“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 Cu2-xSe" heterostructure with double-carbon protective shells (NiSe/Cu2-xSe@C@NCNFs) was successfully constructed for Na+ storage. In this microstructure, N-doped carbon nanofibers (NCNFs) serve as the stem of the twinborn NiSe/Cu2-xSe heterostructure with a built-in electric field, where NiSe improves Na+ absorption and Cu2-xSe enhances Na+ diffusion. The "graft" design enabled the freestanding NiSe/Cu2-xSe@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 Cu2-xSe@C@NCNFs (26 mAh g−1 at 2 A g−1) and NiSe@C@NCNFs (9 mAh g−1 at 2 A g−1).