Phenolate-Mediated Structural Evolution of Lignin toward Hard Carbon Anodes for Sodium-Ion Batteries

Abstract

Hard carbon (HC) anodes are typically described in terms of their final structure. However, this structure is influenced by multiple competing factors, and precursor-level chemical interactions can play an important role in directing its eventual evolution. Here, metal coordination is used to direct lignin precursor organization and control the evolution of lignin-derived HCs. Magnesium gluconate (MgGlu2) introduced into alkaline lignin promotes selective Mg2+ coordination with oxygen-containing functional groups, enabling reorganization of the lignin network prior to carbonization. Density functional theory (DFT) calculations show a thermodynamic preference of Mg2+ for phenolate (Ph–)-rich environments over gluconate ligands and β–O–4 linkages, consistent with the formation of cross-linked precursor configurations. This coordination is preserved during freeze-drying and translates into a distinct carbon microstructure upon pyrolysis. The resulting carbon exhibits a more accessible pore structure and improved cycling stability, retaining 93.6% of its post-solid electrolyte interphase (SEI) capacity after 400 cycles, compared to 8.7% for the pristine counterpart. These results demonstrate that phenolate-directed Mg2+ coordination governs precursor organization and contributes to the structural stability of lignin-derived HCs. More broadly, this work establishes precursor-level coordination as a route to control structure–performance relationships in HC anodes.