Effect of carbon coating on cycle performance of LiFePO4/C composite cathodes using Tween80 as carbon source

Highlights: • The Tween80 addition could enhance cycle stability of LiFePO4 material. • The FTIR spectrum confirms Tween80 surfactant can bond with LiFePO4 particles. • Some chemical bonds between material and carbon layer still exist after sintering. - Abstract: The influence of carbon coating on the cycle performance of LiFePO4/C composite cathodes using polyoxyethylenesorbitan monooleate (Tween80) as carbon source against lithium metal foil anode for Li-ion batteries was investigated in this paper. According to Infrared spectrum analysis (FTIR), the Tween80 surfactant molecules bond to the surface of LiFePO4 and form an adsorption layer, which contribute to the formation of a homogeneous carbon layer tightly coating on the surface of LiFePO4 particles in the process of sintering, due to a strong binding force provided by surface chemical bonds. The transmission electron microscopy (TEM) shows that the carbon layer around LiFePO4 using Tween80 as carbon source still coating on the surface of LiFePO4 after 200 cycles at 5 C rate while the carbon layer shed from the surface of LiFePO4 using glucose as carbon source. As a result, the carbon-coated LiFePO4 using Tween80 as carbon source exhibits much higher capacity retention than the sample using glucose as carbon source. Electrochemical impedance measurement (EIS) reveals that the carbon-coated LiFePO4 electrode using Tween80 surfactant has a lower charge transfer resistance than the electrode using glucose as carbon source electrode after 100 and 200 cycles at 5 C rate