Hard carbon anodes for sodium-ion batteries: from mechanistic reconciliation, precursor-tailored microstructure regulation to industrial scale-up

Hard carbon is the leading anode candidate for sodium-ion batteries, yet its practical deployment remains constrained by three interconnected gaps: the sodium storage mechanism is disputed across competing models, no precursor classification framework links chemical composition to electrochemical function, and predictive structure–property relationships remain largely empirical. This review traces the precursor-to-performance pipeline. We show that the insertion-filling, adsorption-insertion, and adsorption-filling models converge on a multi-stage adsorption-intercalation-filling paradigm, where the intercalation-to-filling balance is governed by carbonization temperature, precursor chemistry, and pore architecture rather than by a single mechanism. We then introduce a three-dimensional precursor classification framework—spanning scalability, structural controllability, and performance ceiling—and deconvolute the component-specific pyrolysis pathways of cellulose, hemicellulose, and lignin to establish how molecular architecture is transmitted into carbon microstructure. Three microstructure engineering levers are critically evaluated: pore-structure manipulation to resolve the low-surface-area/high-closed-pore-volume trade-off, interlayer-spacing regulation via heteroatom doping and pre-oxidation, and defect engineering guided by the conductivity-capacity Pareto Frontier. The rapid industrialization of hard carbon—from laboratory discovery to 10,000-ton-scale manufacturing—is assessed, documenting how precursor diversification and process innovation have driven costs toward parity with graphite anodes. Finally, the current challenges and future development directions of high-performance and industrializable hard carbon anodes are prospected, aiming to provide systematic theoretical guidance and technical references for the further design, modification and large-scale application of hard carbon materials in advanced sodium-ion batteries.