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.

Plasma-engineered FeSe2-encapsulated carbon composites with enhanced kinetics for high-performance lithium and sodium ion batteries
    Abstract
        Graphical abstract
    1 Introduction
    2 Experiment
        2.1 Materials
        2.2 Preparation of FeSe2@C via plasma engineering and selenization process
        2.3 Material characterizations
        2.4 Electrochemical tests
    3 Results and discussion
        3.1 Synthesis mechanism and characterizations
        3.2 Electrochemical properties of as-prepared samples
    4 Conclusions
    Acknowledgements 
    References

• Junmei Luo(School of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea)
• Shufeng Bo(School of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea)
• Seohyun Park(School of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea)
• Beom‑Kyeong Park(School of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea)
• Oi Lun Li(School of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea) Corresponding author