“Grafting” NiSe onto Cu2-xSe with twinborn structure embedded in carbon-based nanofibers to weave freestanding sodium-ion storage electrode

The fabrication of freestanding electrodes for Na + storage is necessary to achieve high energy density. However, the large radius of Na + results in a large volume fluctuation and sluggish reaction kinetics of active materials, particularly at a high active material content, thereby impeding electrochemical performance with undesirable cycling performance or rate capability. In this study, a freestanding electrode based on the “NiSe grafted on Cu 2-x Se” heterostructure with double-carbon protective shells (NiSe/Cu 2-x Se@C@NCNFs) was successfully constructed for Na + storage. In this microstructure, N -doped carbon nanofibers (NCNFs) serve as the stem of the twinborn NiSe/Cu 2-x Se heterostructure with a built-in electric field, where NiSe improves Na + absorption and Cu 2-x Se enhances Na + diffusion. The “graft” design enabled the freestanding NiSe/Cu 2-x Se@C@NCNFs electrode with a high active mass content of 76.1 wt% to exhibit superior electrochemical performance for Na + storage (75 mAh g −1 at 2 A g −1 ) compared to those of Cu 2-x Se@C@NCNFs (26 mAh g −1 at 2 A g −1 ) and NiSe@C@NCNFs (9 mAh g −1 at 2 A g −1 ).