A g-C₃N₄ self-templated preparation of N-doped carbon nanosheets@Co-Co₃O₄/Carbon nanotubes as high-rate lithium-ion batteries' anode materials

A novel N-doped graphene-like carbon nanosheets (CNs) and carbon nanotubes (CNTs)-encapsulated Co-Co₃O₄ nanoparticles (NPs) (CN@Co-Co₃O₄/CNTs) were synthesized successfully by a simple hydrothermal and annealing method with graphite carbon nitride (g-C₃N₄) as self-template. By annealing Co²⁺/g-C₃N₄ under N₂ atmosphere, g-C₃N₄ was transformed into CN/CNTs, and Co²⁺ was reduced into CoNPs which were embedded in CNs. Further annealing in air, a shell of Co₃O₄ was formed around CoNPs. The amount of CNs, CNTs, and CoNPs can be adjusted by changing the ratio of Co²⁺ in Co²⁺/g-C₃N₄. The graphene-like CNs provided a large number of active sites and a large specific surface area for loading lots of small CoNPs uniformly. The CNTs with a diameter of 100 nm could not only improve the conductivity but also provide a buffer space for the aggregation and volume expansion of Co₃O₄. CNTs also enlarged the interlayer distance of CNs, which prevented the re-stacking of CNs and provided great convince for the intercalation and de-intercalation of Li⁺. When applied for anode material of lithium-ion batteries, CN@Co-Co₃O₄/CNTs exhibited a high discharge capacity of 460.0 mAh g^-1 at 5000 mA g^-1 after 300 cycles with a Coulombic efficiency of 98% and excellent higher-rate capacity (401.0 mAh g^-1 at 2000 mA g^-1 and 329.0 mAh g^-1 at 5000 mA g^-1).