Synthesis and electrochemical characterizations of spinel LiMn1.94MO4 (M = Mn0.06, Mg0.06, Si0.06, (Mg0.03Si0.03)) compounds as cathode materials for lithium-ion batteries

Abstract The spinel LiMn 1.94 MO 4 (M = Mn 0.06 , Mg 0.06 , Si 0.06 , (Mg 0.03 Si 0.03 )) compounds are successfully synthesized by citric acid-assisted sol–gel method. The crystal structures and morphologies of synthesized compounds are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. All the compounds possess the cubic spinel structure of LiMn 2 O 4 with space group of Fd-3m. The electrochemical properties of synthesized compounds are investigated by galvanostatic charge-discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that the Si-doping can increase the discharge capacity of LiMn 2 O 4 due to the more expanded and regular MnO 6 octahedra. In particular, for the LiMn 1.94 Mg 0.03 Si 0.03 O 4 compound, the addition of Si 4+ ions can make up for the shortage of Mg-doping in term of the discharge capacity. As a result, the Mg 2+ and Si 4+ co-doping has the effect of synergistic enhancement, which can make full use of the respective advantages of Mg-doping and Si-doping. The optimal LiMn 1.94 Mg 0.03 Si 0.03 O 4 can deliver the initial discharge capacity of 128.3 mAh g −1 with good capacity retention of 92.8% after 100 cycles at 0.5 C in the voltage range of 3.20–4.35 V. Compared with the undoped LiMn 2 O 4 , the co-doped compound also presents superior rate performance, especially the capacity recovery performance.