Doping effect of manganese on the structural and electrochemical properties of Li2FeSiO4 cathode materials for rechargeable Li-ion batteries

Li2MSiO4, with two Li ions per molecule operates on both of the M2+/M3+ and M3+/M4+ redox couples resulting in a higher theoretical capacity that is > 300 mAh.g−1. In this work, synthesis of a Li2Fe1-xMnxSiO4/C (LFMS) composite was done using a sol-gel method. XRD patterns can be indexed in the monoclinic phase with P21/n space group. Li2Fe0.8Mn0.2SiO4/C provides higher discharge potentials and capacities, hence higher energy densities than Li2FeSiO4 of about 60% at 0.1C. (655 Wh.kg−1 vs. 408 Wh.kg−1, respectively). X-ray absorption spectroscopy (XAS) shows that the Fe–O bond length increases by Mn doping in the structure. EIS measurements show that Li-ion diffusion coefficients improved from 8.3 × 10−16 cm2 s−1 to 2.1 × 10−15 cm2 s−1 by Mn doping. The increased Fe–O bond length is correlated with improved lithium ion diffusion and its effect on electrochemical behavior. •Li2Fe1-xMnxSiO4/C has been synthesized by a sol-gel method.•Electrochemical impedance spectra (EIS) analysis shows that Mn doping reduces charge transfer resistance (Rct) .•The Fe–O bond length in the structure is longer with Mn doping.•The study is important to understand the local structural effects on electrochemical performance.