Enhancement of electrochemical performances for LiFePO4/C with 3D-grape-bunch structure and selection of suitable equivalent circuit for fitting EIS results

The LiFePO4/C composite with 3D-grape-bunch structure is successfully synthesized through a novel hydrothermal method. Sucrose is used as in-situ coating carbon source, and the hydroxylated MWCNTs are used as connecting carbon wires which can be embedded into the carbon coating via self-assembling of the hydrophilic groups to form 3D-grape-bunch structure. The influences of the 3D-grape-bunch structure on the morphology, structure and electrochemical performance of the LiFePO4/C composites are investigated by XRD, SEM, TEM, BET, galvanostatic charge/discharge tests, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) tests. Especially, four kinds of the equivalent circuit models usually employed to analysis the EISs of LiFePO4 as cathode material for Li-ion battery are discussed, and the suitable equivalent circuit for fitting EIS of LiFePO4/C composite with 3D-grape-bunch structure is selected. The optimal LiFePO4/C composite with 3D-grape-bunch structure owing to its good conductive network and high graphitic degree (low ID/IG value) of residual carbon exhibits a stable and high reversible capacity of 160.5 mAh g−1 at 0.1C and 108.4 mAh g−1 even at 10.0C, and the cycling capacity retention rate reaches 99.9% over 60 cycles. Moreover, it also exhibits high conductivity, good reversibility and excellent stability in EIS and CV tests.