Double-Reinforced Nano-Sized Ferrosoferric Oxide/Carbon Core–Shell Nanorods Enabling Durable Sodium-Ion Hybrid Capacitors

Sodium-ion hybrid capacitors (SIHCs) represent a promising option for cost-effective grid-scale energy storage due to their combination of high energy and power densities, as well as excellent cycle stability. However, the practical application of SIHCs is hindered by the lack of advanced anode materials that exhibit fast ion diffusion kinetics and robust structures. Herein, a novel design featuring a nano-sized Fe₃O₄ is developed, that is double-reinforced by porous carbon derived from metal-organic frameworks (MOFs) as the inner core support and N, P-co-doped carbon from a polymer decomposition as the outer shell, resulting in a robust pencil-like core–shell structural composite (Fe₃O₄/NPC). The Fe₃O₄ nanograins and abundant surface groups containing N and P reduce the charge/electron transfer distance and provide numerous pseudocapacitive active sites, guaranteeing high energy output and superior rate capability. The optimized core–shell structure and interconnected carbon framework effectively accommodate volume changes, prevent nanoparticle agglomeration, and facilitate ion/electron transport, thereby ensuring structural integrity and rapid kinetics. In testing, Fe₃O₄/NPC demonstrated superior cycling durability, retaining 86.6% of its initial capacity after 2500 cycles in sodium-ion batteries (SIBs). Impressively, the assembled SIHC achieved a notable energy density of 147.1 W h kg⁻¹ and maintained 92% capacity after 8000 cycles.