LiFePO4 batteries with enhanced lithium-ion-diffusion ability due to graphene addition

Abstract

In this study, graphene was added to LiFePO4 via a hydrothermal method to improve the lithium-ion-diffusion ability of LiFePO4. The influence of graphene addition on LiFePO4 was studied by X-ray diffraction (XRD), field emission scanning electron microscopy, transmission electron microscopy, cyclic voltammetry, cycling test, and AC impedance analysis. The addition of graphene to LiFePO4 resulted in the formation of a LiFePO4–graphene composite; XRD observations revealed the composite to have a single phase with an olivine-type structure. Furthermore, LiFePO4 particles in the composite were stacked on the graphene sheet surface, thereby enabling the composite to form an effective conducting network and facilitate the penetration of the surface of active materials by an electrolyte. The lithium-ion-diffusion ability of the LiFePO4–graphene composite was greater than that of pure LiFePO4. Of a number of materials studied [namely, pure LiFePO4, LiFePO4–graphene (1 %), LiFePO4–graphene (5 %), and LiFePO4–graphene (8 %)], LiFePO4–graphene (5 %) delivered the best electrochemical performance with a lithium-ion-diffusion coefficient of 8.18 × 10−12 cm2 s−1 and the highest specific discharge capacity of 149 mAh g−1 at 0.17 C; in contrast, the corresponding values for pure LiFePO4 were 3.01 × 10−12 cm2 s−1 and 109 mAh g−1, respectively. Further, LiFePO4–graphene (5 %) showed a very high specific discharge capacity of 170 mAh g−1 at 0.1 C, which is equal to the theoretical capacity of LiFePO4.