Optimizing Operando Electrochemical and X-Ray Diffraction Parameters for High-Resolution Measurements of Lithium-Based Battery Materials

Advances in lithium-based batteries depend on understanding the mechanisms behind electrode performance. Operando methods are very powerful and useful for decoupling thermodynamic and kinetic (e.g. diffusion) limitations, but impose challenges with data collection and analysis. Electrode composition changes as lithium is inserted or removed (according to the electrochemical conditions); composition resolution is defined by the current, (temperature-dependent) diffusion rate within the electrode, and the time to collect a diffraction pattern. Many material systems of interest require less than 1 atomic percent composition resolution to verify (or question) the expected crystalline phases, and separate composition gradients imposed by slow diffusion from the presence of solid solutions. Solid-solution ranges can be particularly challenging as the cell potential may change quickly over a very small composition range. Some advanced techniques use machine learning to optimize pattern collection time by comparing the measured diffraction pattern with references, and focusing on specific scattering angles to identify materials. 1 Although powerful, this method is not well suited for materials with metastable or unidentified phases, e.g. metal alloys for advanced lithium-based batteries. Here we will describe a robust and low-cost platform for operando electrochemical and crystallographic measurements of lithium-based electrode materials. Electrode composition can be monitored at low to moderate rate electrochemical conditions using laboratory diffractometers and at high rates using some synchrotron-based diffractometers. Our test cell is based on Conflat-style vaccum flanges and a beryllium window; 2 this design enables all standard two electrode electrochemical tests on e.g. lithium-based negative electrode materials with hermetic sealing and controlled pressure. Results from the electrochemical lithiation and delithiation of gold will be presented as a case study. Operando studies of lithium-gold reactivity are of interest since many lithium-gold phases have poorly defined unit cells and solid solution ranges, 3 and gold electrodes are a common choice for in-situ / operando measurements in electron microscopes. A wide range of scattering angle is required to collect a sufficient number of peaks for unit cell indexing, which necessarily increases collection time and impacts composition resolution. We will discuss the influence of detector type and scan time (collection plus movement) on composition resolution as a function of electrochemical conditions (current, potential range, and temperature). Results from point, 1D, and 2D detectors on laboratory and synchrotron diffractometers will be provided, along with general strategies to optimize method resolution. References: Szmanski, N.J. et al., Nature Computational Materials, 9, 31 (2023). Sendetskyi, O. et al, J. Applied Crystallography, 54, 1416 (2021). A. D. Pelton, Bull. Alloy Phase Diagrams, 7, 228–231 (1986). Figure 1. Operando electrochemical X-ray diffraction heat maps before (a) and after (b) adjustment of operational and XRD data collection parameters. Figure 1