Epitaxial and Near-Amorphous CoSn2 Thin Films: Crystallinity-Controlled Li Storage

Abstract Control over microstructure can strongly influence Li storage in alloy-forming anodes, yet isolating structural effects from compositional changes remains challenging. Here, we tailor the crystallinity of ultrathin intermetallic CoSn2 films to compare structure and electrochemical response within a well-defined thin-film model system. Phase-pure epitaxial CoSn2 layers grown by molecular beam epitaxy are contrasted with chemically comparable, near-amorphous nanocrystalline CoSn2 films that retain CoSn2-like local bonding but lack long-range crystalline order. Structural characterization (RHEED, AFM, XRD, TEM, Raman spectroscopy, XRR, and RBS) confirms pronounced differences in long-range order and morphology, while also revealing slight Sn enrichment and native surface oxidation. The near-amorphous film exhibits enhanced mass- and volume-normalized capacity and exceptional cycling stability (>99.99% retention per cycle). These observations are consistent with a significant contribution from grain-boundary- and microstructure-associated interfacial Li storage, although contributions from native Sn/SnOx-related interfaces cannot be fully excluded. More broadly, this work highlights how controlled crystallinity and interface density can govern electrochemical behavior in intermetallic Sn-based systems.