Chemical redox reactions and extended PXRD-characterization of triphylite-type compounds AM0.5Fe0.5PO4 (A = Li, Na; M = Mn, Co)

Abstract

The alkaline content of mixed transition metal phosphates Li(1−y)·zNay·zM0.5Fe0.5PO4 (M = Mn, Co; 0 ≤ y < 1; 0.5 ≤ z ≤ 1) was determined by powder X-ray diffraction data by means of Rietveld analysis and multi-fraction models. The compounds LiM0.5Fe0.5PO4 were synthesized via solid-state reactions. Chemical extraction of Li with bromine in acetonitrile yielded well-crystalline Li0.5M0.5Fe0.5PO4. Subsequent reduction with Na2S in acetonitrile gave a product with a triphylite-type phase of the average composition Li0.5Na0.5M0.5Fe0.5PO4 showing a broad distribution of Li-to-Na-ratios. For Rietveld refinements a set of 11 fractions Li1−yNayM0.5Fe0.5PO4 was used to represent the triphylite-type phase and its composition fluctuation. To obtain samples with compositions close to NazM0.5Fe0.5PO4 (z = 0.5 or z = 1) up to two additional cycles of chemical oxidation and reduction were required. Because of a complex distribution of Li and Na within the triphylite-type phase a model of 121 fractions of Li(1−y)·zNay·zM0.5Fe0.5PO4 was developed to enable proper Rietveld refinements. The scaling factors of the 121 fractions were constrained by a bimodal bivariate normality. The evaluation of the pattern revealed a type of passivation, preventing the compound from complete oxidation or reduction of Fe for higher cycle numbers. Hence, no completely Li-free samples could be obtained.