Heterogeneous interface in hollow ferroferric oxide/ iron phosphide@carbon spheres towards enhanced Li storage

Heterogeneous interface and structural engineering play important roles for electrochemical performance of lithium-ion batteries. Herein, heterostructures of hollow Fe₃O₄/FeP spheres coated with carbon shell (H-Fe₃O₄/FeP@C) are designed to enhance lithium storage performance. As bifunctional anode materials, the H-Fe₃O₄/FeP@C spheres show the good rate performance with 458.4 mAh g^-1 at 5 A g^-1 and long-cyclic performance (630.2 mAh g^-1 at 2.0 A g^-1 after 1000 cycles). Density functional theory calculations demonstrate that the heterogeneous interfaces from (311) plane of Fe₃O₄ and (002) plane of FeP possess high charge density and distinct metallic character, which can improve the conductivity, increase the adsorption energy, provide more active sites and reduce the transfer barrier of ions and electrons. Besides, hollow structure of H-Fe₃O₄/FeP@C not only alleviates the volume expansion during lithiation/delithiation process but also shortens the diffusion distance of Li ions. In addition, the ex-situ X-ray diffraction and X-ray photoelectron spectroscopy are used to reveal the electrochemical Li storage mechanisms of H-Fe₃O₄/FeP@C. This work provides a novel route for design and preparation of Fe-based heterostructures for various energy storage systems in the future.