Synthesis of Porous and Micro-sized LiFePO4/C by a Two- step Crystallization Process and Its Application to Cathode Material in Li-ion Batteries

LiFePO4 with an ordered olivine structure has been recognized as a promising cathode material for advanced Li-ion batteries due to its excellent thermal and structural stability, low cost of starting materials, high reversibility of Li ion insertionextraction, and non-toxicity . However, its practical application has suffered from the inherently poor kinetic properties caused by the low electronic and ionic transfer (σ e = 10– 1 Scm– , DLi+ = 10 cms at R.T.) in the LiFePO4 lattice structure . Recently, an improvement in the kinetic property of the LiFePO4 has been accomplished by reducing its particle size to nanoscale . Conversely, though a nano-structured LiFePO4 shows an improved rate-capability, the gravimetric/volumetric energy density of the LiFePO4 electrode is inevitably reduced; that is, the tap density of the LiFePO4 decreases as the particle size decreases, and the mass fraction of bulky and inactive conducting carbon increases to secure the electrical contact between the LiFePO4 nano-particles . Thus, it is necessary to develop high performance LiFePO4/C with high energy density. To realize LiFePO4/C composites with high energy density and high power density, porous and micro-sized (~10 μ m) LiFePO4/C was synthesized by a novel two-step crystallization process, which involves growth technology using LiFePO4 nano-crystals prepared by a hydrothermal process as seed crystals for secondary particle growth. The effects of synthesis routes (Solution-based and two-step crystallization process) on the crystals structure and electrochemical performance of LiFePO4/C composites were examined. It is demonstrated that the cyclic retention property of the porous and micro-sized LiFePO4/C prepared by the two-step crystallization process is superior to that of the LiFePO4/C prepared by the other process. In addition, the improved kinetic property of the porous LiFePO4/C results from the enlarged reaction sites for Li ion and the short Li ion diffusion length in the porous structure.