A KIRIGAMI-INSPIRED, EXTREMELY STRETCHABLE, HIGH AREAL-COVERAGE MICRO-SUPERCAPACITOR PATCH

Renxiao Xu*, Aaron Hung, Anton Zverev, Caiwei Shen, Lauren Irie, Geoffrey Ding,Michael Whitmeyer, Liangjie Ren, Brandon Griffin, Jack Melcher, Lily Zheng,

Xining Zang, and Liwei LinUniversity of California, Berkeley, USA

ABSTRACTWe present an extremely stretchable micro-

supercapacitor patch (reversible stretchability >282.5%, with <2% change in capacitance retention) with high areal-coverage of functional electronic components (76.2%). Our device is 2.4-9.4 times more stretchable than the state-of-the-art stretchable supercapacitors using the Accordion construct, and enjoys 4 times higherareal-coverage than the previously reported stretchable supercapacitors with the Island-Bridge construct. The Kirigami-inspired design is the key to both high stretchability and high areal-coverage. We envision our stretchable micro-supercapacitor patch (and other power-supply microdevices with a similar design) to be highly desirable in future flexible, stretchable, and wearable systems.

INTRODUCTIONRecently, stretchable electronics systems (i.e., systems

that can maintain high electronic performance whensubject to large bending, twisting, and stretching deformations) have drawn widespread research attentions[1-15]. Most of these systems use the “Island-Bridge” construct, where rigid functional components (i.e., “islands”) with length are separated by spacing among them, and joined by compliant interconnects (i.e., “bridges”) [3-5, 8-13]. In this construct, the areal coverage of functional components can be calculated as =( + ). Since only the “bridges” can be elongated via unraveling while the rigid “islands” remain undeformed, the stretchability of the system can relate

to interconnect stretchability by= (1 ) (1)Therefore, although high stretchability (up to ~200%) may be achieved using this construct, it is at the cost of relatively low areal-coverage (~20% to 60%). Some other systems use the “Accordion” construct, where stretchability of the system results from flattening-out of the wrinkled thin-film electronics, due to the release of pre-strain strates [6, 7, 14, 15]. Ideally, this construct can boast 100% areal-coverage, yet the stretchability is limited (typically less than ~100%). The stretchability and areal-coverage of the state-of-the-art reported stretchable micro-supercapacitors are summarized in Table 1. In contrast, our proposed stretchable micro-supercapacitor patch (SMSP) in this paper represents a new class of construct inspired by Kirigami (paper-cutting), where rationally designed cuts in the patch release strain and make the patch much more deformable by facilitating segment-level rotation, bending and twisting. Using thisconstruct, high stretchability (>282.5%) and high areal coverage (76.2%) can be achieved simultaneously.

Table 1: Stretchability and areal coverage comparison for representative stretchable micro-supercapacitors

Source ConstructRef. 5 Island-bridge 200% 19%Ref. 6 Accordion 30% 100%Ref. 7 Accordion 120% 100%

This work Kirigami >282.5% 76.2%

Figure 1: Different laser powers for graphitic conversion and cutting, and resulting micro structures. Scale bars in the optical images are100 m. b) geometric details of SMSP, with these color codes: black for electrodes, red for cut lines, and green for electrolyte. g-i) The stretchability and twistability comparison between h) SMSP and i) a Kapton sheet.

978-1-5386-4782-0/18/$31.00 ©2018 IEEE 661 MEMS 2018, Belfast, Northern Ireland, UK, 21-25 January 2018

Figure 2: Buckling determinism analysis, featuring the buckling shape and cricitial buckling strain of the first five buckling modes of SMSP. Colors in the figure denote normalized out-of-plane displacement.

DEVICE FABRICATION, CHARACTERI-ZATION AND DEMONSTRATION

Our SMSP can be fabricated by irradiating commercial Kapton film (polyimide, 127 m in thickness) with a commercial CO2 laser source (Universal Laser Systems, wavelength = 10.6 m) at a fixed scanning speed 60 mm/s. Both the electrode patterns and cuts in the SMSP can be defined by the same laser system by using different power settings. The optical images in Fig. 1a show the traces resulting from ONE laser scan at different powers. When the power of the laser is insufficient (below 4.8 W),no obvious change on the Kapton could be observed. Beyond the threshold 4.8 W, laser converts polyimide on and near the Kapton film surface to conductive, porous graphitic networks (laser-induced graphene, or “LIG”, as reported previously [16]) via a photothermal process. As the power increases, larger areas and depths in the film are affected by the heat and converted, so that the resulting porous graphitic trace gets wider and thicker. In our experiment, the optimal laser power is 9.6 W. Using this setting, thick (~70 m), porous yet relatively strong graphitic patterns can be generated. These graphitic structures serve as the electrodes in our supercapacitor.When the power of the laser is further increased (e.g., to 14.4 W), the resulting trace consists of many laminar pieces sticking outwards, due to sudden overheating from the intense laser irradiation. Since these laminae can be easily peeled off even with tiny perturbation, the graphitic networks generated at 14.4 W is vulnerable to fracture, and therefore, not suitable as the electrode material. If the power of the laser is even higher (e.g., at 22.8 W or 28.8 W), the Kapton film is carbonized throughout its thickness, and a continuous Kirigami cut slot is formed. These Kirigami cuts release a patch of pre-designed dimensions from a larger Kapton sheet, and increase the overall compliance of the patch. Finally, a polymer-based electrolyte solution (1g of polyvinyl alcohol + 1g of phosphoric acid + 12 g of deionized water) is applied to selected regions on the patch to make the patch a functionalsupercapacitor.

