The Li-storage capacity of SiOC glasses with and without mixed silicon oxycarbide bonds

In this work we investigate the electrochemical behaviour of two silicon oxycarbide (SiOC) glasses synthesized from the same starting precursor. In one case we perform pyrolysis in an Ar flow, while in the second case, the glass is synthesized under a CO2 flow. The microstructural characterization of the glasses unambiguously demonstrates that the Ar-pyrolyzed material (SiOC-Ar) is a SiOC/C-free nanocomposite with mixed SiCxO4-x 0 <= x <= 4 units, whereas the CO2-pyrolyzed sample (SiOC-CO2) is a SiO2/C-free nanocomposite with exclusively SiO4 units forming the amorphous network. Therefore, in this study we investigate two model systems, addressing the question as to whether the mixed SiCxO4-x units in the SiOC glass play an essential role regarding electrochemical performance. The UV-Raman analysis reveals that the sp(2) carbon present in the mixed bond-containing sample is more disordered/has more defects than the one dispersed in the SiO2 matrix. Apart from the above dissimilarities, the materials present comparable microstructures and a similar amount of free carbon. Nevertheless, SiOC-Ar recovers almost twice higher reversible Li-ion storage capacity than SiOC-CO2 (325 vs. 165 mA h g(-1), respectively). We rationalize this difference in terms of the enhanced Li-ion storage in the more disorder free carbon phase of SiOC-Ar, while this disorder is induced by the presence of the mixed-bond units.