Chemical redox reactions and extended PXRD-characterization of triphylite-type compounds AM0.5Fe0.5PO4 (A = Li, Na; M = Mn, Co). (2nd May 2017)

• Record Type: Journal Article
• Authors: Bauer, Jessica; Haberkorn, Robert; Kickelbick, Guido
• Is Part Of: Powder diffraction. Volume 32(2017)Supplement 1
• Page Start: S74
• Page End: S81
• Publication Date: 2017-05-02
• Subjects: triphylite, -- chemical delithiation, -- chemical sodiation, -- cathode material, -- Rietveld method; Powder metallurgy -- Periodicals; Materials -- Analysis -- Periodicals; 620.1127
• DOI: 10.1017/S0885715617000355
• Languages: English
• ISSNs: 0885-7156

Abstract

Abstract : The alkaline content of mixed transition metal phosphates Li(1− y )· z Na y · z M 0.5 Fe0.5 PO4 ( 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 Li M 0.5 Fe0.5 PO4 were synthesized via solid-state reactions. Chemical extraction of Li with bromine in acetonitrile yielded well-crystalline Li0.5 M 0.5 Fe0.5 PO4 . Subsequent reduction with Na2 S in acetonitrile gave a product with a triphylite-type phase of the average composition Li0.5 Na0.5 M 0.5 Fe0.5 PO4 showing a broad distribution of Li-to-Na-ratios. For Rietveld refinements a set of 11 fractions Li1− y Na y M 0.5 Fe0.5 PO4 was used to represent the triphylite-type phase and its composition fluctuation. To obtain samples with compositions close to Na z M 0.5 Fe0.5 PO4 ( 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 )· z Na y · z M 0.5 Fe0.5 PO4 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.