Fe₇Se₈ Nanostructures with Carbon Skeleton and Core–Shell Design As Anodes for Lithium Ion Batteries

Extensive research has been conducted on high-capacity anodes as potential replacements for graphite in lithium-ion batteries driven by the desire to achieve higher energy density. Among them, transition metal selenides have attracted much attention but suffer from poor structural stability due to the large volume expansion after Li + insertion. In this work, we first encapsulate Fe 7 Se 8 nanoparticles in a carbon skeleton (Fe 7 Se 8 /C) and then coat Fe 7 Se 8 /C with N-doped carbon (NC) to form a core–shell structure with a void gap created between the Fe 7 Se 8 /C core and NC shell (Fe 7 Se 8 /C-NC). The carbon skeleton alleviates the volume expansion of Fe 7 Se 8 nanoparticles and increases the electronic conductivity, while the NC coating layer maintains the structural integrity of Fe 7 Se 8 /C and improves the electrolyte penetration. Because of this special nanostructure, the Fe 7 Se 8 /C-NC electrode shows ultrahigh capacities of 1176, 749.3, and 692.4 mAh g –1 at current densities of 0.1, 1, and 2 A g –1 and maintains high capacities of 457.8 and 324.5 mAh g –1 after 300 and 800 cycles at 2 A g –1, respectively. This multilevel protection strategy makes Fe 7 Se 8 a promising candidate for next generation anodes and provides inspiration for the design of other anodes.