Tailoring of SiOC composition as a way to better performing anodes for Li-ion batteries

• New silicon oxycarbide SiOC materials were studied as anodes for Li-ion batteries.
• The chemical composition and the structure influence the electrochemical activity.
• The SiOC samples exhibit a high recovered capacity and long-life stability.
• The best electrochemical performance was achieved for carbon-rich samples.

Polymer derived silicon oxycarbide (SiOC) ceramics are investigated as potential anodes for lithium ion batteries. Different SiOC ceramics are prepared by pyrolysis (1000 °C and 1400 °C under controlled argon atmosphere) of polysiloxanes ceramic precursors. Preceramic polymers are synthesized using the sol–gel method. Phenyltriethoxysilane (PhTES) and methyltriethoxysilane (MTES) have been used as starting precursors and mixed with different ratios in order to tailor the chemical composition and the structure of the final product. The obtained SiOC ceramics are amorphous with various content of free carbon phase (from approx. 25 to 40 wt.%). The presence of disordered carbons in the ceramic structure is confirmed by the appearance of a well pronounced D band at 1330 cm− 1 in the Raman spectra. Additionally, 29Si MAS-NMR spectra show the presence, in the structure of the materials pyrolysed at 1000 °C, of mixed bond tetrahedra such as: SiO3C, SiO2C2, SiOC3 and SiO4 units. Pyrolysis at an elevated temperature (1400 °C) promotes the phase separation into oxygen rich (SiO4) and carbon rich (SiC4) units with consumption of mixed bonds. Carbon rich SiOC samples exhibit significant reversible capacity and enhanced cycling stability (up to 600 mAh g− 1 measured at a slow current rate of C/20 after 140 cycles of continuous charging–discharging with increasing current density). However, the high irreversible capacity of the first few cycles remains an issue to be solved.