Fabrication of CoFe2O4 and NiFe2O4 nanoporous spheres as promising anodes for high performance lithium-ion batteries

CoFe2O4 and NiFe2O4 nanoporous spheres are fabricated using a facile solvothermal method followed by post-thermal treatment, and their potential applications in lithium-ion batteries are explored in this work. The obtained nanoporous spheres, assembled from a large amount of secondary small particles, have a uniform size distribution with diameters in the range of 300-500 nm. These nanoporous CoFe2O4 and NiFe2O4 are composed of numerous nanoparticles and form a channel structure with a width in the range of 10-20 nm. Furthermore, the growth mechanism of nanoporous spheres is proposed based on the contrast experiments. The important applications of the developed CoFe2O4 and NiFe2O4 nanoporous spheres in lithium ion batteries have been demonstrated by using them as anode materials. At a current density of 1000 mA g(-1), lithium-ion batteries based on CoFe2O4 and NiFe2O4 nanoporous spheres show capacities of 300 mA h g(-1) and 180 mA h g(-1) after 1000 cycles, respectively. Even at a current density of 3000 mA g(-1), their capacities still reach 255 mA h g(-1) and 173 mA h g(-1) after 1000 cycles. Although CoFe2O4 and NiFe2O4 exhibited excellent long cycling life stability at high current densities, it was found that the capacity and the ratio of capacity retention of CoFe2O4 are more superior to those of NiFe2O4. The reasons have been explained from these structural features. It is believed that the aforementioned excellent performance of lithium-ion batteries arises from the unique porous feature of the developed nanoparticles. That is, well-defined porous nanostructures could buffer the volume expansion, shorten the diffusion path during insertion/extraction processes of Li+ in the electrode materials, increase the electrolyte/electrode contact area, and provide more insertion/extraction sites of Li+.