Within this work we try to find out which structural properties of the silicon carbonitride (SiCN) and silicon oxycarbide (SiOC) ceramics determine the reversible and irreversible lithium storage capacities, long cycling stability and define the major differences in the lithium storage in SiCN and SiOC. The experimental observations are addressed with respect to DFT modelling.
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 materials the capacity increases with the amount of carbon to reach a certain threshold value at about 50% of carbon phase. The cycling stability of carbon-poor ceramics is however very low for SiCN and SiOC, while increasing significantly for materials with higher carbon content. For SiOC a clear linear dependence of the insertion capacity on the amount of silicon oxycarbide phase is revealed, while for silicon carbonitride there is no dependence between first cycle insertion capacity and SiCN matrix amount. In contrary to the tendency observed for SiOC, the SiCN lithiation capacity is very low for high (> 70 %) amount of SiCN matrix.
It has been stated that replacing oxygen with nitrogen renders the mixed bond Si-tetrahedra unable to sequester lithium. Lithium is more attracted by oxygen in SiOC network due to more ionic character of Si-O bonds, leading to a high electron density on oxygen. This brings about very high first lithiation capacities, even at low carbon content. With continuing cycling lithium is irreversibly captured within the SiOC network bringing low cycling stability. If oxygen is replaced by nitrogen, the ceramic network becomes much less attractive for lithium ions due to more covalent character of Si-N bonds and lower electron density on the nitrogen atom. It explains a significant difference in electrochemical behavior of carbon-poor SiCN and SiOC materials is observed. For carbon-rich ceramics the free carbon phase starts to play a dominant role bringing about high cycling stability, but also leading both to irreversible and reversible capacities.