Unveiling the Critical Role of Pre-Hydrothermal Effect in Plant Biowaste-Derived Hard Carbon for Superior Rate Capability and Cycle Life in Sodium-Ion Batteries

Leveraging economically viable plant bio-waste-derived hard carbon (HC) anode materials for sodium-ion batteries is logical. Many plants' bio-waste materials are used as HC precursors, but their fabrication process is usually limited by direct carbonization which constrains their large-scale sustainability. Herein, the critical role of the pre-hydrothermal carbonization effect in regulating the structure and interfacial Na⁺ storage mechanism/performance of HC derived from oak leaves (OL) biowaste (OLHC) is reported. The resultant OLHC demonstrates a high-reversible capacity (378 mAh g⁻¹ at 0.1 C), superior rate performance (272.9 mAh g⁻¹ at 10 C), remarkable cycling performance (75% after 8000 cycles at 10 C), and adequate ICE (85%). Advanced ex/in situ characterization combined with theoretical calculations reveals that hydrothermal pre-regulation of OLHC stabilizes the spherical particles, introducing more active sites and promoting surface properties with oxygen dopant-induced defects, which shows uneven surface electrostatic potential and lower activation energy for Na⁺ adsorption thus generates a thin layer of PF₆⁻/NaF-enriched core-shell-like SEI modulation with organic–inorganic composition. This enables fast interfacial Na⁺ diffusion kinetics, contributing to high-capacity retention and stable cycling performance. The studies offer a systematic understanding of the pre-hydrothermal strategy for the structural design of HC from plant-leaves-biowaste with true sustainability and improved performance for SIBs.