Polyacene coated carbon/LiFePO4 cathode for Li ion batteries: Understanding the stabilized double coating structure and enhanced lithium ion diffusion kinetics

Graphical abstract: Schematic diagrams of the double coating process of PAS/carbon coated LiFePO4. -- Highlights: • The LiFePO4 synthesized by hydrothermal route using starch as surfactant has smaller particle size compared with that synthesized by solid state method. • The starch acts as constrained nano-layer restricting the growth of the LiFePO4 particles. • The polyacene/carbon double layers are coated on the surface of LiFePO4 successfully. • The polyacene/carbon double coated LiFePO4 exhibits better capacity retention with 99.7% at 1C after 50 cycles. -- Abstract: Polyacene (PAS)/carbon coated lithium iron phosphate composites were synthesized with starch as surfactant using hydrothermal route at 180 °C for 5 h followed by calcining phenolic resin surface coated LiFePO4 at 750 °C for 6 h. X-ray powder diffraction (XRD) was performed to investigate the structure and phase purity of all samples. The results of Fourier transform infrared spectroscopy (FTIR) and transmission electron microscope (TEM) characterization show that the polyacene (PAS) and carbon double layer are coated on the surface of LiFePO4 successfully. The sample synthesized in hydrothermal route shows small particle size (100–150 nm) as demonstrated in scanning electron microscopy (SEM) images, which can be ascribed to the coated starch nano-layer restricting the growth of the LiFePO4 particles. Electrochemical impedance spectroscopy as well as charge and discharge tests was carried out to investigate the electrochemical performance of all samples. High initial discharge capacity (161.7 mAh g−1 at 0.2C) and good cycling stability are observed in PAS/carbon double coated LiFePO4 synthesized using the hydrothermal route. Compared with the S-PLFP, the lower charge transfer resistance (Rct) and the higher lithium ion diffusion coefficient of the H-PCLFP can be ascribed to the double coating layer (PAS and carbon) on the surface of H-PCLFP and the small grain size