Physicochemical properties of LiAl _x Mn_2 − x O₄ and LiAl_0.05Mn_1.95O_4 − y F _y cathode material by the citric acid-assisted sol–gel method

Abstract

LiAl _x Mn_2 − x O₄ and LiAl_0.05Mn_1.95O_4 − y F _y spinel have been successfully synthesized by citric acid-assisted sol–gel method. The structure and physicochemical properties of this as-prepared powder were investigated by electronic conductivity test, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge test in detail. The electronic conductivity decreases with increasing of the content of doped Al. XRD patterns show that the diffraction of LiAl_0.05Mn_1.95O_4 − y F _y samples is similar, with all the peaks indexable in the Fd3m space group, and a little impurity appears in the LiAl_0.05Mn_1.95O_3.8F_0.2 sample. SEM reveals that all LiAl_0.05Mn_1.95O_4 − y F _y powders have the uniform, nearly cubic structure morphology with narrow size distribution which is less than 500 nm. Galvanostatic charge–discharge test indicates that LiAl_0.05Mn_1.95O₄ has the highest discharge capacity and electrochemical performance among all LiAl _x Mn_2 − x O₄ samples after 50 cycles, and the initial discharge capacity of LiAl_0.05Mn_1.95O_4 − y F _y (y = 0, 0.02, 0.05, 0.1) is 123.9, 124.6, 124.9, and 125.0 mAh g⁻¹, respectively, and their capacity retention ratios are 94.2%, 94.9%, 91.7%, and 89.9% after 50 cycles, respectively. EIS indicates that LiAl_0.05Mn_1.95O_3.98F_0.02 have smaller charge transfer resistance than that of LiAl_0.05Mn_1.95O₄ corresponding to the extraction of Li⁺ ions.