Molten salt assisted synthesis of a lignin derived porous carbon host for lithium–sulfur battery cathodes

Abstract

Lithium–sulfur batteries offer high theoretical energy density and low cost. However, challenges such as the polysulfide shuttle effect and poor conductivity of sulfur have hindered their commercial application. In this work, porous nitrogen-doped carbon materials were synthesized from Kraft lignin using a molten salt method combined with urea and K₂CO₃ for pore generation in a single synthesis step. The use of a KCl/NaCl salt mixture as the reaction medium allowed for control over product morphology and increased yield by retaining volatiles. The effects of varying lignin/urea/K₂CO₃ mass ratios on the characteristics of the produced materials were analyzed. The carbon-based host materials were then combined with sulfur through chemical deposition and melt diffusion. The properties of the host materials and composites were characterized using TGA, SEM, EDS, BET, XPS, and Raman analyses. Electrochemical tests were conducted to study the impact on the electrochemical properties of the lithium–sulfur battery cathode. The analysis revealed that the controlled porosity and functionalization of the host materials significantly influence the distribution and utilization of sulfur during electrochemical testing. By analyzing the effects of host material porosity and nitrogen doping, we improved the electrochemical properties of the cathode material. The best performing composite material exhibited a high initial discharge capacity 1407 mAh per g-S (83% of the theoretical capacity of sulfur) and retained 825 mAh g⁻¹ capacity (average fade of 0.105% per cycle) and 98.7% coulombic efficiency after 200 cycles. In addition, the material displayed good performance at commercially viable mass loading.