Sulfur and\nNitrogen Codoped Hard Carbon with Expanded\nInterlayer Distance as an Effective Anode Material for Sodium-Ion\nBatteries

Development\nof sodium-ion battery (SIB) anodes using\ncost-effective\nand earth-abundant elements has gained a lot of attention nowadays.\nThe most promising SIB anode materials still involve carbon-based\nmaterials. The graphite-based materials used in lithium-ion batteries\ndo not work well for SIB anodes as they have insufficient interlayer\nspacing. Crystallinity, doping functionality, and morphology tuning\nare, therefore, crucial to enhance the anodic performance of carbonaceous\nmaterials for SIBs. Herein, sulfur and nitrogen codoped hard carbon\n(SNHC-X) was prepared as an effective anode material for SIBs. The\ndual doping of sulfur and nitrogen in carbon increases the interlayer\nspacing, defects, and electrical conductivity of SNHC-X, which facilitates\nthe Na<sup>+</sup> adsorption capacity, charge diffusion, and mass\ntransportability. The optimized SNHC-1400 electrode exhibits ∼268\nmA h/g specific capacity at 25 mA/g current density with 77.4% initial\nCoulombic efficiency. The composite also possesses good cyclic stability\nat 100 mA/g current density with ∼86.5% capacity retention\nafter 100 cycles. Furthermore, electrochemical impedance spectroscopy\nof SNHC-1400 shows the promoted Na<sup>+</sup> diffusion dynamics\nbetween the interlayers of microcrystalline structures, thereby boosting\nNa<sup>+</sup> insertion through a diffusion-controlled process. Moreover,\nthe assembled full-cell (SNHC-1400 ∥ NVP) displays excellent\ncycling stability and 231 mA h/g specific capacity at 25 mA/g from\nthe anode side. Thus, sulfur and nitrogen codoping into the carbon\nmaterial may offer a promising prospect for developing sodium-ion\nstorage anodes.