Ultramicropore-Confined Solvation and Interphase Regulation Unlock High-Performance Hard Carbon Anodes for Sodium-Ion...

Hard carbon (HC) anodes are promising for sodium-ion batteries, yet achieving high initial Coulombic efficiency (ICE), large plateau contribution, and fast charge-transfer kinetics remains challenging due to insufficient control of micro-nanostructure and interphase chemistry. Here, we present a precursor-level molecular engineering strategy that simultaneously regulates sodium desolvation and interphase chemistry in HC. An iodine-mediated oxidative cross-linking process converts starch into spherical HC with uniformly distributed ultramicropores and carbonyl-rich surfaces. These nanoconfined pores are proposed to act as molecular sieves, preferentially excluding bulky solvent molecules while allowing PF₆^--coordinated Na⁺ access, thereby favoring anion-enriched electrolyte structures under confinement. Surface carbonyls exhibit strong PF₆^- affinity, which may promote fluorine-rich inorganic interphases on pore surfaces. The synergistic effects of anion-selective adsorption and confined desolvation are suggested to favor thin and robust NaF-rich interphases at external surfaces and within nanoconfined pore regions, contributing to reversible interfacial reactions and rapid Na⁺ storage kinetics. Consequently, the HC delivers an ICE of 88.4%, a reversible capacity of 352.9 mAh g^-1 at 0.1C, excellent rate capability (288.9 mAh g^-1 at 5C), and 95.6% capacity retention over 200 cycles. This work offers a molecular-level design paradigm integrating efficiency, capacity, and kinetics in HC anodes.