Electrolyte Motion–Induced Salt Inhomogeneity (EMSI) Can Occur Across Cell Formats: A Mechanistic Study in Single-layer Pouch Cells and Mitigation Strategies

Electrolyte-motion–induced salt inhomogeneity (EMSI) is increasingly recognized as a failure mode in fast-charging. However, its generality beyond large jelly-roll cells has remained unclear. In this work, we present this effect for the first time in single-layer pouch cells. EMSI arises whenever two conditions coincide: charge-induced pore-volume reduction with pore filling ratio >1 and strong through-plane salt polarization. We reproduce the reported EMSI fingerprint, a reversible, week-scale rise in ohmic resistance accompanied by rate-dependent capacity loss and use it as a diagnostic marker. Direct mapping by ion chromatography and ATR-FTIR spectroscopy reveals centimeter-scale LiPF₆ gradients, with up to ∼3× center-to-edge differences across 2.5 cm after ∼20 equivalent fast-charge cycles. These gradients require nearly a week to dissipate and coincides with edge-localized Li deposition. Their homogenization is tracked by a reversible drop in high-frequency resistance during the cell rest period. A coupled pseudo-3D electrochemical-fluid model reproduces the experimental trends and illustrates how the resistance evolves. Parameter scans of electrolyte amount, charge/discharge rate, temperature, and silicon content chart the onset conditions and motivate actionable mitigation strategies. Together, these results establish EMSI as a general design and testing challenge across cell formats whenever electrode stacks are mechanically constrained.