Ozone induced surface functionalization of hard carbon for sodium-ion batteries: Mechanism, composition, and electrochemical impact

The potential of hard carbon as an anode material for sodium-ion batteries is well-recognized, yet its electrochemical performance remains strongly influenced by incompletely understood surface chemistry. Oxygen functional groups (OFGs) have been reported to exert both beneficial and detrimental effects on solid electrolyte interphase (SEI) formation, initial coulombic efficiency (ICE), and Na⁺ transport, but their specific roles are often obscured by concurrent chemical and structural changes induced by conventional oxidation treatments. Here, ozone is investigated as a controllable gas‑phase oxidant for surface functionalization of hard carbon. By varying the treatment temperature, we distinguish facile functionalization at room temperature, which predominantly targets edge and defect sites, from over‑oxidation at 60–100 °C, which induces pronounced structural degradation and generation of low-molecular highly-oxidized compounds. On this basis, we propose a mechanistically inspired, ozonolysis‑type framework to describe ozone‑induced hard carbon functionalization and structural degradation. Electrochemically, moderate ozone functionalization improves interfacial kinetics and SEI formation without sacrificing reversible capacity, whereas excessive oxidation increases polarization and reduces the ICE. The results of this work refute the common assumption that a high surface oxygen content inherently leads to poor ICE. Instead, both ICE and reversible capacity are governed primarily by the accessibility of the internal porosity.