Melamine-formaldehyde resin-derived hard carbon spheres: Morphology-preserved synthesis and closed pore formation for sodium storage.

Melamine-formaldehyde resin spheres (MFS) often suffer from foaming and structure collapse during pyrolysis. The nitrogen's role in closed pore development of hard carbons for sodium storage is also barely investigated. Herein, through pre-stabilizing of MFS at 150 °C using ammonium chloride (NH4Cl) as a curing agent, we obtained nitrogen-rich (38.3 ± 1.7 at.%) spherical precursor at 400 °C and hard carbon spheres via further 800-1500 °C treatment. During the curing phase, the continuous decomposition of NH4Cl generates hydrogen chloride, which catalyzes sufficient condensation of the molecular structure, enabling the uniform shrinkage of spherical framework during subsequent pyrolysis and carbonization. In addition, the nitrogen release creates numerous defect sites, which hinders the ordered microcrystal development. Above 1000 °C, nitrogen is largely eliminated and intermittent microcrystals crosslink to form closed pores. Ultimately, the closed pore surface area greatly increases from 6 to 736 m2 g-1 as the carbonization temperature rises from 800 to 1500 °C, contributing to a remarkable enhancement in low-potential plateau sodium storage capacity from 15.9 to 206.1 mAh g-1. Therefore, this work provides a reference for preparing MFS derived hard carbon spheres, demonstrating that nitrogen in the precursors favors the formation and development of closed pores during high-temperature carbonization.