In this work, it is reported the fabrication of highly stretchable electrodes on a polydimethylsiloxane substrate. A laser ablation technique is used to design lithium nickel manganese oxide micropillars supported on serpentine Al interconnects. Morphological, mechanical, and chemical analyses have been investigated by scanning electron microscopy, optical microscopy, and energy dispersive X-Ray spectroscopy. We show that unlike compact and continuous electrode thin-films, vertical micropillar structures supported on Al serpentines can be stretched up to 70% without structural damage, which opens a new alternative for the fabrication and development of truly stretchable devices such as stretchable micro-batteries.
Recently, emerging technologies like 3D-printing have doped the development of flexible electronics to design foldable displays, bioprosthesis, wearable devices, etc.1,2 Particularly, stretchable microelectronics able to adopt easily complex shapes like emulating the human body have attracted attention for smart textiles.3,4 To achieve the fabrication of such devices, innovative technologies and new designs involving the use of materials with advanced mechanical properties are required.2,5,6
Due to their elastomeric properties, low cost, and biocompatibility, silicone based polymers are usually used as substrates for stretchable electronics. Especially, polydimethylsiloxane (PDMS) that can be highly elongated is widely used in biomedical applications.7 Very recently, intensive studies on the integration of stretchable metallic interconnections with different geometries onto flexible substrates have been investigated.8 Lacour et al.9 reported for the first time the 100% stretchability of 3 mm wide gold lines on a prestrained substrate during the evaporation without any increase in the electrical resistance. Rogers et al. have proposed a stretchable electrode based on wavy shape metallic interconnects integrated onto elastic substrates.10 In 2013, Xu et al. achieved the fabrication of stretchable Li-ion batteries using metallic serpentines connecting the active electrode materials on flexible substrates.11 Li et al.12 also demonstrated a fabrication method of highly stretchable metal electrodes on PDMS substrate. In their work, Ag wrinkled electrodes were obtained by depositing a silver thin film on top of prestrained PDMS which could reach stretchability up to 200%. This method was used to fabricate stretchable wireless sensors. Very recently, Marchiori et al.13 used a maskless technique relying on the laser patterning of metal tapes for the fabrication of Au and Cu serpentine interconnections encapsulated in PDMS. Compared to the cutting of multilayers composed of polyimide/metal or polyethylene terephthalate/metal,14 higher lateral resolution can be reached owing to the direct patterning of the metal. This method has been used to fabricate organic electrochemical transistors (OECTs) that could be stretched up to 38%. But so far, the serpentine metallic interconnections are mainly used to establish an electrical contact between rigid devices. Nowadays, developing extensible technological bricks (such as sensors, transistors, and energy sources) on metallic interconnections remains a major challenge for stretchable electronics.
In this work, we report the fabrication of lithium nickel manganese oxide (LNMO) micropillar electrodes on Al serpentine interconnects that can be stretched up to 70% without structural damaging. Unlike compact and continuous electrode thin-films, we show that under mechanical strains, arrays of vertical micropillars supported on serpentines are carrying empty spaces that can prevent the formation of cracks and the subsequent electrode delamination. This innovative approach opens interesting perspectives for the fabrication of truly stretchable devices including Li-ion microbatteries, solar cells, transistors, sensors, etc.
First, metallic 30 μm thick aluminum foils were cleaned and sonicated in acetone, ethanol, and deionized water for 10 min each and then dried in a furnace at 100 °C for 30 min. Then, the aluminum foil was laminated onto a glass slide using a thermal release double sided 90 °C Nitto RevAlpha tape to maintain the aluminum foil perfectly planar during the fabrication process. PDMS (Sylgard 184) was prepared at (10:1) and spin-coated on top of the aluminum foil at a speed rate of 300 rpm to obtain a thickness of 350 μm and cured right after at 80 °C for 3 h. The RevAlpha tape was released in a furnace at 100 °C for 15 min; then, the aluminum foil on top of PDMS membrane was laminated upside down on a glass substrate to cast subsequently the active material layer on top of it. The commercial LNMO powder typically serving as a cathode material for Li-ion batteries was purchased from MTI Corp, USA. The powder materials were mixed with carbon black (Super P) and polyvinylidene fluoride (PVDF) in the ratio of 90:5:5 and grounded in a mortar for 20 min. Then, the composite powder was mixed with N-methyl-2-pyrrolidone (NMP) to obtain a paste that was doctor bladed on top of the aluminum foil. The electrode was dried under vacuum at 110 °C for 12 h to achieve a LNMO composite electrode thickness of 100 μm. As described by the sequence given in Fig. 1, a laser ablation technique was used to design both micropillars and serpentines, respectively. The laser treatments were carried out using the LPKF Protolaser S equipment with a radiation of 1064 nm, a frequency of 75 kHz, and a beam diameter of 25 μm. LNMO micropillars of 100 μm × 100 μm × 100 μm with an interpillar distance of 25 μm were obtained with a laser power of 3 W. Then, serpentine lines were patterned using direct laser ablation with a laser power of 10 W before the unwanted electrode regions were peeled off from the PDMS substrate. In order to achieve the maximum stretchability, we used the geometric parameters mentioned in Ref. 10 providing a theoretical stretchability of serpentines in a range between 140% and 180% (w = 400 µm, α = 20°, L = 400 µm, and R = 600 µm). The resulting serpentines are expected to reach an average stretchability of 74% before attaining the fracture point, which is roughly half of the theoretical stretchable value. For the mechanical tests, the stretchable electrodes were encapsulated in a PDMS layer with a thickness of 100 μm. The role of encapsulation is to prevent the out of plane deformation of serpentines from the substrate and the subsequent premature break of lines.15 The morphology of the micropillars was examined by electron microscopy using a CARL ZEISS/Ultra 55 scanning electron microscope (SEM), and the chemical mapping of the surface was analyzed by Energy Dispersive X-Ray Spectroscopy (EDS).
