Visualization of anisotropic-isotropic phase transformation dynamics in battery electrode particles

Abstract

Anisotropy, or alternatively, isotropy of phase transformations extensively exist in a number of solid-state materials, with performance depending on the three-dimensional transformation features. Fundamental insights into internal chemical phase evolution allow manipulating materials with desired functionalities, and can be developed via real-time multi-dimensional imaging methods. Here, we report a five-dimensional imaging method to track phase transformation as a function of charging time in individual lithium iron phosphate battery cathode particles during delithiation. The electrochemically driven phase transformation is initially anisotropic with a preferred boundary migration direction, but becomes isotropic as delithiation proceeds further. We also observe the expected two-phase coexistence throughout the entire charging process. We expect this five-dimensional imaging method to be broadly applicable to problems in energy, materials, environmental and life sciences.

Introduction

Direct observation of anisotropic (isotropic) phase transformation and mapping phase evolution is of importance in design and optimization of functional materials. X-ray tomography allows for characterization of the three-dimensional (3D) internal structure of large-volume structures1,4. This technique has long been used in life, medical and earth sciences to deliver 3D morphology information at micron-scale resolution5,6. In recent years, lens-based full-field transmission x-ray microscopy (TXM) with nanotomography capability has seen growing use in studying energy-storage materials7,8,9,10. Further progress in observing intermediates and chemical phases responsible for materials’ performance requires quantitative measurement methods combining chemical sensitivity and high spatial resolution.

X-ray absorption near-edge structure (XANES) spectroscopy is sensitive to chemical and local electronic change of the probed element, and has been extensively exploited in characterizing fine structural change in a large variety of materials11,12. In combination with TXM, XANES enables mapping and tracking of chemical evolution under in situconditions13,14,15,16,17. Nevertheless, the in situ XANES mapping approach has largely been restricted to two-dimensional (2D) observation as the obtained signal is usually spatially integrated along depth direction. For anisotropic phase transformations, quite common in technologically important materials, the method is limited in its ability to accurately capture phase evolution. Although tomographic scans at dual (below and above the adsorption edge of the studying element) and multiple energies have been achieved to identify the chemical element distribution18,19, it remains very challenging to carry out in situ studies on energy-storage materials, which requires accurately tracking chemical phase evolution in 3D with nanoscale resolution and correlating it to electrochemical performance. To pursue such studies using XANES requires reliable collection of multiple images over a 180°-rotation range at each energy point with sufficient energy resolution with the energy scanned across the absorption edge of the element of interest to produce a spectrum for each voxel of the sample inside a working electrochemical cell. Such an undertaking poses numerous technical and experimental difficulties.

Here, using full-field hard X-ray microscopy, we demonstrate an implementation of in situ XANES nanotomography able to build five-dimensional (5D) data sets tracking phase evolution in lithium iron phosphate particles in a working lithium-ion battery. Olivine lithium iron phosphate (LiFePO4) was selected as a model material because of its well-known two-phase process and representative behaviours for many energy materials20. Many-particle scale intercalation behaviour and atomic-scale phase transformation behaviours have been discussed in previous reports, but 3D features of single-particle phase evolution are yet to be fully established21,22. Figure 1 illustrates the basic principle of our approach, which is demonstrated using our full-field TXM with recently developed automated markerless tomography capability23. One specific feature of the setup is a built-in run-out correction system which enables automated tomography. First, this eliminates the need for a marker mounted on the sample or a special feature inside of the sample, enabling a wider range of samples to be studied and easier sample preparation. Second, manual alignment of hundreds of 2D projection images for 3D reconstruction becomes unnecessary, facilitating increased 3D spatial resolution through rapid collection of many projections and enabling time-resolved studies. Another important feature of the setup is that the image distance, the distance of the CCD detector to the zone plate lens, is automatically adjustable as a function of energy, which ensures that optimal resolution is preserved throughout energy scans. Together, these features make it practically feasible to combine XANES and nanotomography to study in situ phase transformations in 3D at nanoscale resolutions.

Figure 1: 5D XANES tomography.

(a) Schematic of experimental setup. A tomography data set spanning −90° to +90° is collected at each photon energy step (7,102 to 7,192 eV, 2 eV per step) across the near absorption K-edge of iron to produce chemical information for each voxel. By fitting the resulting spectra as a linear combination of spectra of end-phases, phase composition can be assigned to each voxel (shown in the right side of panel). (b) Chemical phase distribution as a function of capacity (or equivalently, time).