Flexible Low-Temperature\nZinc Ion Hybrid Capacitor\nwith Ultra-long Cycle Life Based on Fe-MnO<sub>2</sub> Electrode

Zinc-ion hybrid capacitors (ZIHCs) are expected to become\nthe next\ngeneration of energy storage devices, highly anticipated for their\nbattery-like performance and lower cost. However, because of their\nunmanageable structural deformation and inadequate cycling capabilities,\nthey face significant difficulties and challenges in practical production\nand applications. In this paper, we developed and assembled a flexible\nsolid-state zinc ion hybrid capacitor, which utilizes a gel electrolyte,\nactivated carbon (AC) as the anode, and porous Fe<sup>3+</sup> doped\nMnO<sub>2</sub> (Fe-MnO<sub>2</sub>) as the cathode. The embedding\nof Fe<sup>3+</sup> in MnO<sub>2</sub> broadens the original ion channels\nand provides more electrochemically active sites. The gel electrolyte\nis formed with poly(vinyl alcohol) (PVA) providing a flexible framework,\ncarboxymethylcellulose (CMC) offering rigid support, and enhanced\nlow-temperature performance due to the addition of ethylene glycol\n(EG). This combination results in excellent mechanical properties\nand high ionic conductivity, ensuring stability even under low-temperature\nconditions. As a result, the flexible ZIHC showcases exceptional specific\ncapacitance, achieving a capacitance of 848.8 mF cm<sup>–2</sup> at a current density of 2 mA cm<sup>–2</sup>, and strong\nstability when subjected to bending tests. Notably, even under low-temperature\nconditions of −20 °C, the device maintains consistent\nelectrochemical performance, with a remarkable cycle retention rate\nof 93.75% after 15 000 cycles. This study demonstrates significant\nadvancements in energy storage systems, creating opportunities for\nthe use of flexible devices across a range of applications.