Inherent\nHeteroatom-enriched Amorphous Carbon as \nHigh-performance Electrode for Sodium-ion Battery and Sodium-ion Ultracapacitor

The cassava-peel biomass-derived self-heteroatom-enriched\ndisordered\ncarbon was synthesized by means of carbonization followed by chemical\nactivation using KOH. The obtained carbon was characterized using\nX-ray diffraction (XRD) and Raman and Fourier-transformed infrared\n(FT-IR) spectroscopic studies. The XRD study and Raman results revealed\nthe highly disordered nature of the derived carbon. Morphological\nfeatures of the sample were confirmed using scanning electron microscopy\nand high-resolution transmission electron microscopy analyses. It\nwas observed that the disordered carbon had slit-type pores with a\nhigh surface area of 1032 m2 g−1. The\nX-ray photoelectron spectroscopy analyses confirmed that the disordered\ncarbon was enriched with self-doped heteroatoms (N, O, S). Sodium-ion\nhalf-cell battery was fabricated using the cassava-peel biomass-derived\ndisordered carbon as an anode that exhibited a high discharge capacity\nof 490 mA h g–1 and stabilized at 355 mA h g–1 at a 0.1 C-rate with excellent rate capability and\nreversibility. Further, the disordered biocarbon was examined for\nsymmetric sodium-ion ultracapacitors in both aqueous and non-aqueous\nelectrolytes. The aqueous symmetric sodium-ion ultracapacitor exhibited\na high specific energy of 120 W h kg–1 at a specific\npower of 118 W kg–1, while the non-aqueous symmetric\nsodium-ion ultracapacitor exhibited a high specific energy of 160\nW h kg–1 at a specific power of 425 W kg–1. The symmetric non-aqueous device exhibited almost 100% coulombic\nefficiency even after 10,000 cycles. The enhanced sodium-ion energy\ndensity was attributed to the hierarchical pores and self-doped heteroatoms\nthat promoted diffusive mode of sodium storage, as confirmed by Dunn’s\nmethod as well as Power law. A laboratory prototype non-aqueous sodium-ion\nultracapacitor in the form of CR-2032 coin cell was demonstrated to\npower a commercial red color LED bulb for more than 10 min on single\ncharge.