Synchrotron-based operando X-ray diffraction and X-ray absorption spectroscopy study of LiCo0.5Fe0.5PO4 mixed d-metal olivine cathode

https://doi.org/10.1038/s41598-023-28951-z

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Operando synchrotron-based XAS and XRD characterization

X-ray absorption spectra and simultaneous diffraction patterns were obtained in the operando mode at the European Synchrotron Radiation Facility ESRF’s Swiss-Norwegian Beamline BM31 (Grenoble, France). Home made electrochemical cells with X-ray transparent windows made from glassy carbon were used for cycling²⁹. The resulting cathode powder was placed in the cell with a mass load of 13 mg/cm². The cells were cycled galvanostatically using a BAT-SMALL battery analyzer (Astrol Electronic AG, Switzerland) in the region from 3 to 5 V vs Li/Li⁺ with a cycling rate of C/10. Diffraction patterns were recorded each 5.5 min, while the Fe and Co K-edge absorption spectra—for 9.5 min, resulting in a total measurement time of 15 min per point.

2D diffraction images were performed with 5 s acquisition time by the Dexela 2D CMOS detector (PerkinElmer, USA). The wavelength λ = 0.49796 Å and the sample-to-detector distance of 289.55 mm were calibrated using silicon powder and LaB₆ standards.

Fe and Co K-edge X-ray absorption spectra were measured in a single continuous scan in transmission mode employing a Si(111) monochromator. Ionization chambers were used to monitor intensity before and after assembling, and a Gd₂O₃ reference was measured simultaneously with the sample for energy alignment.The observed spectra were aligned, flattened, and normalized using the Demeter package’s Athena function^30,31,32. Then all spectra were interpolated on the same set of energy points and submitted as the input for the principle component analysis (PCA) via open-source PyFitIt software as described precisely in the Mathematical details of the PCA procedure are presented below, and were not included in the main text, but were appropriately referenced:

Any XANES spectrum can be projected onto an orthogonal basis set of functions. The set of projections over each basis function then describes the entire spectrum. We construct basis set functions in the procedure of principal component analysis applied to the theoretical training set. A theoretical XANES dataset X� is decomposed in the following way:

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In situ synchrotron X-ray absorption measurements

Figure 5 demonstrates the evolution of operando Fe and Co K-edge XANES spectra of LiCo_0.5Fe_0.5PO₄ cathode upon cycling. Though for Fe, there is a very notable chemical shift of the absorption edge, indicating the Fe²⁺/Fe³⁺ redox reaction (Fig. 5a), for Co K-edge, this shift is hardly noticeable (Fig. 5b).

A series of operando XANES spectra for the Fe (a) and Co (b) K-edge in the LiCo_0.5Fe_0.5PO₄ cathode material obtained during the first cycle (the red line is a charging process, and the blue line represents the discharge process).

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This set of spectra was further processed using PCA. Because the XAS spectrum of a multiphase bulk mixture is a superposition of XAS spectra for each phase, PCA can be used to mathematically decompose the sequence of spectra for the electrochemical phase transition process to get the pristine spectra of the relevant phases and their concentrations. Initially, all spectra were interpolated to a single energy range. Singular value decomposition was then used to obtain a certain number of components from the sequence of spectra. In our situation, two abstract spectra were more transformed into chemically meaningful information via target transformation. The following physical constraints were expected: spectra should be normalized, the values of concentrations should be positive, and at least one of the components under consideration should represent either a fully reduced or fully oxidized phase. In this way, two components were successfully identified for both Co and Fe, as shown in Fig. 6.

PCA components extracted from the series of operando XANES spectra for the Fe compared to the experimental spectra for LiFePO₄ and FePO₄ (ref.⁴⁰) (a) and Co (b) K-edge in the LiCo_0.5Fe_0.5PO₄ cathode material, compared to the experimental spectra of the reference compounds (LiCoPO₄@UiO-66)⁷. (c) Cell voltage of Fe during the first cycle dots on a black voltage profile mark the start time for measuring each successive XAS spectrum. (d) PCA phase concentration of the Fe²⁺ and Fe³⁺ components compared to the cell potential. (e) Cell Voltage of Co during the first cycle dots on a black voltage profile marks the start time for measuring each successive XAS spectrum. (f) PCA phase concentration of the Co²⁺ and Co³⁺ components compared to the cell potential.

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There is a significant change between the first and the second components because the first component corresponds to Fe²⁺ and the second—to Fe³⁺ in CoFePO₄, as demonstrated by the comparison with the experimental XAS spectra for LiFePO₄ and FePO₄ in Fig. 6a. For the Co K-edge, in a similar way, the first component can be attributed to the Co²⁺ in LiCo_0.5Fe_0.5PO₄ and the second one—to the Co³⁺ in CoFePO₄ due to a rare study of in situ XAS of LiCoPO₄, the current result compared to our previously reported work LiCoPO₄ coating by UiO-66 meatlorganic framework⁷, as shown in Fig. 6b.

Figure 6d shows that at the first charge, the Fe²⁺ ions are almost completely converted to Fe³⁺, thus, about 50% of Li ions should be extracted from the cathode material. Figure 6e,f demonstrate that the electrode surface remains predominately Co²⁺ in LiCoPO₄, irrespective of the charge process. However, upon discharge process has noted a shift in the center of peaks to the higher energy suggesting the growth of some Co³⁺, with additional loss of various characteristics in the bulk of the material. The determination of several features missing in Co³⁺ compounds suggests that (~ 60)Co²⁺ was not fully oxidized³⁹. There is also a peculiar disturbance in Co phase concentration in the region corresponding to the Fe²⁺ redox activity. Such a reaction of Co on the change in the local atomic and electronic coordination of Fe might be indicative of the homogeneous distribution of d-metals in the structure of cathode material and the homogeneous Li insertion/extraction mechanism. X-ray diffraction is sensitive to the long-range order, and the conclusion about the solid solution mechanism is made based on this experimental technique. On the contrary, X-ray absorption fine structure is sensitive to the local atomic structure within 5–7 Å around the absorber. Therefore the “components” retrieved from PCA or linear combination fits are related to the local order around 3d metals but not to crystalline phases.

To understand both cobalt and iron performance during the charge/discharge process of LiCoFePO₄, ab initio DFT calculations were carried out to estimate the local atomic charges according to the Bader method for different Li stoichiometry in Li_xCoFePO₄ (x = 0, 0.25, 0.5, 0.75 and 1). Figure 7 demonstrates the unit cell averaged atomic charges for cobalt, iron, and oxygen compared to the reference values. The calculation suggests that upon the lithiation, Fe²⁺ should fully convert to Fe³⁺, which is in agreement with the PCA results shown above. However, the suggested Co³⁺/Co²⁺ conversion rate during the lithiation is only slightly above 50%, similar to what was observed in the operando XAS measurements. However, for the oxygen, there is also a substantial change in the Bader charge upon the delitiation, which suggest that there is a reversible anion redox reaction on oxygen, which takes part in the total charge compensation mechanism to make up for the deficiency in Co oxidation.

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