Impact of Nitrogen Functionalities in a Mesoporous Carbon to Use It As a Conductive Material in Cathodes for Li-S Batteries
Authors: Laverde Munera Jennifer , Rosero-Navarro Nataly , Buitrago Robison , Lopez Diana 
Published: July 2022

The Li-S batteries have gradually become a good candidate for energy storage due to its high specific theoretical capacity value (1675 mA h g-1), with an order of magnitude ten times higher than that of ordinary Li-ion batteries. The performance of these devices significantly depends on the physicochemical properties of the materials that compose them. Therefore, it is necessary to obtain cathodes that help to minimize the well-known shuttle effect causing a drastic decrease in the capacity of the batteries due to the formation of insoluble lithium sulfide species. The carbonaceous materials obtained from templates offer the advantage of modulating and incorporating nitrogen to retain lithium sulfides physically and chemically and, in this way, keep a high specific capacity through a large number of cycles. To understand the deactivation process of these batteries, three N-doped carbons were synthesized varying the carbonization temperature (600- 850 °C) to obtain different N-functionalities and thus evaluate their effect on the electrochemical performance of these batteries.

A N-doped mesoporous carbon was synthesized using SBA-15 as a template, as nitrogen and carbon sources, ethylenediamine (EDA), and carbon tetrachloride were used (CCl4). The silica was impregnated with them followed by carbonization variating the temperature (600-850 °C) to incorporate different nitrogen functionalities, the obtained material was labeled as MC-x, with x = carbonization temperature. The S/C composite were formed with the melt-diffusion process at 155 °C for 6 h with 10 wt.% of S8. Raman spectroscopy was used to study the graphitization changes, XPS was used to determine the functional groups present in the materials, and the efficiency in sulfur inclusion within the MC-x pores was studied by XRD, TGA, and Ads-Des isotherms, while by SEM-EDX the S8 distribution on the material was evaluated. The electrochemical evaluation was carried out with the use of discharge tests at 0.1 C and electrochemical impedance spectroscopy (EIS). For the battery assembly, lithium was used as anode and the cathode was formed with the composite, PVDF, and acetylene black in a ratio of 70:15:15 dispersed in N-methyl pyrrolidone. The electrolyte was a 1 M solution of the LiTFSI salt with 0.1 M LiNO3 dissolved in DOL and DME (v/v,1:1).

The nitrogen content decreased with the increase in the carbonization temperature, the XPS analysis revealed that the pyridinic and pyrrolic functionalities decreased at higher temperatures, while the graphitic nitrogen content increase. This result is correlated with that obtained from Raman where the carbonized material at 850°C presents greater graphitization that involves better electronic conductivity. The XPS result for the S/C composites indicates the formation of S-C bonds that helps to stabilize the active material during the redox reactions, however, for MC-600 and MC-700 the XPS show the presence of sulfated sulfur forms that were not detected for MC-850 affecting the electrochemical behavior of these batteries. The EIS results showed a decrease in the resistance to charge transfer when the MC-850 material was used (43 Ω) compared to the one with the lowest carbonization temperature, i.e MC-600 (105 Ω). The battery with MC-850 presented a higher initial discharge (983 mAhg-1), while the lowest value was obtained with MC-600 (520 mA h g-1). After 100 cycles, the same trend was maintained with a final value of 420 mA h g-1 for MC-850. This result is interpreted because of the decrease in the degree of graphitization of the carbonaceous material that leads to a decrease in electronic conductivity. The previous results corroborate the importance of controlling the nitrogen functionalities and content during the synthesis of the carbonaceous material to improve the electrochemical performance.

The authors wish to thank Universidad de Antioquia UdeA and the Microscopy Laboratory at Instituto Tecnológico Metropolitano (ITM) for their support.

Figure 1