Structural and electrochemical properties of Li ₂Mn _0.5Fe _0.5SiO ₄/C cathode nanocomposite

The Li ₂Mn _0.5Fe _0.5SiO ₄ silicate was prepared by blending of Li ₂MnSiO ₄ and Li ₂FeSiO ₄ precursors with same molar ratio. The one of the silicates of Li ₂MnSiO ₄ is known as high capacitive up to ∼330 mAh/g due to 2 mole electron exchange, and the other of Li ₂FeSiO ₄ has identical structure with Li ₂MnSiO ₄ and shows stable cycle with less capacity of ∼170 mAh/g. The major drawback of silicate family is low electronic conductivity (3 orders of magnitude lower than LiFePO ₄). To overcome this disadvantage, carbon composite of the silicate compound was prepared by sucrose mixing with silicate precursors and heat-treated in reducing atmosphere. The crystal structure and physical morphology of Li ₂Mn _0.5Fe _0.5SiO ₄ was investigated by X-ray diffraction, scanning electron microscopy, and high resolution transmission electron microscopy. The Li ₂Mn _0.5Fe _0.5SiO ₄/C nanocomposite has a maximum discharge capacity of 200 mAh/g, and 63% of its discharge capacity is retained after the tenth cycles. We have realized that more than 1 mole of electrons are exchanged in Li ₂Mn _0.5Fe _0.5SiO ₄. We have observed that Li ₂Mn _0.5Fe _0.5SiO ₄ is unstable structure upon first delithiation with structural collapse. High temperature cell performance result shows high capacity of discharge capacity (244 mAh/g) but it had poor capacity retention (50%) due to the accelerated structural degradation and related reaction.