This study explores Na-ions insertion in the “free carbon” phase within two SiCN(O) matrices, prepared with distinct thermal treatments. Structural variations influence electrochemical performance, with SiCN(O)1000 showing higher capacity. Operando Raman spectroscopy reveals microstructural changes during sodium storage resulting in the shifting of the Operando Raman spectra.

Abstract

In this study we investigate the Na insertion process occurring in the “free carbon” phase embedded in two different SiCN(O) matrices with operando Raman spectroscopy. The two SiCN(O) samples have been prepared using two different thermal treatments carried out at 1000 °C (SiCN(O)1000) and 1400 °C (SiCN(O)1400). X-ray diffraction as well as argon adsorption reveal significant structural and morphological differences between the materials. SiCN(O)1000 shows an amorphous nature whereas SiCN(O)1400 reveals the presence of crystalline β-SiC, accompanied by a notable increase in surface area (from 56.7 m²/g to 331 m²/g) and micropore volume (from 0.02 cm³/g to 0.12 cm³/g). These alterations in the ceramic matrix due to thermal treatment affect significantly the electrochemical performance with initial de-sodiation capacities of 112.4 mAh/g and 52.3 mAh/g for SiCN(O)1000 and SiCN(O)1400, respectively. Operando Raman spectroscopy, carried out during the sodiation and de-sodiation of the SiCN(O) ceramics, reveals the microstructural changes occurring to the “free carbon” phase during the storage of sodium ions. As sodium is inserted, a shift in the G-band position is observed in both the samples from about 1600 cm⁻¹ to 1555 cm⁻¹, with a concomitant decrease of the D-band intensity and the distance between defects (L_D) growing from 7.5 nm to 17.5 nm. Upon de-sodiation, SiCN(O)1400 exhibits an inferior storage reversibility compared to SiCN(O)1000. This may be attributed to the irreversible sodium trapping occurring in SiCN(O)1400, highlighted by the reduced efficiency of the electrochemical process.