K₂O Encapsulation-Decomposition Mechanism: Unlocking Closed-Pore Engineering in Hard Carbon Anode for Sodium-Ion...

The chemical mechanism by which KOH activation tailors the closed-pore structure of hard carbons (HCs) for sodium-ion batteries remains an open question. Herein, we report the in situ formation and pivotal role of crystalline K₂O nanodomains (∼79.2 nm) as a chemical template. Advanced characterization via high-resolution synchrotron X-ray diffraction and pair distribution function analysis reveals that these K₂O domains, formed during KOH pretreatment at 600 °C, become nanoconfined within the carbon matrix. Upon carbonization at 1400 °C, their thermal decomposition triggers a concerted chemical-mechanical transformation: it reduces graphitic crystallite size, induces a phase transition (shifting the P6₃/mmc to R3̅m ratio from 41.45:57.95 to 32.87:67.13 wt %), and generates uniform closed pores (avg. 2.79 nm). This K₂O-templated microstructure enables exceptional sodium storage, delivering a reversible capacity of 383.1 mA h g^-1 (262.5 mA h g^-1 from the low-voltage plateau) and outstanding stability over 1000 cycles. This work deciphers the chemical role of a key intermediate in alkali-activation chemistry, providing a foundational principle for precise microstructure design in functional carbons.