Engineering bilayer SEI via ultrasonic fields for long-life lithium batteries

The spatial architecture of the solid electrolyte interphase (SEI) critically affects the cycling stability of lithium batteries. Although electrolyte formulation is widely used to regulate SEI chemistry, external-field control of its spatial organization during formation remains insufficiently explored. Here, ultrasound was applied only during the 1.4 to 1.0 V vs. Li/Li⁺ FEC reduction interval of the first formation discharge to regulate interfacial mass transport and local reaction uniformity, thereby constructing a bilayer SEI on graphite. Areal, depth-resolved ToF-SIMS, supported by TEM, XPS, and KPFM, reveals a compact amorphous inorganic inner layer and an organic-rich outer layer, in contrast to the compositionally mixed mosaic SEI formed without ultrasound. This architecture improves interfacial passivation, charge transfer, rate capability, and cycling stability. Ultrasound-treated cells retain 80.77% capacity after 500 cycles at 1.0 C, whereas untreated cells undergo rapid capacity decay after approximately 250 cycles. These results demonstrate an interfacial strategy for using a physical field to regulate FEC-derived SEI growth without implying ultrasonic molecular scission of FEC or a graphite lattice effect.