Carbon-coated nano-sized LiFe1−x Mn x PO4 solid solutions (0 ≤ x ≤ 1) obtained from phosphate–formate precursors
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LiFe1−x Mn x PO4 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−x Mn x PO4 coated with carbon. The structure and the morphology of the LiFe1−x Mn x PO4 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−x Mn x PO4 is tested in model lithium cells using a galvanostatic mode. All LiFe1−x Mn x PO4 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−x Mn x PO4 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−x Mn x PO4 deliver a good reversible capacity due to the Fe2+/Fe3+ and Mn2+/Mn3+ redox-couples at 3.5 and 4.1 V, respectively.
KeywordsOlivine LiFePO4 Narrow Particle Size Distribution Voltage Profile Precursor Method
Authors are grateful to the financial support from the National Science Fund of Bulgaria (Ch1701/2007). Partial financial support by the National Centre for New Materials UNION (Contract No DCVP-02/2/2009) is also acknowledged. We are grateful of TIMCAL Company for providing carbon additives. The Raman equipment is used in the framework of project Integrated Research Centres at the Universities No DO02-167/2008.
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