Reverse-microemulsion preparation and characterization of ultrafine orthorhombic LiMnO2 powders for lithium-ion secondary batteries

Ultrafine orthorhombic LiMnO2 (o-LiMnO2) were successfully synthesized using a newly developed reverse-microemulsion (RμE) process. To prepare o-LiMnO2 powders with a rock salt structure, precise control of the oxygen content in the heating atmosphere was required. Monophasic o-LiMnO2 was obtained at as low as 700 °C. Not only the reaction temperature was lowered, the reaction duration for synthesizing the desired powders was also markedly shortened via the RμE route. The average particle size of the 900 °C-calcined powders was measured to be around 90 nm. The discharge capacities of the prepared o-LiMnO2 powders significantly increased in the initial stages, and rapidly reached a saturated plateau. The impedance spectroscopy analysis revealed that the chemical diffusion coefficient of lithium ions in o-LiMnO2 was markedly greater than that in LiMn2O4-based materials. The high diffusion rate of lithium ions in o-LiMnO2 is attributed to the high crystallinity as well as the nanosize of the powders synthesized via this RμE process.