<p>The electrostatic effects of separators under the internal electric field are often overlooked, leading to the unreliability of traditional theoretical models. Here we introduce the dielectric constant as a descriptor and develop a high dielectric constant fiber separator primarily composed of phosphorylated cellulose. Under the internal electric field, the intense electron displacement polarization within the high dielectric constant separator enhances the charge transfer kinetics and optimizes the solvation structure, thus mitigating the formation of amorphous organic oligomers at the solid-electrolyte interphase. Furthermore, the separator induces the formation of LiF, thereby forming a robust and low-resistance solid-electrolyte interphase. The separator exhibits high ionic conductivity (0.76 mS cm⁻¹ at 25 °C) and Li⁺ transference number (0.68). Consequently, the Li||LiFePO₄ pouch cell with the prepared separator achieve high specific energy exceeding 350 Wh kg⁻¹ (relative to the mass of pouch cells) under practical quantities of active materials and electrolyte.</p>