Steam explosion of inner birch bark reveals mechanistic links between hemicellulose content, carbonization dynamics, and hard carbon properties for Na-ion storage

The performance of lignocellulosic biomass-derived hard carbon (HC) in sodium-ion batteries (SIBs) is strongly influenced by feedstock purity, macromolecular composition, and carbonization conditions. However, rational strategies to deliberately tailor feedstock properties prior to carbonization remain limited. Here, we demonstrate that mild steam explosion (STEX) pretreatment offers an effective and selective approach to modify inner birch bark (IBB) at both compositional and ultrastructural levels, enabling systematic control over HC formation and electrochemical performance in SIBs. Regulating the severity of STEX pretreatment allows for targeted removal of hemicelluloses and calcium from IBB, which in turn alters the carbonization reaction dynamics and enthalpy, and influences the porosity and nanotexture of the resulting HCs. A controlled STEX treatment (200 °C, 14.5 bar, 1 wt.% H₂SO₄ impregnation) induced partial solubilization of IBB components, predominantly hemicelluloses, while selectively removing nearly 80% of calcium, the most abundant inorganic impurity in the untreated material. Consequently, treated bark yielded HCs with a 265% increase in specific capacity relative to untreated IBB. More importantly, this study establishes, for the first time, clear correlations between the hemicellulose content of IBB, its carbonization dynamics and heat of reaction, and the resulting HC properties and performance in SIBs. These findings reveal a previously unexplored mechanistic link between targeted biomass refining and HC electrochemical performance, positioning STEX as a valuable pretreatment strategy to upgrade underutilized bark residues into high-performance anode materials for sustainable SIBs.