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As a promising matrix material for anchoring sulfur in the cathode for lithium-sulfur (Li-S) batteries, porous conducting supports have gained much attention. In this work, sulfur-containing C-rich SiCN composites are processed from silicon carbonitride (SiCN) ceramics, synthesized at temperatures from 800 to 1100 °C. To embed sulfur in the porous SiCN matrix, an easy and scalable procedure, denoted as melting-diffusion method, is applied. Accordingly, sulfur is infiltrated under solvothermal conditions at 155 °C into pores of carbon-rich silicon carbonitride (C-rich SiCN). The impact of the initial porosity and microstructure of the SiCN ceramics on the electrochemical performance of the synthesized SiCN-sulfur (SiCN-S) composites is analysed and discussed. A combination of the mesoporous character of SiCN and presence of a disordered free carbon phase makes the electrochemical performance of the SiCN matrix obtained at 900 °C superior to that of SiCN synthesized at lower and higher temperatures. A capacity value of more than 195 mAh/g over 50 cycles at a high sulfur content of 66 wt.% is achieved.

Within this work we define structural properties of the silicon carbonitride (SiCN) and silicon oxycarbide (SiOC) ceramics which determine the reversible and irreversible lithium storage capacities, long cycling stability and define the major differences in the lithium storage in SiCN and SiOC. For both ceramics, we correlate the first cycle lithiation or delithiation capacity and cycling stability with the amount of SiCN/SiOC matrix or free carbon phase, respectively. The first cycle lithiation and delithiation capacities of SiOC materials do not depend on the amount of free carbon, while for SiCN the capacity increases with the amount of carbon to reach a threshold value at ~50% of carbon phase. Replacing oxygen with nitrogen renders the mixed bond Si-tetrahedra unable to sequester lithium. Lithium is more attracted by oxygen in the SiOC network due to the more ionic character of Si-O bonds. This brings about very high initial lithiation capacities, even at low carbon content. If oxygen is replaced by nitrogen, the ceramic network becomes less attractive for lithium ions due to the more covalent character of Si-N bonds and lower electron density on the nitrogen atom. This explains the significant difference in electrochemical behavior which is observed for carbon-poor SiCN and SiOC materials.