Preparation And Characterization Of The LiFePO₄/C Composite

Olivine-structured LiFePO₄is considered as a promising cathode material forlithium-ion batteries，due to its advantages such as relatively high capacity,abundant rawmaterials，environmentally benign，inexpensive price and high safety. However，itspower performance is greatly limited by slow diffusion of lithium ions across thetwo-phase boundary and its low electronic conductivity. For high performance LiFePO₄,A green biomimetic way is sought to produce this compound, which is not only simplebut also low in cost. LiFePO₄nuclei/carbon nanocage （LFPN/CNC） mesoporousmicrospheres are synthesized by using yeast cells as both a structural template and abiocarbon source. We find that yeast cells regulate and control the mineralization ofFePO₄nanoparticles at the molecular level. The FePO₄nanoparticles combine with thebiomolecules, become encapsulated inside the protein cage and then self-assembledboth on the yeast cell wall surface and inside the cell by electrostatic interaction andmetabolism regulation. Then by means of an in-situ nanocomposite carbonization andcrystallization heat treatment, LFPN/CNC mesoporous microspheres are synthesized.A microsphere （2⁸μm） is composed of spherical densely aggregated LFPN/CNCparticles （20⁴0nm） and mesoporous biocarbon nets （15.28wt.%） completely coatingthe LiFePO₄surface. A LFPN/CNC particle contains a LiFePO₄core with a size ofabout3⁵nm and a mesoporous biocarbon nanocage with a uniform thickness of about2nm. The hierarchical mesoporous structure （3¹5nm） allows lithium ions to easilypenetrate into the microspheres. The LFPN/CNC mesoporous microspheres provideboth high power and long cycling life. We have revealed the possible lithium pathwaysin LFPN/CNC mesoporous microspheres and established a relation between thestructure and the ionic and electronic transport properties. We prepared this LFPN/CNCmesoporous microspheres successfully by chemical precipitation, sol-gel andhydrothermal method, and finally we discuss the impact on the performance of theLiFePO₄materials with the magnesium ions doping. The materials prepared by thechemical precipitation method shows the most superior performance, and this simpleand potentially universal design strategy is currently being pursued in the synthesis ofan ideal cathode-active material for high power applications.