Abstrakt

Polymer derived ceramics (PDCs) have attracted attention as alternative anode material for Li-ion batteries. It has been found that ternary SiOC and SiCN ceramics obtained through pyrolysis of various preceramic polymers display high reversible capacities of 500 – 650 mAh/g. In this work we try to correlate the electrochemical performance of polymer derived silicon oxycarbide with its chemical composition and microstructural features. Various SiOC ceramics were prepared by pyrolysis of polysiloxanes ceramic precursors. Preceramic polymers were synthesized using the sol-gel method. In this way, by varying the sol-gel precursors, different polymers were obtained. In consequence, the final ceramic composition and microstructure can be tuned and controlled. The aim of the sol-gel synthesis was to obtain polysiloxanes with optimal composition in order to get high free carbon content in the final SiOC ceramic. The free carbon phase is considered to be a major lithium storage site [1], while the ceramic matrix is considered to play a stabilizing role. An increased fraction of free carbon was analyzed in the samples prepared from mixtures containing phenyl- and vinyl-substituted polysiloxanes. Raman spectra confirm the presence of disordered carbons by the appearance of a well pronounced D band. The carbon-rich SiOC samples exhibit higher capacity values and improved cycling stability compared to samples with low free carbon content. The best results are obtained for samples prepared at 1000 °C. An increase of the pyrolysis temperature impairs the electrochemical properties due to progressing carbon organization and crystallization of silicon carbide which is inactive vs. lithium insertion.

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