Effects of sodium substitution on properties of LiMn2O4 cathode for lithium ion batteries

Na-doped Li1.05Mn2O4 cathodes were synthesized using a sol-gel process. The samples were characterized by X-ray diffractometry (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and charge-discharge measurements. The results show that all the samples exhibit the same cubic spinel phase structure without impurity. The lattice constant and unit cell volume decrease with increasing the sodium dopant amount. As the molar ratio of sodium to manganese (x=n(Na)/n(Mn)) increases from 0 to 0.03, the initial discharge capacity of the Li1.05Mn2O4 cathodes decreases from 119.2 to 107.9 mA·h/g, and the discharge capability at large current rate and the storage performance decline dramatically, while cycling performance at room temperature and 55 °C are improved. The CV and EIS studies indicate that reversibility of Li1.05Mn2O4 cathodes decreases and the electrochemical impedance increases with increasing the sodium dopant amount.