Synthesis and optimization of nitrogen-doped activated carbon electrode for enhanced lithium polysulfide adsorption in lithiumsulfur batteries

The polysulfide shuttle effect significantly impedes the commercial viability of lithiumsulfur (Li-S) batteries by causing rapid capacity fading and low coulombic efficiency. Incorporating porous carbon materials as the cathode host is a promising approach to address this issue. Biomass-derived carbon materials offer substantial advantages due to their abundance, renewability, and cost-effectiveness. Palm kernel shell biomass (PKS), generated in vast amounts in Malaysia, holds exceptional promise for conversion into composite electrodes with sulfur for Li-S batteries. This approach not only enhances resource efficiency but also contributes to the circular economy. Enhancing the lithium polysulfide (LiP) adsorption capacity of carbon materials requires modifying surface functionalities and pore properties. Factors such as impregnation ratio, activation temperature, and activation time are critical and require systematic optimization. This study aims to synthesize activated carbon (AC) from PKS with chemical modifications to enhance polysulfide adsorption capacity for improved Li-S battery performance. Potassium hydroxide (KOH) was used as the activation agent, while urea was utilized for nitrogen doping. Response surface methodology (RSM) and artificial neural networks (ANN) were employed to optimize and predict LiP adsorption on the nitrogen-doped activated carbon (NDAC). Although recent studies have focused on using AC cathodes for their strong affinity to lithium polysulfide, there has been limited investigation into the quantitative adsorption of lithium polysulfide with varying ratios of urea doping in AC. Thus, this study aimed to quantitatively assess the polysulfide adsorption capacity uptake for high performance of AC-cathode in lithium sulfur batteries. The results of this study indicate that increasing the urea ratio improves the AC's porosity and enhances LiP adsorption. The NDAC with the biomass-to-urea ratio of 1:3 exhibited the most remarkable adsorption uptake capacity of 12.94 mmol/g, attributed to the well-developed porosity (BET surface area of 1902.99 cm²/g and a pore volume of 0.92 cm³/g) and high nitrogen functionalization on the carbon surface, which contributes to the formation of physical and chemical bonds between polarized lithium polysulfide and the carbon matrix. RSM analysis indicated that the impregnation ratio was the most significant factor affecting LiP adsorption, with optimal NDAC achieved at an activation temperature of 880°C, an impregnation ratio of 2.0, and an activation time of 80 minutes. The experimental results from RSM were used to train the predictive capabilities of the ANN for LiP adsorption. A comparison between ANN and RSM revealed that both methods provided high-quality predictions, with R² values of 0.9901 and 0.9756, respectively. However, ANN exhibited superior prediction accuracy with a lower Mean Squared Error (MSE) of 0.003 compared to RSM's 0.007. The NDAC synthesized under optimal conditions exhibited a large specific surface area of 2303.28 m²/g and a pore volume of 1.27 cm³/g, featuring a variety of pore sizes in the micro and meso ranges. The sulfur composite cathode (NDAC/S) demonstrated excellent electrochemical performance, with an initial discharge capacity of 1043.35 mAh/g at 0.1C and retaining a capacity of 686.52 mAh/g after 100 cycles. This capacity is five time higher than commercial lithium-ion batteries of 278 mAh/g. This outstanding electrochemical performance is due to the synergistic effect of the hierarchical pore structure, large surface area, substantial pore volume, and the presence of doped nitrogen, which provides strong chemical bonding with LiP. Overall, this study provides valuable insights into the utilization of PKS-derived NDAC as a promising host matrix for future Li-S batteries, potentially advancing their commercial viability.