Extended low-voltage plateau capacity of hard carbon spheres anode for sodium ion batteries

Hard carbon is now one of the most promising anode materials for sodium-ion batteries, but achieving high reversible capacity at low voltage plateau (0–0.1 V) is still a major challenge, which can actually enhances the operating voltage as well as the energy density when coupled with cathodes. In this work, a series of hard carbon spheres (HCS) with controlled architectures for sodium ion batteries (SIBs) are prepared by carbonizing synthetic phenolic resin over a wide temperature range from 900 to 2800 °C. HCS treated at 1900 °C (HCS-1900) has a pure hard carbon structure with appropriate graphitic interlayer distance (0.358 nm) and microcrystal size (La ~ 1.61 nm, Lc ≈ 3.07 nm). It delivers a reversible capacity of 295 mAh g−1 and an ultra-large capacity of 248.2 mAh g−1 (84% of the reversible capacity) at low-voltage plateau, in which an intercalation mechanism is proposed for Na ion storage. Exquisite differential scanning calorimetry (DSC) analysis suggests that soliated hard carbon has better thermal stability than that of metallic Na.