Synthesis and electrochemical characterization of LiFePO 4/C composites prepared by the microemulsion method

Abstract

Electroactive LiFePO 4/C composite powders were successfully prepared by the microemulsion process under controlled conditions. A series of residual carbons on the surface of prepared LiFePO 4 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 LiFePO 4/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 LiFePO 4/C composites after calcinating the cores. The molecular structures of surfactant affect the formation of micelle size and further influence the behavior of the LiFePO 4/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 LiFePO 4 after calcination of the cores. Small aqueous cores formed smaller particles with lower I D/ I G values after pyrolysis yielding composites with higher discharge capacities.