Acid, Alkaline, or Salt? Impact of the Activators on the Sodium Storage Properties of Lignin-Derived Hard Carbon

Lignin is considered as one of the most potential precursors for hard carbon. The chemical activation is effective for creating hierarchical pores in lignin-derived hard carbons. Nevertheless, the impact and mechanism of different activators on the sodium storage properties are not well studied. Herein, three different activators (H₃PO₄, KOH, and ZnCl₂) were compared in the activation of lignin. The 1.0 mol/L H₃PO₄-activated lignin samples (1.0 H) are doped with P atoms, resulting in a widened layer spacing, a rich nanopore structure, and moderate defect concentration. Thus, storage sites have been improved, and the Na⁺ transport distance has been shortened. As a result, the sample delivers high electrochemical performance. The discharge specific capacity was 342 mAh g^-1 at 0.05 C, and initial Coulombic efficiency (ICE) was maintained at 87%. In addition, 1.0 H exhibits excellent rate performance with a capacity recovery rate of 98% when the C-rate is reduced from 1 to 0.1 C and retains 84% of its capacity after 100 cycles.