Yolk–Shell NiS₂ Nanoparticle-Embedded Carbon Fibers for Flexible Fiber-Shaped Sodium Battery

Fiber-shaped rechargeable batteries hold promise as the next-generation energy storage devices for wearable electronics. However, their application is severely hindered by the difficulty in fabrication of robust fiber-like electrodes with promising electrochemical performance. Herein, yolk–shell NiS₂ nanoparticles embedded in porous carbon fibers (NiS₂⊂PCF) are successfully fabricated and developed as high-performance fiber electrodes for sodium storage. Benefiting from the robust embedded structure, 3D porous and conductive carbon network, and yolk–shell NiS₂ nanoparticles, the as-prepared NiS₂⊂PCF fiber electrode achieves a high reversible capacity of about 679 mA h g⁻¹ at 0.1 C, outstanding rate capability (245 mA h g⁻¹ at 10 C), and ultrastable cycle performance with 76% capacity retention over 5000 cycles at 5 C. Notably, a flexible fiber-shaped sodium battery is assembled, and high reversible capacity is kept at different bending states. This work offers a new electrode-design paradigm toward novel carbon fiber electrodes embedded with transition metal oxides/sulfides/phosphides for application in flexible energy storage devices.