NiCo ₂ S ₄ Nanosheet–Nanoneedle Homojunctions as Electrode Material for High-Performance Supercapacitors

Engineering the interfacial properties of electrode materials at the atomic level is a promising yet challenging route to break performance limits for next-generation supercapacitors. Herein, we demonstrate a rational strategy of crystal facet engineering to construct an intrinsic homojunction within nickel cobalt sulfide (NiCo 2 S 4, NCS), where one-dimensional (220)-oriented nanoneedles (NN) are seamlessly integrated with two-dimensional (111)-oriented nanosheets (NS), forming a unique NCS-NN/NS homojunction. The obtained electrode delivers an outstanding specific capacitance of 12.1 F cm –2 (1630.6 F g –1 ) at 2 A g –1 and maintains 8.12 F cm –2 at 10 A g –1, showcasing excellent rate capability. Moreover, it exhibits remarkable long-term stability with 87.6% capacitance retention after 5000 cycles. Experimental analyses combined with first-principles calculations reveal that the enhanced performance originates from the synergistic effects of the facet homojunction. The interfacial charge transfer establishes a built-in electric field that facilitates charge transport, while the resultant electron-deficient surface optimizes the adsorption energetics for hydroxide ions, thereby accelerating the surface redox kinetics. This work provides a profound insight into performance enhancement through crystal facet engineering within homojunctions, offering a design principle for high-performance energy storage materials.