Gian Domenico Sorarù

Polymer‐derived ceramics (PDCs) are a class of advanced materials obtained by pyrolysis in a controlled atmosphere of an organosilicon polymer. Their functional as well as mechanical properties originate from the peculiar nanostructures developed during the pyrolysis. Herein, we investigate the formation of transient microporosity in a model PDC (methyl‐silsesquioxane) obtained in Ar, Ar–5%H 2 , CO 2 , and air. It is shown that a common evolution can be detected up to 700°C. At this temperature, the structure of the material in terms of chemical bonds is marginally changed (only redistribution reactions take place), but the medium‐range order is clearly modified moving the system to a more disordered state (detected by small angle x‐ray scattering [SAXS]) and causing the formation of a large amount of open micropores sized at about 1.2–1.7 nm. In the 700–800°C range, the proper ceramization starts causing the formation of a new class of small (around 1 nm) open micropores. These partially annihilate at 900°C in Ar and Ar–H 2 (i.e., in the second part of the ceramization process), whereas they totally collapse in CO 2 due to the formation of a more silica‐like SiOC (less polymerized and viscous). Finally, SAXS points out the persistence of relatively large closed nanovoids of about 1 nm at 1250°C for the samples treated in Ar and Ar–H 2 . These might explain anomalies in terms of density, elastic modulus, and thermal conductivity of this class of ceramics as reported in the literature.

Lithium–sulfur batteries are among the promising high‐capacity candidates owing to the superior theoretical capacity of sulfur, when compared with conventional cathodes such as LiCoO 2 . However, several issues must be addressed before these batteries can be considered fully operational. Major issues regard the insulating nature of sulfur and the so‐called shuttle effect of soluble polysulfides, which dramatically reduces the cathode capacity upon cycling. Herein, three carbon‐containing polymer‐derived ceramic aerogels are characterized belonging to the Si‐C‐O and Si‐C‐N systems, infiltrated with sulfur to work as cathodes for Li‐S batteries. The electrochemical performances are evaluated in relation to the microstructural and chemical features of such materials. In particular, the effect of the pore size of the ceramic matrices on the shuttling behavior of polysulfides is investigated. Despite the high initial specific capacities exceeding hundreds of mAh g −1 , all types of cathodes show stable capacities in the 60–120 mAh g −1 range after 100 cycles.