Abstract
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 °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.
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