Ultra Flexible Thick Electrodes for 3D Free Form Li Ion Batteries

The rapid development of flexible electronics including roll up displays, smart phones and wearable sensors requires flexible and reliable batteries. High-energy density Li ion battery with high mechanical properties should be integrated into the limited space for reliable operation of flexible electronics tasks. The form factor of various complex products also requires non regular type of free form batteries. In this study, we have developed a stainless-steel (SS) fiber reinforced thick electrode that has a minimum bending radius of 3mm and attain a high energy density of 500Wh/L. This novel electrode structure was fabricated through reinforcing thermal-induced polymer-rich electrode by SS fiber current collector. The polymer rich electrode made through thermal induced phase separation process was suggested for flexible thick electrode. The LiNi0.6Mn0.2Co0.2O2 (NMC622) powder and graphite powder were used as active materials. The SS fiber 3D current collector was prepared using a facile method from a bundle of SS fibers. The electrochemical property and mechanical properties of the battery with the novel electrode were analyzed by comparing with battery of polymer rich electrode coated on conventional metal foil current collectors. At 2C, the specific capacity of the SS fiber electrode battery (135 mAhg−1) is twice as high as that of the foil electrode battery(60 mAhg−1). The cycling performance of half-cells (NMC/Li) was also analyzed at 0.1C for 100 cycles, revealing that the foil electrode battery fails after 30 cycles, whereas the SS fiber electrode battery maintains a relatively stable capacity throughout its life cycle. The energy density of the SS fiber electrode battery increases until the electrode is 400 μm thick, where it attains the maximum energy density (500 Wh/L), which is higher than that of the foil electrode battery and previously reported flexible electrode batteries. The electrochemical performance of the flexible pouch cell with SS electrode was tested before and after repeated 2000 bending cycles. A high capacity of 144 mAhg−1 is retained after the 2000 cycles, which is not significantly less than that before bending (158 mAhg−1). The SS fiber electrode has the structural flexibility to fit any electrode configuration owing to it free and flexible shape. 3D free form battery is fabricated using a scalable and facile method by designing special molds. Pyramid and doughnut shaped electrodes were fabricated and then the hemispherical battery was assembled in polymer packaging and successfully powered an LED light.