C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S
S C I E N T I F I C H I G H L I G H T S
9 4 H I G H L I G H T S 2 0 2 3 I
X-rays probe pyrochlore iron hydroxy fluorides as affordable lithium-ion cathode materials
A new, cost-effective synthesis method for pyrochlore iron (III) hydroxy fluorides (Pyr-IHF) for economical stationary energy-storage materials demonstrates high capacity retention over 600 cycles without complex electrode design. Operando X-ray diffraction was used to investigate crystalline water and its effect on electrochemical performance in Pyr-IHF.
The shift from fossil fuels to renewable energy sources necessitates cost-effective stationary rechargeable batteries for energy balance. Iron (III) fluoride-based compounds are being explored as economical lithium-ion (Li-ion) battery cathodes due to their abundant constituents and high capacity [1]. Pyrochlore iron (III) fluorides (Pyr-FeF3) with large hexagonal channels show promise for enhanced Li-ion diffusion [2]. However, a cost-effective Pyr-FeF3 synthesis method has been elusive. In this study, a straightforward, ambient-condition synthesis method was developed for Pyr-IHF from an ethanol-dissolved precursor, achieving over 80% capacity retention after 600 cycles. Investigation of water content within Pyr IHF channels confirmed its impact on capacity and Li-ion diffusion. This research advances low-cost materials for renewable energy integration.
The synthesis of Pyr-IHF involved dissolving FeF3(H2O)2 ⋅ H2O in ethanol, followed by water-induced precipitation. The as-synthesised Pyr-IHF consisted of size-uniform spherical nanoparticles with a cubic pyrochlore structure, featuring 3D interconnected hexagonal channels. Wavelet-transformed extended X-ray absorption fine structure (EXAFS) analysis, obtained at beamline BM31, was used in combination with infrared spectroscopy to determine an average coordination environment of FeF4(OH)2 octahedra. Rietveld refinement of synchrotron X-ray diffraction (XRD) performed at beamline BM01 indicated the presence of water of crystallisation in the Pyr-IHF structure, contributing to larger lattice parameters and suggesting a chemical formula of FeF2(OH) ⋅ 0.7 H2O for as-synthesised Pyr-IHF.
The structural changes during heat-treatment of as- synthesised Pyr-IHF were analysed using operando XRD obtained at beamline BM01 (Figure 72). The analysis revealed two distinct stages of lattice parameter contraction during heating, with the first contraction occurring between 75-125°C and the second between 220-280°C. The initial contraction was attributed to the release of disordered water molecules from within the channels. The second unit cell contraction was linked to the release of the water of crystallisation, confirmed by changes in the occupancy of crystalline water. In the presence of residual moisture, Pyr-IHF could be partially rehydrated during cooling. The appearance of small-angle
Fig. 72: Structural changes in Pyr-IHF during heat-treatment.
a-c) Operando XRD of the heat- treatment of as-synthesised Pyr-IHF.
d,e) Temperature evolution of Pyr-IHF lattice constant (d), and
crystalline water content inside the channels (e). f) Fitted scattering
exponent obtained from SAXS. g) Schematic representation of the H2O loss in Pyr-IHF during
heat-treatment.