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 (SiOC_MK) and polysiloxane Polyramic RD‐684a (SiOC_RD) 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 SiOC_MK/Sn and 651 mAhg^?1 for SiOC_RD/Sn. The superior cycling stability and rate capability of SiOC_RD/Sn as compared to SiOC_MK/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 SiOC_MK/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 SiOC_RD/Sn, alloying up to Li₇Sn₂ is registered, whereas for SiOC_MK/Sn Li₂₂Sn₅ stoichiometry is reached. The suppression of Li₂₂Sn₅ phase in SiOC_RD/Sn is rationalized by an expansion restriction of the matrix and thus prevention of a higher Li content in the alloy. For SiOC_MK/Sn on the contrary, the matrix severely ruptures, providing an unlimited free volume for expansion and thus formation of Li₂₂Sn₅ phase.