A Highly Flexible, Non-Flammable Li-Ion Battery via Co-Optimization of Electrode Cohesive and Adhesive Strength

With the development of flexible and wearable electronics, the thickness of current wearable devices can scale down to hundreds of microns and the flexibility of the devices can be less than 1 mm bending radius [1]. Therefore, to match the progress of wearables, the design of flexible batteries needs to consider form factor, flexibility. However, traditional batteries (i.e. Coin cell, pouch cell, cylindrical cell) have not scaled down in a similar manner as their device counterparts, which makes the final size of the integrated system is limited. In addition, regarding flexibility, traditional battery structures fail to match the flexibility of devices they power, compromising the advantage of wearable devices. In past few decades, researchers have developed various types of flexible batteries for powering wearable biomedical devices, including fibrous batteries, and rubbery batteries. However, they are still limited in flexibility, stability, and output performance.<br/><br/>According to research [2,3], a battery fails during flexing in two modes: cohesive failure and adhesive failure. As for the cohesive mode, stress concentration and propagation happen within electrode layers due to its complex particle-network structures, which causes non-uniform Li intercalation and deintercalation processes. As for the adhesive failure mode, electrode layers delaminate from current collector layers, thus significantly increasing their contact resistance. Both modes strongly hurt battery stability and electrochemical performance.<br/><br/>Based on our previous work[4], we demonstrate a “land-bridge” electrode design to reduce cohesive stress concentration within electrode layers and a “Site-pinning” approach to increase adhesive strength between electrodes and current collectors. The coupled optimization of these two methods gives the battery achieve &gt; 1000 times bending under 5 mm bending radius, 1 mAh/cm²capacity density, &lt; 300 um total thickness.<br/><br/>First, as for “land-bridge” design, a 8x8 electrode matrixes are casted on a 600 nm current collectors in PDMS singulation walls. By taking the advantage of low Young’s Modulus of PDMS and the “land-bridge” design, the bending stress within electrode layer is significantly reduced. In addition, we also develop a 5 um flexible SU8 layer between electrodes and current collectors to improve the adhesive strength. We prove that the hydrogen bond between SU8 layer and electrode forms hydrogen bonding enhances the electrode adhesive strength to flat current collector. Finally, to address the safety concerns of the Li-ion battery for wearables, an ionic liquid electrolyte (1M Lithium bis(fluorosulfonyl)imide (LiFSI) in 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Py₁₄TFSI)) is used in this work to provide non-flammability.<br/><br/>[1] Hanna, A. et al. Extremely Flexible (1mm Bending Radius) Biocompatible Heterogeneous Fan-Out Wafer-Level Platform with the Lowest Reported Die-Shift (&lt;6 µm) and Reliable Flexible Cu-Based Interconnects. in 2018 IEEE 68th Electronic Components and Technology Conference (ECTC) 1505–1511 (IEEE, 2018). doi:10.1109/ECTC.2018.00229.<br/>[2]Chang, J., Huang, Q., Gao, Y. & Zheng, Z. Pathways of Developing High-Energy-Density Flexible Lithium Batteries. Advanced Materials 33, 2004419 (2021).<br/>[3] Liang, G., Zhu, J., Chen, A., Yang, Q. & Zhi, C. Adhesive and cohesive force matters in deformable batteries. npj Flex Electron 5, 1–4 (2021).<br/>[4]Ouyang, G., Whang, G., MacInnis, E. & Iyer, S. S. Fabrication of Flexible Ionic-Liquid Thin Film Battery Matrix on FlexTrateTM for Powering Wearable Devices. in 2021 IEEE 71st Electronic Components and Technology Conference (ECTC) 1620–1626 (2021)