Structural characterization of the lithium silicides Li15Si4, Li13Si4, and Li7Si3 using solid state NMR

Abstract

Local environments and lithium ion dynamics in the binary lithium silicides Li₁₅Si₄, Li₁₃Si₄, and Li₇Si₃ have been characterized by detailed variable temperature static and magic-angle spinning (MAS) NMR spectroscopic experiments. In the ⁶Li MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200 K, whereas at higher temperatures partial or complete site averaging is observed on the ms timescale. The NMR spectra also serve to monitor the phase transitions occurring in Li₇Si₃ and Li₁₃Si₄ at 235 K and 146 K, respectively. The observed lithium isotropic shift ranges of up to approximately 50 ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl–Klemm–Busmann concept for the electronic structure of these materials. The ²⁹Si MAS-NMR spectra obtained on isotopically enriched samples, aided by double-quantum spectroscopy, are well suited for differentiating between the individual types of silicon sites within the silicon frameworks, and in Li₁₃Si₄ their identification aids in the assignment of individual lithium sites via²⁹Si{⁷Li} cross-polarization/heteronuclear correlation NMR. Variable temperature static ⁷Li NMR spectra reveal motional narrowing effects, illustrating high lithium ionic mobilities in all of these compounds. Differences in the mobilities of individual lithium sites can be resolved by temperature dependent ⁶Li MAS-NMR as well as ⁶Li{⁷Li} rotational echo double resonance (REDOR) spectroscopy. For the compound Li₁₅Si₄ the lithium mobility appears to be strongly geometrically restricted, which may result in a significant impediment for the use of Li–Si anodes for high-performance batteries. A comparison of all the ⁶Li and ⁷Li NMR spectroscopic data obtained for the three different lithium silicides and of Li₁₂Si₇ previously studied suggests that lithium ions in the vicinity of silicon clusters or dimers have generally higher mobilities than those interacting with monomeric silicon atoms.