Coordination Polymers-Derived Three-Dimensional Hierarchical CoFe₂O₄ Hollow Spheres as High-Performance Lithium Ion Storage

Hierarchical CoFe₂O₄ (CFO) hollow spheres were successfully synthesized via solvothermal method and calcination treatment. The obtained CFO completely inherited the hollow structure and spherical morphology of its precursor of cobalt-based ferrocenyl coordination polymers (Co-Fc-CPs). The three-dimensional (3D) porous hierarchical hollow structure can not only promote the permeation of electrolyte and shorten the lithium-ion transfer distance but also provide a cushion for the volume change during insertion/extraction of lithium ions. To improve the electrochemical properties, the CFO was combined with two forms of carbonaceous materials to controllably obtain 3D CoFe₂O₄@C (CFO@C) and CoFe₂O₄@reduced graphene oxide (CFO@rGO) composites. Compared with bare CFO and CFO@C, CFO@rGO exhibited a superior electrochemical performance, achieving a high specific capacity of 933.1 mA h g^-1 at a current density of 100 mA g^-1 after 100 cycles and showing an outstanding cycling life with a capacity of 615.6 mA h g^-1 at 1000 mA g^-1 after 600 cycles. In situ X-ray diffraction technique was applied to investigate the lithium storage mechanism during discharge/charge processes. This work provides a new approach to prepare hierarchical hollow bimetallic oxides composites for lithium-ion anode materials.