Synthesis and electrochemical characterization of LiFePO4/C composites prepared by the microemulsion method

Electroactive LiFePO4/C composite powders were successfully prepared by the microemulsion process under controlled conditions. A series of residual carbons on the surface of prepared LiFePO4 materials originating from the pyrolysis of various salt concentrations, Hydrophile-Lipophile-Balance (HLB) values, and structures of organic surfactants were characterized. All samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectral analysis, BET surface area, cyclic voltammetry (CV) and charge–discharge tests. The performance of the LiFePO4/C composite powders displayed a strong relationship between the emulsion conditions and structures of surfactants. Lower salt concentrations of the aqueous cores or a smaller HLB numbers cause the formation of small aqueous cores resulting in smaller particle sizes of LiFePO4/C composites after calcinating the cores. The molecular structures of surfactant affect the formation of micelle size and further influence the behavior of the LiFePO4/C composite. The OP-7 surfactant contains more branched structures on the lipophilic groups causing the formation of small aqueous cores and particle sizes of LiFePO4 after calcination of the cores. Small aqueous cores formed smaller particles with lower ID/IG values after pyrolysis yielding composites with higher discharge capacities.