Anion-Programmed Lewis-Acidity-Amplified Cellulose Separators for Dual Interphase Regulation in Li||NCM811 Batteries

Separators in lithium metal batteries are typically treated as passive ion-transport membranes, despite their strategic position between two unstable electrode-electrolyte interfaces. Inspired by Lewis acid-base regulation in liquid/ solid state electrolyte systems, we report an anion-programmed, separator-level Lewis acid-base engineering strategy that uses sequential Lewis acid-base interaction to amplify another Lewis-acidity environment. In this design, ZrF₆ ^2- anions immobilized on cellulose-bound Zr⁴⁺ sites (ZrF-CNF) redistribute electron density around the Zr⁴⁺-centered coordination environment, amplifying Lewis acidity while introducing polar fluorinated domains. Such ZrF-CNF separator couples Li⁺ desolvation/transport with PF₆ ^- enrichment/activation, thereby directing anion-derived interphase formation at both electrodes. In Li||LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) cells, ZrF-CNF separator enables homogeneous Li deposition and a gradient LiF-rich SEI on lithium metal, while forming a thin, dense LiF-rich CEI on NCM811 particles. This dual-interface regulation delivers stable Li||Li cycling over 2400 h at 1 mA cm^-2/1 mAh cm^-2, and 80.1% capacity retention over 800 cycles in Li||NCM811 cells at 1C. Furthermore, a 2.1 Ah pouch cell with ZrF-CNF separator delivers an energy density of 383.1 Wh kg^-1 and retains 94% capacity after 100 cycles at 0.3C. This work moves cellulose separators beyond physical ion transport and establishes a chemically programmable biopolymer framework for stabilizing energy-dense lithium metal batteries.