Freestanding Lamellar Porous Carbon Stacks for Low-Temperature-Foldable Supercapacitors.

High-performance supercapacitors (SCs) are important energy storage components for emerging wearable electronics. Rendering low-temperature foldability to SCs is critically important when wearable devices are used in a cold environment. However, currently reported foldable SCs do not have a stable electrochemical performance at subzero temperatures, while those that are performing are not foldable. Herein, a freestanding pure-carbon-based porous electrode, namely, lamellar porous carbon stack (LPCS), is reported, which enables stable low-temperature-foldable SCs. The LPCS, which is fabricated with a simple vacuum filtration of a mixture of carbon fibers (CFs), holey reduced graphene oxides (HRGOs), and carbon nanotubes (CNTs), possesses a lamellar stacking of porous carbon thin sheets, in which the CFs act as the skeleton and the HRGOs and CNTs act as binders. The unique structure leads to excellent compression resilience, high foldability, and high electronic and ionic conductivity. SCs made with the LPCS electrodes and ionic liquid electrolyte show a high energy density (2.1 mWh cm-2 at 2 mA cm-2 ), low-temperature long lifetime (95% capacity after 10 000 cycles at -30 °C), and excellent low-temperature foldability (86% capacity after 1000 folding cycles at -30 °C).