Active Material-Free Continuous Carbon Nanotube Fibers with Unprecedented Enhancement of Physicochemical Properties for Fiber-Type Solid-State Supercapacitors
Authors: Yu Hayoung , Kim Jeong-gil , Kim Nam Dong , Kim Seung Min , Jeong Hyeon SU 
Published: December 2024

The continuous evolution of wearable and flexible devices has driven the need for resilient energy storage systems that can uphold their power and energy density amidst varying mechanical deformations. Among the diverse energy storage mediums, fiber-type solid-state supercapacitors (FSSCs) have been spotlighted for their potential in wearable energy storage, thanks to their inherent capability to blend into diverse structural forms, mirroring the versatility of traditional fibers [1]. In this context, Carbon nanotube fibers (CNTFs) produced through a wet-spinning process based on the liquid crystalline (LC) phase emerge as an ideal electrode material, given their remarkable electrical conductivity, mechanical robustness, and unmatched flexibility [2].
While the excellent physical properties of CNTFs is undeniably compelling, their electrochemical inactivity has been a persistent hurdle in realizing their full potential in FSSC applications. Most prior studies have aimed to circumvent this by modifying the fiber’s surface with active materials, such as metal oxides [3] and conductive polymers [4], to serve as energy storage sites. Despite its successes, this approach has been plagued by issues like active material detachment during long-term usage and, more importantly, increasing processing costs due to the need for additional materials and process.
In this study, we have pioneered a new approach to enhance the electrochemical activity of LC spun CNTFs without the need for additional post-processing or active materials, addressing the challenge of the sp2 carbon surface's limitations. Through strategic surface functionalization and subsequent LC phase development of CNTs, an unprecedented high-concentration dope (i.e., 160 mg/mL) for wet-spinning was realized. Leveraging high-concentration dope, the as-spun functionalized CNTF (F-CNTF) exhibits more aligned and densely packed structures compared to raw CNTF, resulting in significantly improved mechanical and electrical properties. While the raw CNTF exhibited a specific capacitance of 4.2 F/g at 0.5 A/g, the electrochemically activated F-CNTF, bearing enhanced physicochemical properties, demonstrated a high specific capacitance of 139.4 F/g at the same charge-discharge rate, all without any active materials or post-processing. For practical application, we integrated symmetric FSSCs using F-CNTFs into a textile and used it as a power source for a 1.5 V digital clock, which was operating effectively for over 15 minutes. Additionally, wrist straps integrated with F-CNTF FSSCs could function effectively as a power source even when folded, rolled, or worn on the wrist. This study not only highlights the potential for efficient, mass-produced fiber-type energy storage electrodes but also marks a significant departure from the conventional, non-economical sheath-core strategy, paving the way for more sustainable and efficient energy storage solutions in wearable technology.
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