Completely crystalline carbon containing graphite-like crystal enables 99.5% initial coulombic efficiency for Na-ion batteries

Abstract

Hard carbon, also known as hardly graphitizable carbon, shows great potential for Na-ion batteries (NIBs), but its practical application is greatly hindered by the low initial coulombic efficiency (ICE). Here we report the growth of the completely crystalline carbon from hard carbon precursor under sufficient graphite template-induced conditions. The most crucial growth stage is realized at the low temperature slightly higher than pyrolysis peak temperature (e.g. 380 °C for eggshell membrane), in which sufficient biomass pyrolysis is realized; graphite crystal template can efficiently induce the decomposed carbon atoms and atomic fragments orderly align in parallel fashion enabling their orderly mobility and conversion of sp3-hybridized carbon to sp2-hybridized carbon, promoting the formation of crystal structure. Further high temperature calcination (e.g. 1300 °C) is used to improve the crystal quality, leading to the formation of completely crystalline carbon. The graphite-like crystal possesses a geometrically similar unit-cell to graphite but proportionally enlarged unit-cell constants and interplanar distance of (002) plane. When used as an anode in NIBs, it achieves an ICE up to 99.5% with large reversible capacity of 321.7 mAh/g. Furthermore, it demonstrates only low-voltage plateaus below 0.5 V without high-voltage slope region, showing the same electrochemical behaviors as Li-graphite counterpart.