High nitrogen content carbons: Morphological and chemical changes with synthesis temperature and application in lithium-sulfur batteries

We present a new two-step synthesis method to prepare nitrogen-doped carbons with micro, meso and macroporosity. We modified the classical polycondensation of resorcinol-formaldehyde, by adding a large excess of melamine in basic medium. A series of materials were prepared by varying the maximum carbonization temperature in the range 600-900 degrees C, and are denoted NCC-X, where X denotes that maximum temperature. NCC-X showed a high nitrogen content, ranging from 32.9% to 10.3%. Scanning electron microscopy showed macropores in the order of 100-600 nm, with sizes decreasing with temperature, reaching a minimum for NCC-800, and then increasing again. N-2 adsorption-desorption isotherms showed the presence of micro and mesopores for all samples, with a maximum surface area of 505 m(2) g(-1) for NCC-800. CO2 adsorption isotherms showed that all NCC-X materials present ultrami-cropores. NCC-X were incorporated as host materials for elemental sulfur in lithium-sulfur batteries. The increased narrow micropore volume of materials pyrolysed at higher temperature seems to promote an initial higher cell capacity. Conversely, the much higher N content and the higher amount of N in pyridinic environments were identified as the reasons for the higher cycling stability of the cells prepared with NCC-600-7h and NCC-750. (C) 2020 Elsevier Ltd. All rights reserved.