Anionic Covalent Organic Framework Separator Enabling Guided K ⁺ Desolvation‐Transportation Toward Dendrite‐Free Potassium Metal Batteries

To mitigate interphase instability and suppress dendrite formation on potassium metal anodes, employing nanochannel separators with fixed charges offer an appealing solution. Nevertheless, their precise design based on covalent organic framework (COF) to simultaneously harness facilitated cation desolvation, expedited selective ion transport, and promoted inorganic‐rich solid‐electrolyte interphase (SEI) formation remains lacking. Herein, we report an innovative integration of anionic COF crystals with cellulose fibrous scaffold to form a customized membrane separator in a scalable fashion. The engineered fixed‐charge microenvironment allows to reduce solvent coordination of K + and facilitate interfacial desolvation, while promoting potassium bis(fluorosulfonyl)imide dissociation to build a KF‐rich SEI that homogenizes interfacial ion flux and ensures uniform K deposition. Benefiting from the guided cation desolvation‐diffusion, our separator affords a K + transference number of 0.87 and enables stable K plating/stripping for over 3000 h at 0.5 mA cm −2 /0.5 mAh cm −2 . The thus‐assembled full cells deliver 82 mAh g −1 after 1000 cycles at 1.0 A g −1 . Notably, as the temperature decreases from 30 to −20°C, 89% of the capacity could be retained. This work pioneers a separator design strategy for durable and sustainable potassium metal batteries.