Flexible and Fast Chargeable Lithium-Ion Battery Based on Percolative Network-Based Electrospun Nickel Microfibers and Electrosprayed Nanotextured Anode Materials

High-performance flexible energy-storage devices have great potential as power sources for portable and wearable electronics. The major obstacle for developing such next-generation flexible batteries is the absence of flexible electrodes that can simultaneously meet high stability, high energy-density, and fast charging features. In this study, we present a novel approach employing an electrospun nickel microfiber (NiMF)-based flexible current collector. Not only does the current collector developed here show high mechanical properties, high electrical conductivity, and lightweight design, but the percolative network thereof also enables high flexibility and fast-charging speed with high stability. For the anode coating material, the mixture of lithium titanium oxide (LTO), carbon nanotubes (CNTs), polyvinylpyrrolidone (PVP), and polyvinylidene fluoride (PVDF) with a mixing ratio of 85:5:1:9 was used. In particular, the use of the electrostatic spraying method herein enabled the manipulation of the coating-thickness, -uniformity, and -texturing of the anode material being deposited on the randomly-entangled individual NiMFs. In case of the NiMF-based LTO electrode, despite being coated with the same loading mass of active materials, its total weight was only 25 mg because of an extremely low weight of the NiMF current collector relative to its volume. On the other hand, the Ni foil-based LTO electrode had a total weight of 475 mg. This difference in weight enabled the NiMF-LTO electrode to achieve a significant energy-density increase of more than 20 times compared to the Ni foil-LTO electrode. The resistance of the NiMF-LTO electrode was measured to be 6.3 Ω, which was significantly lower than that of the Ni foil-LTO electrode, which was 261 Ω. In addition, the NiMF-LTO electrode showed a 1.5% change in resistance after 100 cycles of bending test, while the Ni foil-LTO electrode yielded a large change of 10% with the formation of cracks. In the half-cell test, as increasing the current rate (C-rate) from 0.1 to 2 C and subsequently reducing to 1 C, the corresponding discharge capacity (<i>C</i>_dis) of the NiMF-LTO case decreased from 166 to 75 mAh/g, followed by a recovery to 145 mAh/g, showing a high retention rate of 99.5%. In contrast, the <i>C</i>_dis of the Ni foil-LTO case exhibited a significant decrease from 165 to 4.5 mAh/g and then recovered to 10.5 mAh/g, showing a lower retention rate of 37%. Moreover, the NiMF-LTO case demonstrated a high stability with a capacity decay rate of only 0.05% over 100 cycles at 1 C. This advancement was attributed to the low impedance possibly identified by the charge transfer and ion transfer resistances (<i>R</i>_ct and <i>R</i>_it). At 0.1 C, the <i>R</i>_ct of the NiMF-LTO electrode-based half-cell was measured to be 142 Ω, lower than that of the Ni foil-LTO case (297 Ω). This difference indicates that the percolative structure of the NiMF-LTO electrode facilitated the efficient transportation of lithium ions from the electrolyte to electrode. Furthermore, the <i>R</i>_it, corresponding to the concentration gradient of lithium ions within the electrode, was measured to be 100 mΩ for the NiMF-LTO case, while that measured from the Ni foil-LTO case was higher as 704 Ω. This also indicates that the high uniformity and texturing (or surface area) of the LTO layer electrosprayed on the NiMFs enhanced the ion diffusion rate within the electrode. The approach and materials employed here are expected to show great promise for developing next-generation flexible batteries with offering high and stable electrochemical and mechanical features.