¹¹⁹Sn and ⁷Li Solid-State NMR of the Binary Li-Sn Intermetallics: Structural Fingerprinting and Impact on the Isotropic ¹¹⁹Sn Shift via DFT Calculations

Abstract

We report the ¹¹⁹Sn and ⁷Li solid-state nuclear magnetic resonance (NMR) spectroscopic characterization of all thermodynamically stable intermetallic phases of the binary Li–Sn system. The isotropic ¹¹⁹Sn shifts (sum of the isotropic chemical and hyperfine shifts) of the Li–Sn intermetallics are found to be spread over a broad spectral range from 7300 to −500 ppm, allowing a clear Li–Sn phase identification. DFT calculations showed that the hyperfine interaction (Fermi-contact and spin-dipole contributions) constitutes the major ¹¹⁹Sn magnetic shielding contribution for the Sn-rich Li–Sn intermetallics, which is significantly reduced for Li–Sn intermetallic phases with low and intermediate Sn-content. A full characterization of the effective ¹¹⁹Sn magnetic shielding anisotropies for all Li–Sn intermetallic phases was achieved using the static broad-band ¹¹⁹Sn Wideline Uniform Rate Smooth Truncation (WURST) Carr–Purcell–Meiboom–Gill (WCPMG) NMR experiment. These experiments further highlight the potential of the WCPMG NMR technique as it enables the acquisition of the full spectral range observed for the Li–Sn intermetallic phases in a single, static NMR experiment (B₀ up to 7 T), where information about crystallinity and local ordering is directly available from the ¹¹⁹Sn NMR lineshapes. Such structural fingerprinting possibilities are clear advantages when compared to ⁷Li NMR that will be of interest for studies of Sn-containing active materials in lithium-ion-based batteries, allowing a clear distinction between amorphous and crystalline (de)lithiation products in addition to the possibility to probe for amorphization during (dis)charge processes.