Reactive Dissolution–Thermal Conversion Enables Closed‐Pore Filling in Hard Carbon toward Potassium Storage

ABSTRACT Hard carbon (HC) is a leading anode candidate for potassium‐ion batteries (PIBs), yet its low capacity and high operating potential limit energy density and practical application. Herein, a reactive dissolution–thermal conversion strategy is developed to engineer HC microstructure at the molecular level. Phosphoric acid dissolves cellulose at room temperature to form P─O─C crosslinks as a structural scaffold, while subsequent mild air oxidation induces ring opening to generate thermally labile C═O and C─O─C groups that act as gas‐sculpting agents during carbonization. Their synergy creates larger closed pores, and concurrent phosphorus doping expands the interlayer spacing, thereby facilitating K + pore filling and markedly enhancing the low‐voltage plateau capacity. As a result, the optimized HC anode delivers a reversible capacity of 341.2 mAh g − 1 , including 242 mAh g − 1 below 0.4 V, with 73.9% capacity retention after 1000 cycles. Coupled with a K 2 Mn[Fe(CN) 6 ] cathode, the full cell exhibits an average discharge voltage of 3.61 V, a specific energy of 312.8 Wh kg − 1 (based on the total mass of K 2 Mn[Fe(CN) 6 ] and HC), and 83.1% capacity retention after 1000 cycles at 0.5 C. This work provides a molecular‐level design strategy for HC anodes toward high‐energy PIBs.