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 Fe 3 O 4 nanosheets are embedded in nitrogen-doped carbon nanofibers (hexa-Fe 3 O 4 @N-CNF), boosting the Li + diffusion kinetics and maintaining cycling stability. The ultrathin hexa-Fe 3 O 4 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-Fe 3 O 4 during Li + storage, thereby enhancing the cyclic stability of hexa-Fe 3 O 4 @N-CNF. The compressed anode of hexa-Fe 3 O 4 @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.