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

Abstract

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 °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. N2 adsorption–desorption isotherms showed the presence of micro and mesopores for all samples, with a maximum surface area of 505 m2 g−1 for NCC-800. CO2 adsorption isotherms showed that all NCC-X materials present ultramicropores. 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.