Abstract:

Lithium-sulfur batteries (Li-S) have attracted extensive attention as energy storage devices owing to their high theoretical capacities (1675 mAh/g) and specific energy densities (2600 Wh/kg). However, the poor electronic conductivity of elemental sulfur and the "shuttle effect" of the intermediates polysulfides (Li₂S _n, 4≤n≤8) occurring during the discharge/charge processes lead to the lower utilization of active sulfur and an irreversible capacity loss of the cathode materials. Therefore, finding a cost-effective, recyclable, and thermally stable carrier matrix is crucial in improving the utilization of elemental sulfur and enhancing the electrochemical performance of Li-S batteries. In this work, a three-dimensional porous carbon-sulfur nanosphere composite material was prepared using the facile chemical method through natural lignin as the carbon source. First, the carbon nanospheres were prepared using sodium lignosulfonate as a carbon source through extraction and carbonization. Then, elemental sulfur was successfully impregnated into the voids of lignin-based carbon nanospheres in the melting process to obtain the LS-C/S nanosphere composites. When used as the cathode material for the Li-S batteries, the nanocomposite material with 59.41% sulfur content delivered the first discharge/charge capacities of 800.3 mAh/g and 758.8 mAh/g at the current density of 0.1 C, showing the Coulombic efficiency of 94.8%. The capacity stabilized at 647.4 mAh/g after 200 repeated discharge/charge cycles with a capacity retention rate of 84.3%, corresponding to an average capacity loss of 0.0785% per cycle. Additionally, after multiple high-rate discharge/charge cycles, the specific capacity of the LS-C/S nanocomposites still recovered and stabilize at 620 mAh/g, showing excellent reversible rate capability. The lignin-based carbon nanospheres with high specific surface areas and porous structures effectively promote the transport of Li⁺ and e^-, suppress the "shuttle effect" of lithium polysulfides, and improve the utilization of sulfur materials. Hence, the composite electrode showed superior cycling stability and rate performance when employed as a cathode for Li-S batteries.

Key words: lithium-sulfur batteries, lignin, carbon nanosphere, cathode material, electrochemical performance