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

Abstract The spinel LiMn 2−2x Mg x Si x O 4 (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 · H 2 O, Mn(CH 3 COO) 2  · 4H 2 O, Mg(NO 3 ) 2  · 6H 2 O and C 8 H 20 O 4 Si as raw materials. The crystal structures and morphologies of LiMn 2−2x Mg x Si x O 4 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 LiMn 2 O 4 and no impurity peaks were observed. Equimolar Mg 2+ and Si 4+ ions could completely occupy the octahedral (16d) sites to substitute Mn 3+ and Mn 4+ ions, respectively, which significantly improved the structural stabilization and suppressed the Jahn–Teller distortion. The effects of equimolar Mg 2+ and Si 4+ ions co-doping on the electrochemical properties of LiMn 2−2x Mg x Si x O 4 were investigated by galvanostatic charge–discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that the equimolar Mg 2+ and Si 4+ ions co-doped LiMn 2 O 4 presented better cycling retention and rate performance. For the optimal LiMn 1.90 Mg 0.05 Si 0.05 O 4 , 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 LiMn 2 O 4 .