Figure 1b shows the geometric details of the SMSP, featuring the graphitic electrode patterns (black), the Kirigami cut lines (red), and the regions to apply electrolyte (green). The overall size of the patch is 36.5 mm by 28 mm, which is comparable to a US quarter (Fig. 1c). The pre-designed alternating, offset Kirigami cuts transforms the Kapton sheet into five deformable units mechanically (and electrically) joined together. Each deformable unit consists of two equivalent parallel plate-like capacitors in series. In total, the SMSP has ten of

these capacitors in series connection.In order to show the significant increase in

deformability resulting from the pre-designed Kirigami cuts, we stretched and twisted the SMSP (Fig. 1e) and an intact Kapton sheet of the same size (Fig. 1d). As expected, the SMSP can be easily stretched to three times of its initial length and twisted by ~90 degrees with ease, whereas the intact Kapton sheet can only be twisted by less than 30 degree with negligible increase in length.

We then performed cyclic voltammetry test on our SMSP with an electrochemical workstation (Gamry Reference 600). When the scan rate is set as 100 mV/s, the SMSP (in its undeformed configuration) yields specific capacitance 2.52 mF/cm2. As expected, this value is comparable to that reported in Ref. 16, where similar laser induced graphitic electrodes are used.

BUCKLING DETERMINISM ANALYSISTraditionally, structural buckling is associated with a

state of chaos. In many cases across length scales, buckling is treated as the first route to failure, ranging from bridgesand pressure vessels, down to MEMS structures [17-19]. Inrecent years, many researchers have started to make use of controlled buckling instability, especially in development of mechanical metamaterials and stretchable electronic systems [20]. In order to rationally utilize the structural buckling to achieve stretchability in our SMSP, we need to confirm that this buckling is deterministic and predictable. Here, we performed finite element analysis (FEA) using the linear perturbation module of a commercial FEA package (ABAQUS). This analysis generates the buckling shapes and critical buckling strains corresponding tothe first five modes, as is shown in Figure 2. The shape of Mode 1 is characterized by all the five deformable units rotating in the same direction, and the cut slot in each unit slightly opening. We find that the critical buckling strain for this mode ( = 0.72%) is significantly lower than those of the other four modes (by 28% to 67%). Therefore, Mode 1 is the only dominant mode expected to occur, as islater confirmed by experiments.

POST-BUCKLING DEFORMATIONFigure 3 shows the deformation of our SMSP at four

elongation stages (70.6%, 141.3%, 211.9% and 282.5%) as predicted by FEA (left column) and observed in experiments (right column). The modeling results agree very well with the optical images in geometric details. As predicted, SMSP deforms following the route of Mode 1.When the SMSP is stretched beyond the onset of buckling, each deformable unit further rotates, and the cut slots in

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Figure 3: FEA predictions (left) and optical images (right) of the SMSP at four deformation stages. Center column: strain distributions at three selected sites with large deformation. Colors in the figure denote maximum principal strain (absolute values) in the laser induced graphitic electrodes.

each unit opens up further, such that the initially straight segments pop out of plane and bend considerably to accommodate the applied stretch. Both the rotation (twisting) and the bending contribute to the overall stretchability of the SMSP. Because of this, the electrodes are not elongated by much even when the SMSP is stretched by 282.5%.

Magnified views show the strain mapping at three representative regions with the most significant strain concentration (center column). Even in these regions, the maximum principal strain (<3.0%) on the electrodes is

always lower than material failure strain (4.2%), even for 282.5% of elongation. Therefore, we expect no degradation in electronic performance induced by the deformation of SMSP. In experiments, we indeed see that the five cyclic voltammetry curves corresponding to different elongation stages overlap well (Figure 4a). Despite significant overall stretching deformation (0% to 282.5%, Fig. 4b), only less than 2% change in capacitance retention is observed. This <2% change in capacitance is smaller than most reported stretchable supercapacitors, despite that our elongation is greater. All of these results

Figure 4: a) Cyclic voltammetry curves for the same SMSP in undeformed and four elongated configurations. The scan rate is fixed at 200 mV/s. b) Capacitance retention vs. elongation of SMSP.

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above prove that our SMSP can maintain full performance under extreme deformations.

CONCLUSIONSThis paper presents a stretchable micro-supercapacitor

patch that achieves high stretchability and areal coverage at the same time, facilitated by a Kirigami-inspired design. This stretchable patch can be easily fabricated by laser induced graphitic conversion and cutting. The performance of the supercapacitor can be maintained even with extreme deformation. Our future work will aim at developing stretchable micro supercapacitors with optimized structural designs and improved electrochemical performances.

ACKNOWLEDGEMENTSThis work is supported in part by Berkeley Sensor and

Actuator Center. The authors would like to thank Mr. Yichuan Wu and the staff at Jacobs Institute for Design Innovation and UCB QB3 Biomolecular Nanotechnology Center for helpful discussions. The work of R. Xu is partially supported by the Anselmo Macchi Fellowship at UC Berkeley.

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CONTACT*R. Xu, tel: +1-510-672-3581; renxiaoxu@berkeley.edu

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