Flame-retardant polymer-enabled space-confined carbonization toward quasi-spherical hard carbon for high-rate sodium storage

Starch-derived hard carbons suffer from volatile release and insufficient carbon-layer rearrangement during pyrolysis, leading to structural collapse, small graphite domains, and poor sodium storage performance. Here, we report a space-confined carbonization strategy enabled by the flame-retardant polymer p-phenylenediamine polyphosphate (PPD-PP), which coats starch and stabilizes its structure during pyrolysis. Nitrogen- and phosphorus-containing species released from PPD-PP catalyze polysaccharide dehydrogenation and aromatization, forming a protective carbon layer that preserves the granular morphology and directs the growth of long-range nanographite-like domains. The resulting quasi-spherical hard carbon (QSHC) integrates reduced defects and controlled microporosity with a mesoporous framework that facilitates rapid ion transport and enhances intrinsic conductivity. As a result, QSHC delivers a high capacity of 180 mAh g⁻¹ at 1 A g⁻¹, with plateau contributions of up to 70%. Full cells pairing QSHC with Na₃V₂(PO₄)₃F₃@C cathodes achieve an energy density of 239 Wh kg⁻¹ at a power output of 7.1 kW kg⁻¹, while retaining 95% capacity after 120 cycles. This work demonstrates a simple and scalable route for engineering biomass-derived carbons and provides new insights into the rational design of high-rate hard carbon anodes for practical sodium-ion batteries.