Dimensionally Modulated, Single-Crystalline LiMPO4 (M= Mn, Fe, Co, and Ni) with Nano-Thumblike Shapes for High-Power Energy Storage

Abstract

We demonstrate an efficient and rapid microwave irradiated solvothermal method to prepare nanostructured lithium metal phosphates LiMPO(4) (M = Mn, Fe, Co, and Ni) within a short reaction time (5-15 min) at temperatures as low as 300 degrees C without requiring any post annealing at elevated temperatures. The highly viscous, high-boiling tetraethyleneglycol used as the solvent not only provides a reducing atmosphere to prevent the oxidation of M(2+) to M(3+) but also inhibits the agglomeration of the nanoparticles formed. The enhanced reaction rates facilitated by the dielectric volumetric heating of the microwave absorbing reactants led to the formation of highly crystalline, phase-pure LiMPO(4) powders. The samples are characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and electrochemical measurements in lithium cells. High-resolution TEM studies reveal the formation of single-crystalline LiMPO(4) with nano-thumblike shapes. The dimensionally modulated nano-thumblike shapes with the lithium diffusion direction (b axis) along the shorter dimension are particularly beneficial to achieve high-power capability in lithium ion cells. Subsequent networking of the single-crystalline LiMPO(4) nano-thumps with multiwalled carbon nanotubes by a simple solution-based mixing at ambient temperatures to overcome the electronic conductivity limitations offers excellent electrochemical performance in lithium ion cells.