Spatially Resolved X-ray Fluorescence for Quantifying Electrolyte Salt Distribution in Lithium-Ion Batteries

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Abstract
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.

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