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

Polymer derived silicon oxycarbide (SiOC) ceramics are investigated as potential anodes for lithium ion batteries. Different SiOC ceramics are prepared by pyrolysis (1000 degrees C and 1400 degrees 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, Si-29 MAS-NMR spectra show the presence, in the structure of the materials pyrolysed at 1000 degrees C, of mixed bond tetrahedra such as: SiO3C, SiO2C2, SiOC3 and SiO4 units. Pyrolysis at an elevated temperature (1400 degrees 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.