Microwave-assisted polyol synthesis of LiMnPO4/C and its use as a cathode material in lithium-ion batteries

Abstract

We synthesized LiMnPO4/C with an ordered olivine structure by using a microwave-assisted polyol process in 2:15 (v/v) water–diethylene glycol mixed solvents at 130°C for 30min. We also studied how three surfactants—hexadecyltrimethylammonium bromide, polyvinylpyrrolidone k30 (PVPk30), and polyvinylpyrrolidone k90 (PVPk90)—affected the structure, morphology, and performance of the prepared samples, characterizing them by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, charge/discharge tests, and electrochemical impedance spectroscopy. All the samples prepared with or without surfactant had orthorhombic structures with the Pnmb space group. Surfactant molecules may have acted as crystal-face inhibitors to adjust the oriented growth, morphology, and particle size of LiMnPO4. The microwave effects could accelerate the reaction and nucleation rates of LiMnPO4 at a lower reaction temperature. The LiMnPO4/C sample prepared with PVPk30 exhibited a flaky structure coated with a carbon layer (∼2nm thick), and it delivered a discharge capacity of 126mAh/g with a capacity retention ratio of ∼99.9% after 50 cycles at 1C. Even at 5C, this sample still had a high discharge capacity of 110mAh/g, demonstrating good rate performance and cycle performance. The improved performance of LiMnPO4 likely came from its nanoflake structure and the thin carbon layer coating its LiMnPO4 particles. Compared with the conventional polyol method, the microwave-assisted polyol method had a much lower reaction time.