Comparison of microwave assisted solvothermal and hydrothermal syntheses of LiFePO4/C nanocomposite cathodes for lithium ion batteries

Highly crystalline LiFePO4 nanorods have been synthesized within a short reaction time of 5-15 min at < 300 degrees C by a novel microwave-solvothermal (MW-ST) process and a microwave-hydrothermal (MS-HT) process. In order to improve the electrical conductivity, both an ex situ carbon coating by heating at 700 degrees C with sucrose the LiFePO4 obtained by the MW-ST method and an in situ carbon coating by carrying out the MW-HT process in presence of glucose (MW-HT carbonization) followed by heating at 700 degrees C have been pursued. The products have been characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and charge-discharge measurements in lithium cells. The MW-ST method offers smaller size nanorods (25 +/- 6 nm width and up to 100 nm length) compared to the MW-HT method (225 +/- 6 nm width and up to 300 nm length). Annealing at 700 degrees C improves the rate capability and cyclability significantly without much particle growth due to an improvement in the structural order of carbon and electronic conductivity. Moreover, the LiFePO4/C nanocomposite obtained by the MW-ST method offers higher initial discharge capacity than that obtained by the MW-HT method due to a smaller particle size, illustrating that both lithium ion diffusion and electronic conductivity play a critical role in controlling the electrochemical properties.