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

ABSTRACT 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, ZrF 6 2− anions immobilized on cellulose‐bound Zr 4+ sites (ZrF‐CNF) redistribute electron density around the Zr 4+ ‐centered coordination environment, amplifying Lewis acidity while introducing polar fluorinated domains. Such ZrF‐CNF separator couples Li + desolvation/transport with PF 6 − enrichment/activation, thereby directing anion‐derived interphase formation at both electrodes. In Li||LiNi 0.8 Mn 0.1 Co 0.1 O 2 (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.