Plasma-engineered FeSe₂-encapsulated carbon composites with enhanced kinetics for high-performance lithium and sodium ion batteries

Iron selenides with high capacity and excellent chemical properties have been considered as outstanding anodes for alkali metal-ion batteries. However, its further development is hindered by sluggish kinetics and fading capacity caused by volume expansion. Herein, a series of FeSe₂ nanoparticles (NPs)-encapsulated carbon composites were successfully synthesized by tailoring the amount of Fe species through facile plasma engineering and followed by a simple selenization transformation process. Such a stable structure can effectively mitigate volume changes and accelerate kinetics, leading to excellent electrochemical performance. The optimized electrode (FeSe₂@C₂) exhibits outstanding reversible capacity of 853.1 mAh g⁻¹ after 150 cycles and exceptional rate capacity of 444.9 mAh g⁻¹ at 5.0 A g⁻¹ for Li⁺ storage. In Na⁺ batteries, it possesses a relatively high capacity of 433.7 mAh g⁻¹ at 0.1 A g⁻¹ as well as good cycle stability. The plasma-engineered FeSe₂@C₂ composite, which profits from synergistic effect of small FeSe₂ NPs and carbon framework with large specific surface area, exhibits remarkable ions/electrons transportation abilities during various kinetic analyses and unveils the energy storage mechanism dominated by surface-mediated capacitive behavior. This novel cost-efficient synthesis strategy might offer valuable guidance for developing transition metal-based composites towards energy storage materials.