Electrolyte Motion–Induced Salt Inhomogeneity in Multilayer Pouch Cells is Affected by Mechanical Constraints and Electrode Tabs

Abstract Electrolyte motion–induced salt inhomogeneity (EMSI) can limit fast-charging performance in lithium-ion cells, yet its underlying mechanisms remain insufficiently understood. Here, we investigate EMSI in multilayer pouch cells with 50 wt. % of a silicon:carbon composite (chemical Si:C) and 50 wt. % graphite in the negative electrode by varying electrolyte fill volume, electrolyte formulation, and mechanical constraints. Long-term fast-charging cycling shows how these factors affect EMSI-induced capacity fade and ultimately trigger cell failure. We show that electrolyte pumping occurs not only under rigid confinement but also under soft confinement, where it gives rise to a counterintuitive, reversed EMSI pattern characterized by edge-enriched and center-depleted salt distributions governed by confinement compliance and the applied state-of-charge window. Under rigid constraints regions of electrodes under high pressure, for example near tabs, show localized salt depletion. Overall, this study establishes EMSI as a multifaceted phenomenon arising from the coupled interplay of electrochemical, mechanical and cell format factors, rather than a simple consequence of anode-driven expulsion of low-concentration electrolyte during fast charging. These results offer actionable guidelines for improving fast-charging durability across Li-ion cell formats