Quantitative Dielectric Constant Engineering of the Separator for Reversible Lithium Metal Batteries.

Current research on separators for lithium metal batteries lacks unified theoretical frameworks and essential quantitative parameters, limiting reversibility, interfacial stability, and lifespan. Herein, we reveal a pronounced nonlinear correlation between the separator dielectric constant and ion transport kinetics. Specifically, an organic-aqueous interfacial reaction is employed to drive polycondensation on the polypropylene separator, enabling wide-range tuning of the separator dielectric constant (2.7-9.1). The dielectric constant serves as an effective descriptor of separator polarization under the internal electric field of the cell. The separator with an intermediate dielectric constant (∼5.4) optimizes the trade-off among ion transport kinetics, SEI stability, and lithium deposition, whereas excessively low or high dielectric constants favor organic-rich SEI formation and restricted transport kinetics, respectively. This strategy enables a Li||LiFePO4 pouch cell (416.4 Wh kg-1, excluding packaging weight) to retain 76.88% of its initial capacity after 1000 cycles.