Large‐Area, All‐Solid, and Flexible Electric Double Layer Capacitors Based on CNT Fiber Electrodes and Polymer Electrolytes

This work presents a scalable method to produce robust all‐solid electric double layer capacitors (EDLCs), compatible with roll‐to‐roll processes and structural laminate composite fabrication. It consists in sandwiching and pressing an ionic liquid‐based polymer electrolyte membrane between two carbon nanotube (CNT) fiber sheet electrodes at room temperature, and laminating with ordinary plastic film. This fabrication method is demonstrated by assembling large‐area devices of up to 100 cm² with electrodes fabricated in‐house, as well as with commercial CNT fiber sheets. Freestanding flexible devices operating at 3.5 V exhibit 28 F g⁻¹ of specific capacitance, 11.4 W h kg⁻¹ of energy density, and 46 kW kg⁻¹ of power density. These values are nearly identical to control samples with pure ionic liquid. The solid EDLCs could be repeatedly bent and folded 180° without degradation of their properties, with a reversible 25% increase in energy density in the bent state. Devices produced using CNT fiber electrodes with a higher degree of orientation and therefore better mechanical properties show similar electrochemical properties combined with composite specific strength and modulus of 39 and 577 MPa SG⁻¹ for a fiber mass fraction of 11 wt%, similar to a structural thermoplastic and with higher specific strength than copper.