Core-shell carbon composite material as anode materials for lithium-ion batteries

The core–shell structured humic acid (HA)-based carbon composites were synthesized through liquid-phase coating coupled with further carbonization treatment at 900 °C. The crystal structures and morphologies of samples were explored by scanning and transmission electron microscopy, X-ray diffraction, and Raman spectra, revealing that graphitized HA (GHA) has the same microstructure as microcrystal graphite, and intercalation/de-intercalation of lithium ion can be profitable. Meanwhile, a uniform carbon shell and distinct boundary between the GHA core and carbon shell can be noticed in a carbon coating GHA (CGHA) composite. By electrochemical characterizations, the first charge and discharge capacities of the 10% CGHA composite electrode are 463.0 and 597.1 mAh/g at a current rate of 0.1 C, respectively, with an initial coulombic efficiency of 77.5%. Moreover, 10% CGHA composite electrode continues to deliver a reversible specific capacity of 251.7 mAh/g at the current rate of 2 C even after 50 cycles. An enhanced rate capability is attained, with a reversible specific capacity of 173.5 mAh/g at a high current rate of 5 C. The electrochemical performance is improved because the amorphous carbon coating layer can effectually enhance the electrical conductivity, buffer the volume expansion during charging and discharging, and maintain the stability of the core–shell structure. These results suggest that the GCHA composites can be applied as a potential electrode material to prepare excellent performance and low-cost lithium-ion batteries (LIBs).