The tilted and cross-sectional SEM images of the stretchable electrodes fabricated onto PDMS are shown in Figs. 2(a) and 2(b), respectively. Serpentines with a width of 400 μm and a buckle period of 13 mm can be clearly observed on the flexible substrate. Vertical and periodically spaced LNMO micropillars supported on a continuous Al layer have been also successfully achieved. Compared to conventional lithographic processes, the laser patterning technique is a direct writing approach avoiding the use of photoresists and chemical etching steps that could react with the active material. Besides its simplicity, the control of the laser beam parameters allows the selective ablation of the stacked layers with a resolution in the micrometer range, i.e., etching is limited to the LNMO layer. This is crucial with the perspective of using Al as the cathode current collector. In order to verify the formation of independent micropillars and the preservation of the Al layer, a chemical analysis of the surface was investigated by EDS. Figure 2(c) shows the chemical mapping obtained from a rectangular region of a serpentine carrying 14 pillars. The signals of C and O arise from the both PDMS and composite LNMO, while the Ni, Mn, and F signals are solely attributed to LNMO. This chemical mapping reveals the presence of clearly defined squares corresponding to distinct LNMO pillars as well as the complete removal of LNMO up to the Al layer, which has not been ablated by the laser beam.
The mechanical properties of the micropillars supported on Al serpentines have been studied and compared to that of a continuous composite LNMO layer that was 100 μm thick. The SEM images taken without constraint [Figs. 3(a) and 3(b)] and under a 50% tensile strain [Figs. 3(c) and 3(d)] reveal the superior mechanical behavior of the micropillars. Actually, continuous LNMO serpentines suffer from appearance of multiple cracks and delamination of the active material, while vertical micropillar structures carrying empty spaces can bear the mechanical deformations. In addition, adhesion of micropillars on Al is absolutely not affected by the mechanical constraint. It can be pointed out that the serpentines are slightly detached from the PDMS because the samples have not been encapsulated in order to facilitate SEM observations.
In order to show the advanced stretchable properties of the micropillar structures supported on serpentines, PDMS encapsulated electrodes that were stretched up to 70% and then relaxed have been examined by optical microscopy [Figs. 4(a) and 4(b)]. From these images, it is confirmed that micropillars are almost not affected by the deformation even under the maximum stretched condition. Hence, the morphology of the microstructured electrode remains unchanged after applying the mechanical strain suggesting that this electrode can stand multiple and cyclic strain constrains. One the other hand, continuous LNMO active electrodes are seriously damaged as they are attested by delamination of the layer and the presence of debris. Clearly, the electrodes undergo irreversible damages making them unsuitable for potential use as self-supported stretchable electrodes.
In summary, the fabrication of LNMO micropillars supported on Al serpentine interconnects has been achieved by the laser patterning technique. Morphological and chemical characterizations confirm the formation of independent micropillars while preserving the underlying Al film. We show that unlike compact and continuous electrode thin-films, vertical micropillar structures supported on serpentines can be stretched up to 70% without structural damaging owing to the presence of empty spaces that can prevent the formation of cracks and the electrode delamination. The present approach based on the fabrication of microstructured electrodes supported on serpentine interconnects can be extended to a wide range of materials, which opens promising perspectives for the conception of truly stretchable devices such as stretchable micro-batteries.
This work was supported by Institut Mines Telecom and Ecole des Mines de Saint-Etienne.