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

Abstract

Lithium-ion batteries based on high-voltage cathode materials, such as LiCoPO₄, despite being promising in terms of specific power, still suffer from poor cycle life due to the lower stability of common non-aqueous electrolytes at higher voltages. One way to overcome this issue might be decreasing the working potential of the battery by doping LiCoPO₄ by Fe, thus reducing electrolyte degradation upon cycling. However, such modification requires a deep understanding of the structural behavior of cathode material upon lithiation/delithiation. Here we used a combination of operando synchrotron-based XRD and XAS to investigate the dynamics of d-metal local atomic structure and charge state upon cycling of LiCo_0.5Fe_0.5PO₄ mixed d-metal olivine cathode material. Principal components analysis (PCA) of XAS data allowed the extraction of spectra of individual phases in the material and their concentrations. For both Co and Fe two components were extracted, they correspond to fully lithiated and delithiated phases of Li_xMPO₄ (where M = Fe, Co). Thus, we were able to track the phase transitions in the material upon charge and discharge and quantitatively analyze the M²⁺/M³⁺ electrochemical conversion rate for both Fe and Co. Rietveld's refinement of XRD data allowed us to analyze the changes in the lattice of cathode material and their reversibility upon (de)lithiation during cycling. The calculation of DFT and Bader charge analysis expects the oxygen redox procedure combined with d-metals redox, which supplements iron charge variations and dominates at high voltages when x < 0.75 in Li_xCoFePO₄.

Figure 1

Rietveld refinement of the synchrotron XRD pattern of as-synthesized LiCo_0.5Fe_0.5PO₄.

Figure 2

First two charge/discharge profiles of LiCo_0.5Fe_0.5PO₄.

Figure 3

Operando synchrotron XRD patterns of LiCo_0.5Fe_0.5PO₄ upon the first two charge–discharge cycles in the 2θ range: (a) from 5.3 to 8°, indicating the shift to the lower 2θ values at this region, (b) from 9.5 to 12° indicating the shift to lower 2θ values at this region. The red line represents the charging process, and the blue line is the discharge process.

Figure 4

Evolution of (a) lattice parameters and (b) unit cell volume obtained from the operando XRD compared to the cell potential and Li loss upon the cycling of LiCo_0.5Fe_0.5PO₄.

Figure 5

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).

Figure 6

PCA components extracted from the series of operando XANES spectra for the Fe compared to the experimental spectra for LiFePO₄ and FePO₄ (ref.) (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). (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.

Figure 7

Unit cell averaged local atomic (Bader) charges for Fe (a), Co (b) and O (c) as a function of x in Li_xCo_0.5Fe_0.5PO₄ (x = 0, 0.25, 0.5, 0.75, and 1). The values for each species were averaged over non-equivalent atomic positions. Reference charge values (dashed lines) were obtained from calculations for reference compounds with different Co and Fe oxidation states.