Mombrini, Isabella; (2024) Advanced Diffraction Methods for Characterization of Li-ion Battery Materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The experiments done in this work were conducted at the high-energy ID15A beamline at European Synchrotron Radiation Facility. ID15A provides the opportunity to take advantage of the new storage ring concept (Extremely Brilliant Source) that increases the brilliance and coherence of the X-ray beams produced by a factor of 100 compared to present-day light sources, providing exceptional spatial and temporal resolution. Two advanced synchrotron XRD methods, X-ray diffraction computed tomography (XRD-CT) and multi-channel-collimator XRD (MCC-XRD), have been employed to investigate the behaviour of commercial LIBs in real-time and three dimensions. Both techniques were used to test pristine and aged 18650 cells to elucidate the principals cause of capacity fading under multiple cycling. XRD-CT allows for the reconstruction of a cross-sectional map of temperature and lithiation distribution across the volume of the battery cell. By analysing the diffraction patterns of the copper current collector obtained in each image voxel, it is possible to accurately determine temperature map during open-circuit cooling by analysing the expansion and contraction of copper current collector was produced. This enabled to identify hotspots and non-uniform thermal behaviour, critical for understanding and addressing thermal management challenges in LIBs. It was observed that a 20-minute discharge on an energy-optimised cell (3.5 Ah) resulted in internal temperatures >70 °C, whereas, a faster 12-minute discharge on a poweroptimised cell (1.5 Ah) resulted in substantially lower temperatures (< 50 °C). However, when comparing the two cells under the same electrical current, the peak temperatures were similar, e.g., a 6A discharge resulted in 40°C peak temperatures for both cell types. We observe that the operando temperature rise is due to heat accumulation, strongly influenced by the charging protocol e.g., constant-current (CC) and/or constant-voltage (CV); mechanisms that worsen with cycling, as degradation increases the cell resistance. Moreover, in the same dataset, by analysing electrodes diffraction patterns, it is possible to calculate the state-of-charge (SoC) variations (lithiation and phase transitions) within the cell during cycling. Cross sectional maps were produced to show the lithiation state across the volume during operation to determine the influence of the cycling protocol applied to the cell and performance after aging. The capacity of the negative electrode decreased after the ageing process and the experiment by 8.8%. The degradation is showed comparing the pristine and aged cell in the fully charged state. In the pristine cell, the fully lithiated LiC6 stage was reached in 64% of the voxels at the end of the CV step, 32% of the voxels reached Li0.95C6 and 2% Li0.90C6 at the end of the charge. Therefore, the majority of the pristine cell reached LiC6 state without significant heterogeneity across the volume. On the other end, the aged cell did not reach a fully lithiated LiC6 state. 25% of the voxels in the area showed LiC6 formation, while 71% of the voxels displayed Li0.8<x<0.9C6. The cell’s damaged centre showed a lower lithiation state, with Li0.70C6 and the most inactive zones had voxels with Li0.60C6. The second method used was a single-point diffraction measurement with a multi-channel-collimator (MCC-XRD) to resolve the temperature within arbitrarily selected internal locations for real-time quantifications during operation. Multi-Channel Collimator (MCC) allows high speed (sub-second), high angular resolution and high signal-to-noise XRD measurements from a given spatial location (gauge volume). During cell charge and discharge, in addition to thermal strain due to the Joule heating effect, the Cu current collector develops mechanical elastic strain caused by stress build up inside the cell, originated by different volumetric expansion/contraction of anode and cathode. The MCC-XRD analysis was designed with the specific purpose of separating the mechanical from thermal strain and explore the temperature during stages that precede the peak temperatures. As proof of concept, the MCC-XRD is used to analyse a LiFun pouch cell to study the formation cycles with a particular focus into resolving the electrode depth. Moreover, results from a stack of prismatic cell will be presented to prove the validity of the technique on more complex systems such as battery modules.

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