Organic radical‑boosted ionic conductivity in redox polymer electrolyte for advanced fiber‑shaped energy storage devices

As wearable electronics proliferate, the demand for flexible, safe, and high-performance power sources has never been greater. Fiber-shaped energy-storage devices (FSESDs) are particularly attractive for integration into textiles, yet their progress is hindered by the limited ionic conductivity of conventional solid polymer electrolytes. Now, a Korea-based team led by Prof. Jinwoo Lee (KAIST), Prof. Yongho Joo and Dr. Nam Dong Kim (KIST) reports a versatile redox polymer electrolyte that harnesses the fast self-exchange chemistry of 4-hydroxy-TEMPO (HT) to deliver record conductivity and device performance without any traditional active materials.

Why This Redox Polymer Electrolyte Matters

• Ionic Conductivity: HT acts simultaneously as a plasticizer and a redox shuttle, transforming a rigid PVA matrix into a rubbery, highly amorphous network. A quasi-solid HT electrolyte achieves an exceptional 73.5 mS cm⁻¹ at room temperature—>20× higher than PVA–LiClO₄ alone.
• Energy & Power: Symmetric FSESDs using carbon-nanotube/graphene fiber electrodes deliver 25.4 Wh kg^-1 at 25 000 W kg^-1, retaining 17.1 Wh kg^-1 even at 97 000 W kg^-1, outperforming halide-, metal-ion- and hydroquinone-based redox electrolytes.
• Mechanical Durability: The device keeps 91.2 % capacitance after 8 000 bending cycles and 83 % after 10 000 charge/discharge cycles, while tolerating knotting, twisting and crumpling.
• Thermal Resilience: Stable operation from 25 °C to 85 °C with full recovery, owing to the thermal robustness of the nitroxide radical.

Innovative Design & Mechanism

• Self-Exchange Hopping: Equal populations of N–O• and N⁺=O species enable ultrafast bimolecular electron transfer (Marcus-Hush) with an activation energy of only 0.13 eV, providing a dedicated “radical highway” for Li⁺ transport.
• Structure Engineering:
  – CNT core fiber (30 µm) offers 1.2 N tex^-1 strength and 2 945 S m² kg^-1 conductivity.
  – Vertically-graphene shell (69 µm) introduces 3-D porous, oxygen-rich surfaces that anchor HT and shorten ion-diffusion paths.
• Optimal Formulation: T-10 (1 M HT in PVA–LiClO₄) maximizes amorphous domains, minimizes crystallization, and balances charge-carrier proximity, yielding the lowest bulk resistance (8.5 Ω) and charge-transfer resistance (25 Ω) in the series.

Device Performance in Detail

• CV at 1 000 mV s^-1 retains symmetrical redox peaks, evidencing rapid kinetics.
• Specific capacitance: 183 F g^-1 (@ 50 A g^-1), ~8× that of HT-free control.
• Rate capability: 71 % retention when current rises to 200 A g^-1.
• Series/parallel modules scale linearly, demonstrating wearable-pack potential.

Challenges & Future Outlook

The team emphasizes that rational tuning of radical-to-radical-cation ratio and suppression of HT crystallization are critical for maintaining high conductivity. Next steps include:

• Exploring other TEMPO derivatives (amino-, carboxy-) and alternative polymer hosts for even lower Eₐ and wider voltage windows.
• Integrating with high-capacity fiber electrodes (MXene, CP@NiCo-LDH) to push energy density beyond 50 Wh kg^-1.
• Developing continuous coating processes for kilometer-scale fiber production compatible with textile machinery.

This work spotlights how molecular-level design of redox mediators can simultaneously resolve the conductivity, flexibility and safety bottlenecks of solid electrolytes, paving a practical route toward next-generation wearable power systems. Watch for further fiber-device innovations from the KIST-KAIST collaboration!