Hexagonal Fe₃O₄ Nanosheets Confined within Nitrogen-Doped Carbon Nanofibers for Fast Ion Diffusion Kinetics and Stable Cycling in a Flexible Quasi-Solid-State Lithium-Ion Hybrid Capacitor

Iron-based anode materials usually exhibit an unsatisfactory rate performance and poor cycle stability during Li+ storage. Herein, ultrathin hexagonal Fe3O4 nanosheets are embedded in nitrogen-doped carbon nanofibers (hexa-Fe3O4@N-CNF), boosting the Li+ diffusion kinetics and maintaining cycling stability. The ultrathin hexa-Fe3O4 nanosheets possess a short Li+ diffusion pathway, while N-CNF displays excellent conductivity, both of which contribute to the rapid insertion/extraction of Li+. The iron-induced thermal-catalyzed N-CNF manifests enhanced mechanical stability due to its increased graphitization degree, which effectively alleviates the mechanical stress resulting from the volume change of the hexa-Fe3O4 during Li+ storage, thereby enhancing the cyclic stability of hexa-Fe3O4@N-CNF. The compressed anode of hexa-Fe3O4@N-CNF presents a high mass loading (3.3 mg cm–2) and is coupled with a flexible cathode (activated carbon casted onto N-CNF) to assemble the quasi-solid-state lithium-ion hybrid capacitor (LIHC), enabling the attainment of high volumetric power/energy density (66.5 W h L–1 at 117.9 W L–1 and 38.5 W h L–1 at 11785.7 W L–1). When the LIHC is bent at 180°, its Li+ storage performance illustrates a negligible degradation. Additionally, LIHC features a low self-discharge rate (∼14.2 mV h–1). Overall, the long-term cycle stability, high power/energy density, low self-discharge rate, as well as mechanical robustness render this LIHC a suitable candidate for flexible energy storage applications.