2017.02

Chemical redox reactions and extended PXRD-characterization of triphylite-type compounds AM_0.5Fe_0.5PO₄ (A = Li, Na; M = Mn, Co)

Jessica Bauer, Robert Haberkorn^*, Guido Kickelbick

Powder Diffraction 32/S1 (2017) S74-S81

Article first published online: 02 May 2017

DOI: 10.1017/S0885715617000355

Abstract

The alkaline content of mixed transition metal phosphates Li_(1−y)·zNa_y·zM_0.5Fe_0.5PO₄ (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 LiM_0.5Fe_0.5PO₄ were synthesized via solid-state reactions. Chemical extraction of Li with bromine in acetonitrile yielded well-crystalline Li_0.5M_0.5Fe_0.5PO₄. Subsequent reduction with Na₂S in acetonitrile gave a product with a triphylite-type phase of the average composition Li_0.5Na_0.5M_0.5Fe_0.5PO₄ showing a broad distribution of Li-to-Na-ratios. For Rietveld refinements a set of 11 fractions Li_1−yNa_yM_0.5Fe_0.5PO₄ was used to represent the triphylite-type phase and its composition fluctuation. To obtain samples with compositions close to Na_zM_0.5Fe_0.5PO₄ (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)·zNa_y·zM_0.5Fe_0.5PO₄ 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.