Stable SiOC/Sn Nanocomposite Anodes for Lithium-Ion Batteries with Outstanding Cycling Stability

Silicon oxycarbide/tin nanocomposites (SiOC/Sn) are prepared by chemical modification of polysilsesquioxane Wacker-Belsil PMS MK (SiOCMK) and polysiloxane Polyramic RD-684a (SiOCRD) with tin(II)acetate and subsequent pyrolysis at 1000 degrees C. The obtained samples consist of an amorphous SiOC matrix and in-situ formed metallic Sn precipitates. Galvanostatic cycling of both composites demonstrate a first cycle reversible capacity of 566 mAhg(-1) for SiOCMK/Sn and 651 mAhg(-1) for SiOCRD/Sn. The superior cycling stability and rate capability of SiOCRD/Sn as compared to SiOCMK/Sn is attributed to the soft, carbon-rich SiOC matrix derived from the RD-684a polymer, which accommodates the Sn-related volume changes during Li-uptake and release. The poor cycling stability found for SiOCMK/Sn relates to mechanical failure of the rather stiff and fragile, carbon-poor matrix produced from PMS MK. Incremental capacity measurements outline different final Li-Sn alloy stages, depending on the matrix. For SiOCRD/Sn, alloying up to Li7Sn2 is registered, whereas for SiOCMK/Sn Li22Sn5 stoichiometry is reached. The suppression of Li22Sn5 phase in SiOCRD/Sn is rationalized by an expansion restriction of the matrix and thus prevention of a higher Li content in the alloy. For SiOCMK/Sn on the contrary, the matrix severely ruptures, providing an unlimited free volume for expansion and thus formation of Li22Sn5 phase.