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 P2(1)/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 x 10(-1)6 cm(2) s(-1) to 2.1 x 10(-15) cm(2) s(-1) by Mn doping. The increased Fe-O bond length is correlated with improved lithium ion diffusion and its effect on electrochemical behavior.