A simple route to fiber-shaped heterojunctioned nanocomposites for knittable high-performance supercapacitors

Fiber-shaped supercapacitors with high energy density have been an active subject of research due to their promising prospect for use in portable and wearable electronics. Herein, we report on a robust two-step strategy for crafting a MgS nanowire-draped NiCo2S4 nanosheet network (i.e., NiCo2S4@MgS nanocomposites) in situ grown on ultrafine flexible stainless steel microwires to render knittable supercapacitors with markedly enhanced performance. The two-step route involves the formation of oxide compounds, followed by their conversion into NiCo2S4@MgS nanocomposites. In sharp contrast to pure NiCo2S4 nanosheets, NiCo2S4@MgS nanocomposites facilitate a rapid charge transport between NiCo2S4 nanosheets and MgS nanowires due to the presence of the interconnected MgS network and manifest a more than two-fold discharging time over that of NiCo2S4. Notably, fiber-shaped asymmetric supercapacitors (denoted as FASCs), assembled by intertwining a NiCo2S4@MgS positive electrode and a FeOOH negative electrode electrodeposited on the same type of stainless steel microwires, deliver a remarkable specific volumetric capacity of 134.4 mA h cm-3, a high energy density of 107.5 mW h cm-3, and a good power density of 1.7 W cm-3 at 1 mA cm-2. More importantly, the FASCs also demonstrate great stability with 87.5% performance retention after 5000 cycles. Such hair-like FASCs enable the successful charging of an electronic bracelet, and can power light-emitting diodes (LEDs) after being woven into fabrics. As such, the two-step strategy in this study may represent a viable means of yielding a variety of metal-containing oxide, sulfide, and nitride networks on stainless steel microhairs for high-performance and light-weight wearable electronics.