Multiconstituent synthesis of LiFePO₄/C composites with hierarchical porosity as cathode materials for lithium ion batteries

Monolithic, three-dimensionally ordered macroporous and meso-/microporous (3DOM/m) LiFePO₄/C composite cathodes for lithium ion batteries were synthesized by a multiconstituent, dual templating method. Precursors containing sources for lithium, iron, and phosphate, as well as a phenol-formaldehyde sol and a nonionic surfactant were infiltrated into a colloidal crystal template. Millimeter-sized monolithic composite pieces were obtained, in which LiFePO ₄ was dispersed in a carbon phase around an interconnected network of ordered macropores. The composite walls themselves contained micropores or small mesopores. The carbon phase enhanced the electrical conductivity of the cathode and maintained LiFePO₄ as a highly dispersed phase during the synthesis and during electrochemical cycling. Monoliths containing 30 wt % C were electrochemically cycled in a 3-electrode cell with lithium foil as counter and reference electrodes. No additional binder or conductive agent was used. The capacity was as high as 150 mA h g^-1 at a rate of C/5, 123 mA h g^-1 at C, 78 mA h g^-1 at 8C, and 64 mA h g^-1 at 16C, showing no capacity fading over 100 cycles. In spite of the low electronic conductivity of bulk LiFePO₄ (10^-9-10^-10 S cm^-1), the monolithic LiFePO₄/C composite was able to support current densities as high as 2720 mA g^-1.