In-situ encapsulation of core-shell structured Co@Co3O4@CNOs as anode for lithium-ion batteries with high rate performance

Abstract Core-shell structured material consisting of Co@Co3O4 core encapsulated in carbon nano-onions (CNOs) shell using 1-hexyl-3-methyl imidazolium dilyanamide ([HMIm]N(CN)2) as carbon source were synthesized by high-temperature carbonization and hydrogen peroxide (H2O2) oxidation. In the process of high-temperature carbonization, cobalt ions were in-situ reduced to metallic cobalt, producing core-shell structured Co@CNOs. Subsequently, Co@Co3O4@CNOs was obtained using H2O2 oxidation, in which parts of the metallic cobalt were in-situ oxidized to Co3O4, and simultaneously the carbon material was not lost. The structural characterizations indicating that the Co@Co3O4@CNOs was composed of three crystalline phases, with a higher degree of graphitization and a larger specific surface area. When applied to lithium-ion batteries anode, Co@Co3O4@CNOs showed superior reversible capacity (865 mAh g−1 after 500 cycles at 0.5C) and rate performance (403 mAh g−1 after 1500 cycles at 10C). Its superior electrochemical performance was attributed to CNOs, nitrogen doping, rich mesopores, and the synergistic effect between Co3O4 and conductive metallic cobalt, uniformly dispersed Co@Co3O4 not only have excellent structural stability but also have improved electrical conductivity and ion transfer.