Porous 3D graphene-based biochar materials with high areal sulfur loading for lithium–sulfur batteries

Abstract

The low electronic conductivity of sulfur and the high solubility of polysulfides seriously hinder the practical application of lithium–sulfur (Li–S) batteries. Therefore, the incorporation of sulfur into carbon-based materials is considered as a suitable solution. Here, a porous pomelo biochar/graphene composite (PBG) was prepared via a simple and green method combining hydrothermal carbonization and KOH activation. The obtained material was used as a host to encapsulate sulfur for the cathode of Li–S batteries, and the three-dimensional pore structure with enhanced conductivity is beneficial for the utilization of sulfur and absorption of soluble polysulfides. As a result, the PBG-S composite (63.3 wt% sulfur) delivered an initial discharge capacity of 1053 mA h g⁻¹ at 0.1C (1C = 1675 mA g⁻¹) and retained 418 mA h g⁻¹ at 3C, even with a high sulfur loading of 4.0 mg cm⁻². In addition, the performance of the composite was further improved by reducing the content of sulfur to an appropriate ratio in the PBG-S composite. The optimized PBG-S composite (48.6 wt% sulfur) exhibited a high initial discharge capacity of 1368 mA h g⁻¹ at 0.1C and retained 638 mA h g⁻¹ at 3C, and the discharge capacity remained as high as 664 mA h g⁻¹ and 354 mA h g⁻¹ even after the 200^th and 600^th cycles at 1C, respectively. The results indicated that PBG, with excellent electrochemical properties, is an ideal electrode material for lithium–sulfur batteries, and can also be prepared economically on an industrial scale.