Flexible Low-Temperature Zinc Ion Hybrid Capacitor with Ultra-long Cycle Life Based on Fe-MnO₂ Electrode

Zinc-ion hybrid capacitors (ZIHCs) are expected to become the next generation of energy storage devices, highly anticipated for their battery-like performance and lower cost. However, because of their unmanageable structural deformation and inadequate cycling capabilities, they face significant difficulties and challenges in practical production and applications. In this paper, we developed and assembled a flexible solid-state zinc ion hybrid capacitor, which utilizes a gel electrolyte, activated carbon (AC) as the anode, and porous Fe³⁺ doped MnO₂ (Fe-MnO₂) as the cathode. The embedding of Fe³⁺ in MnO₂ broadens the original ion channels and provides more electrochemically active sites. The gel electrolyte is formed with poly(vinyl alcohol) (PVA) providing a flexible framework, carboxymethylcellulose (CMC) offering rigid support, and enhanced low-temperature performance due to the addition of ethylene glycol (EG). This combination results in excellent mechanical properties and high ionic conductivity, ensuring stability even under low-temperature conditions. As a result, the flexible ZIHC showcases exceptional specific capacitance, achieving a capacitance of 848.8 mF cm^-2 at a current density of 2 mA cm^-2, and strong stability when subjected to bending tests. Notably, even under low-temperature conditions of -20 °C, the device maintains consistent electrochemical performance, with a remarkable cycle retention rate of 93.75% after 15 000 cycles. This study demonstrates significant advancements in energy storage systems, creating opportunities for the use of flexible devices across a range of applications.