Operando X-ray Absorption Spectroscopy on Empyrean

Introduction

Nickel-rich layered oxides such as NMC 811 (LiNi0.8Mn0.1Co0.1O2) are among the most widely used cathode materials for high-energy density lithium-ion batteries. Their electrochemical performance and long-term stability are closely linked to changes in transition-metal oxidation state and local atomic structure during cycling.

Understanding these processes requires characterization techniques capable of probing materials under realistic operating conditions. X-ray Absorption Spectroscopy (XAS) is particularly well suited for this purpose, providing complementary information on both electronic structure and local atomic coordination through analysis of the X-ray Absorption Near Edge Structure (XANES ) region.

Traditionally, operando XAS experiments have been largely restricted to synchrotron facilities. The Empyrean platform extends these capabilities into the laboratory with integrated software to control battery cycling and XAS measurements, enabling direct observation of electrochemical processes during battery operation. In this study, operando Ni K-edge XANES measurements are used to monitor the evolution of nickel oxidation state in NMC 811 during charge and discharge cycling.

Experimental Setup

Operando measurements were performed in transmission mode using a standard (unmodified) bi-layer pouch cell containing an NMC 811 cathode. The cell was cycled between 3.0 V and 4.2 V at a C/5 rate while Ni K-edge XAS spectra were collected continuously.

The experimental configuration enables simultaneous electrochemical cycling and XAS acquisition, providing direct insight into the evolution of both electronic and local atomic structure under realistic operating conditions. Approximately one spectrum was acquired per hour, allowing real-time monitoring of the charge-discharge process.

Operando Tracking of Nickel Redox and Structural Evolution

The evolution of the Ni K-edge XANES spectra reveals a systematic shift of the absorption edge toward higher energies during charging, corresponding to progressive oxidation of nickel. During discharge, the edge position returns toward lower energies, demonstrating the reversibility of the redox process.

The measured edge shift of approximately 2 eV is consistent with the expected Ni2+/Ni3+/Ni4+ redox chemistry that governs charge compensation in NMC 811. Tracking the edge position throughout the cycle provides a direct measure of nickel oxidation state evolution and its correlation with cell voltage.

In addition to the edge shift, subtle changes are observed in the near-edge spectral features during cycling. These variations reflect changes in the electronic structure and local chemical environment surrounding nickel as lithium is extracted and reinserted. While the observed spectral evolution is consistent with changes occurring within the cathode material during cycling, quantitative determination of local structural parameters would require a dedicated EXAFS analysis extending further beyond the absorption edge.

The results demonstrate the ability of laboratory-based operando XANES measurements to follow electrochemical processes in real time and provide direct insight into nickel redox behavior during battery operation.

Figure 1. Operando Ni K-edge XANES measurements collected during cycling of an NMC 811 pouch cell. (a) XANES heat map showing the evolution of the absorption edge throughout charge and discharge. (b) Extracted edge-position trend demonstrating the reversible oxidation and reduction of nickel, with an approximately 2 eV edge shift correlated with cell voltage. (c) Expanded XANES heat map highlighting changes in near-edge spectral features during cycling.

Implications for Battery Materials Research

Understanding the relationship between oxidation state and local structure is essential for improving the performance, safety, and lifetime of nickel-rich cathode materials. Operando XAS enables these processes to be monitored directly rather than inferred from post-mortem characterization.

The availability of operando XAS on a laboratory platform offers several advantages:

• Direct observation of redox mechanisms during cycling
• Simultaneous access to electronic and structural information
• Routine and repeatable experiments without synchrotron scheduling constraints
• Faster iteration during materials development and optimization
• Flexible investigation of novel electrode materials and cycling protocols

By enabling operando studies within the laboratory environment, researchers can rapidly evaluate material behavior and accelerate the development of next-generation energy-storage materials.

Conclusions

The Empyrean platform enables laboratory-based operando X-ray Absorption Spectroscopy for direct investigation of battery materials under realistic operating conditions. Using NMC 811 as an example, reversible changes in nickel oxidation state were successfully tracked during electrochemical cycling via analysis of the Ni K-edge XANES spectra.

The observed edge shifts and evolution of near-edge spectral features demonstrate the capability of laboratory XANES measurements to monitor redox processes in real time during battery operation. These results highlight the potential of laboratory-based operando XAS as a powerful tool for studying electrochemical mechanisms in energy-storage materials and for complementing synchrotron-based investigations.