Mahla Bakhshi

Electrolyte motion–induced salt inhomogeneity (EMSI) is a critical degradation mechanism that can limit lifetime of large-format lithium-ion cells. Here, ex-situ scanning micro–X-ray fluorescence (µ-XRF) spectroscopy is employed to map electrolyte salt concentration across entire electrodes harvested from 18 650 cells. Lithium hexafluoroarsenate (LiAsF ₆ ) is used as a spectroscopically favorable salt, enabling arsenic fluorescence as an indicator of local salt concentration. Transport properties of LiAsF ₆ are shown, via Advanced Electrolyte Model calculations, to closely match those of LiPF ₆ in relevant carbonate solvents, validating its use as a surrogate salt for this technique. We confirm that the measured As distributions reflect true electrolyte salt concentration by tracking their growth and relaxation over cycling and rest, and by showing consistent patterns across the separator–anode–cathode stack, indicating a cell-level redistribution of salt. The maps reveal that the axial salt gradient along the entire length of the anode is strongly non-uniform. We observe pronounced tab-correlated banding and localized salt-depleted regions beneath the tab, which we interpret as signatures of pressure-dependent electrolyte pumping and tab-driven modulation of EMSI. This work establishes µ-XRF mapping of LiAsF ₆ as a powerful new technique to quantify electrolyte salt distributions with high spatial resolution.

Energy-dense, fast-charging lithium-ion batteries are in high demand due to their essential role in various applications, from consumer electronics to electric vehicles.¹ However, one significant challenge with fast charging is that it can accelerate lithium plating which is a key degradation mechanism that reduces battery life and performance.² Gaining a deeper understanding of this degradation process in commercial batteries, along with its root causes, is essential for advancing battery technology. ³ Recent research has focused on this critical issue, revealing new insights into the aging mechanisms at play during fast charging.⁴ Solchenbach et al.,have shown that fast charging can induce motion of the electrolyte, leading to salt concentration imbalances, or "salt inhomogeneity".⁴ This salt imbalance leads to localized lithium plating on the anode over cycling and ultimately decrease the cell lifespan. This finding suggests that addressing electrolyte motion induced salt inhomogeneity, “EMSI”, is important not only for mitigating lithium plating and extending the lifespan of fast-charging Li-ion batteries, but also for improving cell design and testing methodologies, particularly for large-format commercial cells under high compression.⁴ The salt concentration across the electrode in their work was measured using ion chromatography (IC) and liquid chromatography-mass spectrometry, LC-MS, both of which are costly and not widely accessible in most research facilities. Additionally, the process of punching coin-shaped samples, diluting them with precision in solvent, and performing subsequent analysis on the extracted solvent is cumbersome. Furthermore, even small punched samples provide only averaged results over the entire area of the coin. For instance, punching 15 mm diameter coins across the electrode limits the spatial resolution to only maximum four individual sampling points along the electrode height for commercially available 18650 and 21700 cells. At each sampling point, the salt gradient is averaged over an area of approximately 1.7 cm² (the coin area), which cannot resolve variations in the salt gradient over smaller scales. This lack of spatial resolution and complex analysis technique makes it challenging to effectively quantify the salt inhomogeneity in the electrode. Given the importance of EMSI effects for large-format battery aging studies, we have developed a novel technique that can effectively quantify the salt concentration gradient across the electrode with higher spatial resolution. Here, we present a custom-designed measurement rig featuring a probe head with multiple wire electrodes, enabling the mapping of salt concentration from ionic conductivity derived through impedance measurements. A sketch of the measurement rig developed is shown in Figure 1 as well as the preliminary results on the cathode electrode of an uncycled 18650 LG cell and a fast charged 18650 Samsung cell. References P. Grey and D. S. Hall, Nat Commun, 11, 6279 (2020). Waldmann, B.-I. Hogg, and M. Wohlfahrt-Mehrens, Journal of Power Sources, 384, 107–124 (2018). Münster, M. Diehl, J. E. Frerichs, M. Börner, M. R. Hansen, M. Winter, and P. Niehoff, Journal of Power Sources, 484, 229306 (2021). Solchenbach, C. Tacconis, A. Gomez Martin, V. Peters, L. Wallisch, A. Stanke, J. Hofer, D. Renz, B. Lewerich, G. Bauer, M. Wichmann, D. Goldbach, A. Adam, M. Spielbauer, P. Lamp, and J. Wandt, Energy Environ. Sci., 17, 7294–7317 (2024). Figure 1. a) A simplified illustration of a measurement setup featuring a probe head equipped with an array of wire electrode probes positioned on top of a separator and a battery electrode. b) Preliminary results showing the LiPF₆ concentration profile across the cathode electrode of an uncycled 18650 LG MJ1 cell (green) and a fast-charged, cycled 18650 Samsung 35E cell (red). The profiles reveal differences in salt concentration for the fast-charged cell, with the minimum concentration observed at the bottom region (near the negative terminal). c) Li-plating pattern on the anode electrode of the fast-charged, cycled 18650 Samsung cell, showing that the extent of Li-plating follows the measured salt concentration trend. The most severe Li-plating is observed at the bottom of the electrode, where the salt concentration is the lowest. Figure 1