Synthesis and Performance of LiFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$PO$_{4}$ in Lithium-ion Battery

Olivine-type lithium transition metal phosphates (i.e. LiFePO$_{4})$ have been intensively investigated as promising electrode materials for rechargeable lithium-ion batteries. There have been attempts to improve energy density and voltage quality of phosphate based electrode. In this study, we have partially substituted Fe$^{\mathrm{II}}$/Fe$^{\mathrm{III}}$ redox center with Mn$^{\mathrm{II}}$/Mn$^{\mathrm{III}}$ in LiFePO$_{4}$ that provides over 600 mV higher voltage. We prepared various compositions of LiFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$PO$_{4}$ (x$=$0, 0.2, 0.4, 0.6, 0.8 and 1) between the two end members (LiFePO$_{\mathrm{4}}$ - LiMnPO$_{4})$. Due to intrinsic low electronic conductivity of lithium transition metal phosphates, we coat these materials with a uniform conductive carbon through a unique sol-gel process developed in our laboratory. In addition, we made a composite of the carbon coated phosphate with carbon nano-tubes to develop a highly conductive matrix electrode. We report the materials structure, morphology, electrical conductivity and electrochemical performances of LiFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$PO$_{4}$ using XRD, Raman spectroscopy, SEM, TEM, XPS, electrical conductivity and galvanostatic charge/discharge measurements.