Synthesis and reversible Li-ion intercalation of a novel chromium-doped iron phosphate with an α-CrPO4 structure

Abstract

Li-ion batteries based on environmentally friendly, cheap, and available elements are in high demand for sustainable economic, societal, and technological development. In this paper, we report on a novel series of iron-based phosphates, α-Fe_1−xCr_xPO₄, adopting the α-CrPO₄-type structure, capable of Li-ion intercalation in a high potential region. These are obtained by thermal decomposition of structurally related KTiOPO₄-type NH₄Fe_1−xCr_xPO₄F precursors under oxidizing conditions. α-Fe_0.75Cr_0.25PO₄ particles reveal a porous “rugby-ball” morphology with a preferred < 100 > orientation along the longest dimension, as examined by scanning transmission electron microscopy and electron diffraction. The crystal structure is refined based on synchrotron powder X-ray diffraction, assisted by ⁵⁷Fe Mössbauer and infrared spectroscopy techniques. The carbon-coated α-Fe_0.75Cr_0.25PO₄/C electrode material exhibits a single-phase de/intercalation mechanism of Li⁺ intercalation with an 88 mA h g⁻¹ discharge specific capacity at C/10 and the Fe³⁺/Fe²⁺ redox potentials approaching ∼3.54 and 3.78 V vs. Li⁺/Li, being the highest among the known Fe-based phosphates. The extended cycling at 1C reveals ∼98.5% capacity retention after 250 cycles with 99.8(3)% coulombic efficiency. This research highlights the applied synthesis route and lays the groundwork for designing and perfecting new iron-based phosphates with promising electrochemical properties as electrode materials for metal-ion batteries.