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 degrees C. HCS treated at 1900 degrees C (HCS-1900) has a pure hard carbon structure with appropriate graphitic interlayer distance (0.358 nm) and microcrystal size (L-a similar to 1.61 nm, L-c approximate to 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 pro- posed for Na ion storage. Exquisite differential scanning calorimetry (DSC) analysis suggests that soliated hard carbon has better thermal stability than that of metallic Na.