Hard Carbon Microsphere with Expanded Graphitic Interlayers Derived from a Highly Branched Polymer Network as Ultrahigh Performance Anode for Practical Sodium-Ion Batteries

Growing attention has been attached to hard carbon in sodium-ion batteries (SIBs). However, hard carbon from individual precursors tends to exhibit an inferior rate capability due to its limited interlayer distance. Here, a coupled strategy is designed to prepare hard carbon microspheres (HCMSs) via the pyrolysis of a highly branched polymer network formed instantaneously between two interactive precursors during the atomization of the spray drying process. The combined precursors with a tunable cross-linked structure prefer to generate a large interlayer spacing (0.399 nm) and abundant closed pore structure by suppressing the graphitization of precursors during the carbonization, relative to the individual precursor, which contributes greatly to the ion diffusion kinetics. Benefiting from the unique structure, HCMS exhibits an impressively high reversible specific capacity of 373.4 mA h g^-1 in SIBs and high initial Coulombic efficiency of 88%, retaining 90.2% of the initial capacity even after 150 cycles, which presented comparable capacities with commercial graphite in lithium-ion batteries. Besides, excellent rate capability was also demonstrated with HCMSs (250 and 117 mA h g^-1 at 300 and 600 mA g^-1). Notably, the interlayer distance and closed pore structure are tunable just by adjusting the ratio of the two precursors. The tunable and extendable fabrication process, together with its amazing high carbon yield of 48 wt % (1400 °C) and high tap density close to 0.8 g cm^-3, makes this strategy promising in the practical application for SIBs.