Carbon-coated nano-sized LiFe1−xMnxPO4 solid solutions (0 ≤ x ≤ 1) obtained from phosphate–formate precursors

LiFe1−xMnxPO4 solid solutions in the whole concentration range (0 ≤ x ≤ 1) are obtained at 500 °C by a phosphate–formate precursor method. The method is based on the formation of homogeneous lithium–iron–manganese phosphate–formate precursors by freeze-drying of aqueous solutions containing Li(I), Fe(II), Mn(II), phosphate, and formate ions. Thermal treatment of the phosphate–formate precursors at temperatures at 500 °C yields nano-sized LiFe1−xMnxPO4 coated with carbon. The structure and the morphology of the LiFe1−xMnxPO4 compositions are studied by XRD, IR spectroscopy, and SEM analysis. The in situ formed carbon is analyzed by Raman spectroscopy. The electrochemical performance of LiFe1−xMnxPO4 is tested in model lithium cells using a galvanostatic mode. All LiFe1−xMnxPO4 compositions are characterized with an ordered olivine-type structure with a homogeneous Fe2+ and Mn2+ distribution in the 4c olivine sites. The morphology of LiFe1−xMnxPO4 consists of plate-like aggregates which are covered by in situ formed carbon. Inside the aggregates nano-sized isometric particles with narrow particles size distribution (between 60 and 100 nm) are visible. The structure of the deposited carbon presents a considerable disordered graphitic phase and does not depend on the Fe-to-Mn ratio. The solid solutions LiFe1−xMnxPO4 deliver a good reversible capacity due to the Fe2+/Fe3+ and Mn2+/Mn3+ redox-couples at 3.5 and 4.1 V, respectively.