Double-Shelled Fe-Fe₃C Nanoparticles Embedded on a Porous Carbon Framework for Superior Lithium-Ion Half/Full...

Cost-effective and environmentally friendly Fe-based active materials offer exceptionally high energy capacity in lithium-ion batteries (LIBs) due to their multiple electron redox reactions. However, challenges, such as morphology degradation during cycling, cell pulverization, and electrochemical stability, have hindered their widespread use. Herein, we demonstrated a simple salt-assisted freeze-drying method to design a double-shelled Fe/Fe₃C core tightly anchored on a porous carbon framework (FEC). The shell consists of a thin Fe₃O₄ layer (≈2 nm) and a carbon layer (≈10 nm) on the outermost part. Benefiting from the complex nanostructuring (porous carbon support, core-shell nanoparticles, and Fe₃C incorporation), the FEC anode delivered a high discharge capacity of 947 mAh g^-1 at 50 mA g^-1 and a fast-rate capability of 305 mAh g^-1 at 10 A g^-1. Notably, the FEC cell still showed 86% reversible capacity retention (794 mAh g^-1 at 50 mA g^-1) at a high cycling temperature of 80 °C, indicating superior structural integrity during cycling at extreme temperatures. Furthermore, we conducted a simple solid-state fluorination technique using the as-prepared FEC sample and excess NH₄F to prepare iron fluoride-carbon composites (FeF₂/C) as the positive electrode. The full cell configuration, consisting of the FEC anode and FeF₂/C cathode, reached a remarkable capacity of 200 mAh g^-1 at a 20 mA g^-1 rate or an energy density of approximately 530 Wh kg^-1. Thus, the straightforward and simple experimental design holds great potential as a revolutionary Fe-based cathodic-anodic pair candidate for high-energy LIBs.