Tracking Battery Oxidation State Changes with Laboratory XANES

Initially developed in synchrotron facilities, XANES has demonstrated its powerful capability in analyzing battery materials, especially for determining the chemical states of the battery cathode materials. Sigray QuantumLeap is the first laboratory system that provides synchrotron-like XANES capabilities with a microfocus X-ray tube source. In the present study, LiFePO4 battery cathode specimens in the fresh and aged states were investigated using Sigray QuantumLeap with a ~0.5 eV energy resolution. The results yielded a change in the XANES spectra between the two specimens, indicating a change in oxidation state as a function of aging.

Introduction

Batteries are ubiquitous in modern world, which have been deployed in different aspects of our daily life, from various portable electronic devices to electric vehicles and large-scale stationary energy storage1. The core of any battery is a type of electrochemical reaction, involving changes of the chemical states of the electrode materials. Consequently, determining the influence of different material preparations, cell assemblies, and operation conditions on the chemical state of electrode material is critical to understand and optimize the battery design. X-ray absorption near edge spectroscopy (XANES) is a powerful technique used frequently for chemical state analysis. XANES measures the slight changes in the absorption characteristic of elements based on its chemical state.

A New Laboratory Approach

The Sigray QuantumLeap X-ray absorption spectroscopy system is the first laboratory instrument with synchrotron-like XANES capabilities. The system provides a wide range of X-ray energies, ranging from 2.1 – 12 keV, for both modes of X-ray absorption spectroscopy: XANES and EXAFS. For XANES analysis, the system provides high energy resolution of <0.5 eV and spatial resolution down to 100 μm for investigating heterogeneous samples.

In this study, the capability of QuantumLeap to differentiate the chemical states of Fe in LiFePO4 batteries is demonstrated.

Schematic for XANES measurements in off-Rowland geometry using Sigray QuantumLeapTM: The x-rays from the Sigray microfocus source (S) are collected and focused by the condenser (C) on to the sample (A) placed within the Rowland circle (R). The x-rays with different energies (shown with different colors) and focused by the Johansson crystal (Cr) at different locations on the CCD detector (D) enabling simultaneous detection of x-rays with multiple energies. See text for further details.

Figure 1. Schematic for XANES measurements in off-Rowland geometry using Sigray QuantumLeap: The X-rays from the Sigray microfocus source (S) are collected and focused by the condenser (C) on to the sample (A) placed within the Rowland circle (R). The X-rays with different energies (shown with different colors) and focused by the Johansson crystal (Cr) at different locations on the CCD detector (D) enabling simultaneous detection of X-rays with multiple energies. See text for further details.

Experiment and Results

Lithium iron phosphate (LiFePO4) batteries, which use LiFePO4 in the cathode material, have been commercially mass-produced for applications in portable equipment, transportation vehicles, utility scale stationary electric energy storage and others. It has been presented as a low cost, low toxicity, and intrinsically safer cathode material2.

In the present study, two LiFePO4 cathodes were analyzed: the pristine marked as “NEW” and the cycled marked as “AGED.” The cathodes were directly delayered from the current collector and placed on the sample stage. Sigray’s patented multi-target microfocus source coupled to a high efficiency condenser lens was employed to focus the X-rays onto the sample. The transmitted X-rays were collected by a Johansson crystal spectrometer operated in an Off-Rowland circle geometry for simultaneous acquisition of XANES data centered around the K-α absorption edge of Fe (7.11keV). The dispersed X-rays from the crystal were collected utilizing a high DQE 2D direct detection CCD. A data collection schematic is shown in Figure 1.

The XANES spectra were collected at 0.5 eV pixel energy resolution. The evolving iron oxidation states were clearly captured by the changed absorption features, especially the energy shifts, shown on the XANES spectra, which is consistent with synchrotron data available in the literature3. By comparing samples in these two states, researchers are afforded unique insight into the long-term oxidation state evolution of these electrodes. X-ray radiation, non-destructive in nature, may thus be applied could conceivably be thus be used to track this evolution over time, examining the changing oxidation states throughout the lifetime of a long cycle-life LiFePO4 battery.

XANES spectra of LiFePO4 cathodes at different charge/discharge states. QuantumLeap-210 scaled acquisition time and energy resolution for each cathode sample are 3 hours @ ~0.5 eV.

Figure 2. XANES spectra of LiFePO4 cathodes at different charge/discharge states. QuantumLeap-210 scaled acquisition time and energy resolution for each cathode sample are 3 hours @ ~0.5 eV.

Summary

Sigray QuantumLeap has demonstrated synchrotron-like capabilities for analyzing chemical states of lithium iron phosphate battery electrode materials, with high throughput and energy resolution of <0.5 eV for XANES. The difference between pristine and aged batteries was identified through a shift in the absorption profile, which indicated a change in the oxidation state. Further studies employing this technique may track the evolution of a single electrode over time, using the non-destructive nature of X-rays to enable precise tracking of the oxidation state evolution throughout the lifetime of a single battery electrode.

References

  1. US Department of Energy report: “Basic Research Needs for Electrical Energy Storage” 2007
  2. JB Goodenough, et al. “The Li-ion rechargeable battery: a perspective.” Journal of The American Chemical Society Vol. 135, Issue 4 (2013): 1167-1176
  3. A Deb, et al. “Structural investigations of LiFePO4 electrodes and in situ studies by Fe x-ray absorption spectroscopy.” Electrochimica Acta Vol. 50 (2005): 5200-5207

This information has been sourced, reviewed and adapted from materials provided by Sigray, Inc.

For more information on this source, please visit Sigray, Inc.

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