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

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 angstrom, c = 9.6 angstrom, alpha = beta = 90 degrees, gamma = 120 degrees) with d-spacing of 4.8 angstrom 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.