., H.W., and F.L.M. acknowledge support from the King Abdullah University of Science and Tech- nology (KAUST) Investigator Award (No. KUS-l1-001-12). Karim Zaghib from Hydro-Que ´bec is gratefully acknowledged for the generous supply of the LTO electrode materials. Y.Y. acknowl- edges financial support from a Stanford Fellowship. Supporting Information Available: Additional figures and ex- perimental details. 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