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BIT’s progress in building omnidirectional crumpled in-plane micro-supercapacitors

News Source & Photographer: School of Chemistry and Chemical Engineering

Editor: News Agency of BIT

Translator: Cheng Jie, News Agency of BIT

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On May 27, Wang Ying, a doctoral candidate of the School of Chemistry and Chemical Engineering of Beijing Institute of Technology (BIT), published a research paper as the first author in the journal Science Advances under the title “Fixture-free omnidirectional prestretching fabrication and integration of crumpled in-plane micro-supercapacitors”. BIT is the first communication unit. Zhao Yang, special researcher of the School of Chemistry and Chemical Engineering of BIT, Professor Qu Liangti of the Department of Chemistry of Tsinghua University, and Liu Feng, associate researcher of the Institute of Mechanics, Chinese Academy of Science, are co-corresponding authors. This research has been supported by the National Key R&D Program, the National Natural Science Foundation of China and the Beijing Municipal Natural Science Foundation Project.

Miniaturized energy storage devices are becoming increasingly important for the next generation of renewable energy, and have compatibility that traditional power supply devices cannot achieve in the microelectronics device industry. Micro-supercapacitors (MSCs), especially in-plane MSCs, are one of the most competitive alternatives, not only because of their small size, fast charging and discharging speed, long cycle life, and high safety, but also because of their diaphragm-free construction that can meet multi-directional rapid ion diffusion and avoid the occurrence of short circuits. According to the empirical observation of the development of in-plane MSCs, in addition to the electrode material, the main factors affecting the charge storage capacity of microelectrodes also include specific spatial constraints, microelectrode interface effects and other microscopic morphologies, which mainly depend on the reasonable structural design and preparation technology of microelectrodes. At present, the more mature two-dimensional in-plane microelectrode preparation methods mainly include: screen printing, inkjet printing, electrodeposition, filtration, laser direct writing, etc., which can build patterned thin film electrodes on the flat substrate. However, due to space and device size limitations, the electrochemical capacity is still not enough for practical applications. Although three-dimensional in-plane MSCs are developed to compensate for the lack of area specific capacitance, the accumulation of electrode thickness greatly hinders the preparation accuracy of the device and lead to sluggish dynamic behavior of MSCs.

For the optimization of in-plane MSCs processing technology, it is necessary to consider how to expose more electrode materials to the electrolyte as much as possible in the micro-scale range. In recent years, prestretching strategies have been extensively studied in stretchable devices, i.e., thin film materials overlaid on uniaxial or biaxial prestretching elastomers are compressed and deformed after the substrate relaxes, forming a tightly stacked crumpled structure. The experimental results show that the presence of abundant stacked structures can provide more exposed surface area within the same space than block structures. Therefore, we attempt to combine the screen printing method with a prestretching strategy (Figure 1) with the aim of generating a tightly packed active electrode surface within a limited pattern area. Without the need for a complex fixture, we choose a latex balloon in the expanded state as the omnidirectional prestretching elastic substrate, compensating for the unevenness of the tensile direction in the traditional prestretching system. Carbon nanotube (CNT) slurry with excellent mechanical properties and high electrical conductivity is further selected as the compressible electrode material. The CNT slurry is scraped onto the electrode mask on the surface of the balloon and then patterned electrodes are formed by removing the template. It is worth mentioning that the inflated balloon can provide sufficient support for the adhesion of the masking template and electrode material, effectively reducing the difficulty of operation. After pre-coating a layer of gel electrolyte, the balloon is deflated and contracted to easily obtain a crumpled MSC, realizing a step of transformation from macroscopic operation to micro-device production. In addition, microdevices prepared by omnidirectional contraction stress have significant form retention, which can achieve a diversification of device designs.

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Figure 1 Preparation and Structural Characterization of Crumpled MSCs

The results show that the single crumpled MSC has a small area of about 0.02 cm2 and a high area specific capacitance of 13.5 mF cm-2, which are 13 times smaller and 45 times larger than the initial state device, respectively. At the same time, it also has an ultra-high electrode volume specific capacitance of 9.3 F cm-3 (about 3.6 times that of the initial state device), which is superior to the previously reported CNT-based micro energy storage device. Subsequently, we further explore the mechanism of improving the electrochemical properties of MSCs after wrinkles from both experimental and theoretical simulations (Figure 2). The results show that the dense crumpled structure improves the electrical conductivity of the carbon nanotube film and reduces the ion adsorption energy of the carbon nanotube film, thereby providing more effective active surface area and enhancing the electrochemical activity of the electrode. Using this strategy, an ultra-small energy storage device of 0.1 mm2 is further implemented (Figure 3), where the width and electrode gap of a single fork finger are only 44 and 20 microns, respectively. This is the smallest size device that can be obtained using the screen printing method at present, without the need for complex precision machining.

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Figure 2 Electrochemical Lifting Mechanism

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Figure 3 Ultra-small Size of the Crumpled MSC

Finally, we use an elastic balloon substrate and a custom spherical mask to develop a large-scale spherical integrated microdevice (Figure 4), which provides a breakthrough idea for processing challenging curved microdevices. A set of miniature device arrays measuring just 3.9 cm2 can generate a high output voltage of 100V. After that, the flexible and mechanically stable miniature device can withstand a series of extreme deformations and has great potential in wearable, flyable power supply systems, which is in stark contrast to other miniature flexible energy storage devices. In short, these findings provide expandable design ideas for developing high-performance, shape-appropriate, and wearable flexible miniature energy storage devices.

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Figure 4 Demonstration of Integration and Diverse Applications of MSCs Arrays

Paper link: https://www.science.org/doi/10.1126/sciadv.abn8338


Author Bio attached:

Wang Ying, a 2019 doctoral student in the School of Chemistry and Chemical Engineering, is supervised by Professor Qu Liangti and the second supervisor is Special Researcher Zhao Yang. Since 2019, he has been pursuing a doctorate degree in this research group. During this period, he published one paper in each of Science Advances, Advanced Functional Materials, and Journal of Energy Chemistry as the first author, and one paper in the journal ChemSusChem as a co-author.

Yang Zhao is a special researcher and doctoral supervisor at the School of Chemistry and Chemical Engineering of BIT. He published more than 40 SCI papers as the first or corresponding author, including Nat. commun., Sci. Adv., JACS, Angew. Chem. Int. Ed., Adv. Mater., Energy Environ. Sci., ACS Nano, etc. He have published more than 90 SCI papers in total, 4 highly cited ESI papers, more than 10,000 citations, and 4 authorized patents, of which 1 achievement has been industrialized. He has presided over a number of National Natural Science Foundation of China and Beijing Municipal Natural Science Foundation projects, and has participated in a number of national major basic research and development (973) program projects and key research and development program projects. Selected as the 2017 Beijing Natural Science Foundation Outstanding Young Talents.