Siliconoxycarbides (SiOC) are emerging as promising electrode material for Li-ion batteries. They show capacities of up to 800 mAh/g and reasonable cycling stability. SiOCs are amorphous networks that are typically obtained by pyrolysis of various precursors. Depending on the choice of the precursor and the pyrolysis temperature, SiOCs with different chemical compositions and thus different morphologies can be obtained, including such that contain an additional free carbon phase. Moreover, Si or Sn nanoparticles can be embedded into SiOC networks, either by mixing or by modification of the precursor. In this way, further increased Li capacities have been demonstrated.
Currently our understanding of structure and electrochemical behaviour of SiOCs is still limited, in particular with regard to atomic resolution. In this contribution, density functional theory (DFT) calculations are used to model lithiation of SiOC from a an atomistic and electronic perspective. We first address the challenge of designing realistic structure models. For pristine SiOC, the lack of the existence of a crystalline phase prevents straightforward structure-generation using a cook-and-quench approach. In recent literature, SiOC models that were obtained by replacing O by C in amorphous SiO2 networks have been presented. However, in such models C is always 2-fold coordinated, while at least in crystalline forms, C prefers a 4-fold coordination in the vicinity of Si. We therefore develop a strategy that allows to construct models that consist of Si and C that are both 4-fold coordinated while O is 2-fold coordinated. We compare their thermodynamic stability with that of models where C is 2-fold coordinated. Eventually, we study Li insertion in SiOC and present information about Li storage sites, energetic evolution (including voltage profiles), structural evolution (including specific volume changes) and elastic properties. We compare our results to experiment and previous modelling studies.