Effects of equimolar Mg (II) and Si (IV) co-doping on the electrochemical properties of spinel LiMn2−2xMgxSixO4 prepared by citric acid assisted sol–gel method

The spinel LiMn2−2xMgxSixO4 (x = 0, 0.01, 0.03, 0.05 and 0.07) cathode materials were successfully synthesized by a citric acid assisted sol-gel process with LiOH · H2O, Mn(CH3COO)2 · 4H2O, Mg(NO3)2 · 6H2O and C8H20O4Si as raw materials. The crystal structures and morphologies of LiMn2−2xMgxSixO4 were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. All the obtained samples with good crystallinity were identified as the cubic spinel structure of LiMn2O4 and no impurity peaks were observed. Equimolar Mg2+ and Si4+ ions could completely occupy the octahedral (16d) sites to substitute Mn3+ and Mn4+ ions, respectively, which significantly improved the structural stabilization and suppressed the Jahn–Teller distortion. The effects of equimolar Mg2+ and Si4+ ions co-doping on the electrochemical properties of LiMn2−2xMgxSixO4 were investigated by galvanostatic charge–discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that the equimolar Mg2+ and Si4+ ions co-doped LiMn2O4 presented better cycling retention and rate performance. For the optimal LiMn1.90Mg0.05Si0.05O4, the initial discharge capacity was 126.9 mAh g−1 and remained 123.5 mAh g−1 after 100 cycles at 0.5 C in the voltage range of 3.20 ∼ 4.35 V. The capacity retention could reach 97.3%, which was far higher than that of the undoped LiMn2O4.