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XRD-CT: A non-invasive probe of lithium distribution in high-power 18650-type Li-ion batteries at µm- scale

The spatial lithium distribution in the positive (LiFePO4) and negative (graphite) electrode of a high-power cylinder-type Li-ion battery was studied using X-ray diffraction computed tomography. Irregularities in lithium distribution were found to be related to the specific current distribution in the studied cell type.

Lithium-ion batteries play an important role in our daily life. Their outstanding performance makes them the major energy storage technology for portable electronics and electric drivetrains. Real-life cells exhibit complex interactions between chemical, mechanical, morphological and transport properties, which are crucial for optimisation, estimation and prediction of cell parameters and behaviour during standard operation and misuse. Li-ion batteries exhibit non-uniform distributions of lithium, electrolyte, salt and other components that highly depend on the state- of-charge (SOC) of the cells and properties like capacity, current, temperature, pressure, etc. Cell ageing leads to the development of a heterogeneous distribution of battery properties [1]. These features are often difficult to predict by modelling and require dedicated experimental input, based on multiscale and multiparameter characterisation.

A commercial 18650-type high-power Li-ion battery utilising LiFePO4|C chemistry was selected. Spatially- resolved neutron diffraction experiments with multi-mm resolution [2] revealed a pronounced non-uniformity of lithium concentration in the graphite anode in fully charged state. The lithium heterogeneity was found to be defined by the current distribution along the electrode stripe, which is a direct consequence of the cell design.

The poor spatial resolution used in the neutron experiment made it impossible to resolve separated adjacent electrode windings with a thickness of approximately 100 µm.

Micron-sized beams of high-energy photons were applied in X-ray diffraction radiography (XRDR) [3] and XRD-CT experiments. The lithium distribution along the cylinder axis was probed using XRDR at beamline ID11, while characterisation of the lithium distribution in a horizontal cut at the middle of cylindrical cell was performed using XRD-CT at beamline ID15. The XRD-CT data set yielded 277 200 collected 2D diffraction patterns in total. From the reconstructed diffraction intensities, the spatial distribution of the cell materials (LixFePO4 cathode, LixC6 phases from the anode, PP/PE separator, signal from Cu and Al current collectors and steel cell housing) was extracted. As shown in Figure 109a, the achieved 30-µm resolution was sufficient enough to resolve the different electrode windings, with only a minor overlap of neighbouring materials contributing to the same voxel.

Exploiting the biphasic behaviour in the lithiation of the LiFePO4 cathode and graphite anode along with the known structural behaviour, the lithium concentrations in both electrodes could be unambiguously calculated. The determined lithium distribution in the charged graphite anode displayed in Figure 109b was in good agreement with the results from spatially resolved neutron powder diffraction experiments [1,4]. In both cases, a well-defined concentration plateau was observed in the middle of the electrode strip with systematically lower values at both ends. With the excellent spatial resolution of XRD-CT, two important properties were found. Firstly, the highest lithiation for the winding was close to the current lid, where the current collector is electrically connected. Secondly, the presence of a heterogeneous lithium distribution across the electrode thickness was observed, reflected in the systematically higher lithium concentration at the

Fig. 109: a) Layout of the studied 18650-type cell in the central plane as deduced from XRD-CT experiments, with different cell components highlighted. Spatially resolved lithium content (corresponding to degree-of-lithiation (DoL) distribution) is shown in b) the anode and c) the cathode of the studied 18650-type cell.