Nanocrystalline materials obtained by using a simple, rapid method for rechargeable lithium batteries

Nanocrystalline oxides with either spinel (s.g. Fd3m) or layered (s.g. R3m) structures suitable as cathodic materials for lithium cells were prepared by using a simple, rapid method based on the thermal decomposition of mixed nanocrystalline oxalates formed by grinding hydrated salts and oxalic acid. Their structural and textural properties were determined by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR) and N2 adsorption measurements. Well-crystallized spinels of formulae viz. LiMn2O4 and LiNi0.5Mn1.5O4 with a thin sheet-like morphology and average particle size at ca. 30 nm were obtained by heating at temperatures as low as 400 °C for a short time. On the other hand, pure layered oxides (LiCoO2 and LiNi0.5Co0.5O2) required higher temperatures (800 °C), which resulted in greater particle sizes (average size ca. 100 nm). The electrochemical properties of these materials in lithium cells were studied from cyclic voltammetry and galvanostatic measurements. Cells made from the spinels exhibited good rate performance and the delivered capacities changed little over the charge-discharge rate range from C/4 to 4C (C is defined as the theoretical capacity delivered in 1 h). By contrast, the capacity values for the cells made from the layered oxides are strongly affected by the charge-discharge rates. Their increased particle size may be the origin of the poorer cell performance observed.