Intergrowth of Graphite‐Like Crystals in Hard Carbon for Highly Reversible Na‐Ion Storage

Abstract Hard carbon usually refers to non‐graphitizable carbon, which does not transform into a crystal structure even at high temperatures. Although hard carbon is considered as the most promising anode material for Na‐ion batteries (NIBs), its practical application is still hindered by its low initial coulombic efficiency (ICE). Here, the intergrowth of large area graphite‐like crystals in hard carbon is reported using cotton as precursor and graphite as crystal template by calcination under their intimate contact condition. Extensive characterizations provide abundant evidence that graphite‐like crystal is a new carbon allotrope belonging to the hexagonal system (a = b = 3.528 Å, c = 9.6 Å, α = β = 90°, γ = 120°) with d‐spacing of 4.8 Å for (002) plane. Its unit cell dimensions and d‐spacing of (002) plane are all enlarged by the same factor of 1.428 compared to those of the graphite phase. When applied in NIBs, the hard carbon with graphite‐like crystals shows an extremely high ICE of 95.0% with a large reversible capacity of 343 mAh g −1 , which reaches the same level as graphite used in Li‐ion batteries. The graphite‐like crystal breaks the traditional concept of hard carbon, while the graphite‐template induced epitaxial growth mechanism may provide a new way for obtaining novel crystalline carbon allotropes.