Ion‐Specific Templating in Deep Eutectic Solvent Directs Carbon Architectures for High‐Performance Potassium‐Ion Batteries

Converting sustainable biomass into a high-performance carbon anode for potassium-ion batteries (KIBs) remains mostly an empirical pursuit, hindered by an inability to tune pore hierarchy and defect density. Herein, we uncover a design principle in which the identity of the metal ion within a deep eutectic solvent (DES) governs the carbonization pathway through a precise balance of coordination and decomposition chemistry. Using a choline chloride-urea-MCl₂ (M = Ca, Mg, Zn), we show that only Ca²⁺ guides the formation of a nitrogen-doped carbon with a kinetically ideal structure for K⁺ storage, featuring expanded interlayer spacing (0.376 nm), optimized defect density, and a favorable pyrrolic-N configuration. This Ca²⁺-regulated carbon anode achieves a superior combination of reversible capacity as high as 320 mAh g⁻¹, exceptional rate capability of 212 mAh g⁻¹ at 1 A g⁻¹, and long-term stability (89% retention after 2000 cycles). This work transforms DES from passive green solvents into programmable reaction media, offering a universal principle for predictively designing biomass carbons for potassium-ion storage.