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 119 Sn and 7 Li solid-state nuclear magnetic resonance (NMR) spectroscopic characterization of all thermodynamically stable intermetallic phases of the binary Li−Sn system. The isotropic 119 Sn 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 119 Sn 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 119 Sn magnetic shielding anisotropies for all Li−Sn intermetallic phases was achieved using the static broad-band 119 Sn 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 (B 0 up to 7 T), where information about crystallinity and local ordering is directly available from the 119 Sn NMR lineshapes. Such structural fingerprinting possibilities are clear advantages when compared to 7 Li 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.

Identification of LixSn Phase Transitions During Lithiation of Tin Nanoparticle-Based Negative Electrodes from Ex Situ 119Sn MAS NMR and Operando 7Li NMR and XRD

“…As discussed above, this finding can be explained with the phase transition of Li 2 Sn 5 to LiSn within region II of the lithiation profile of Sn-NP-based electrodes. By further charging the Sn-NP-based negative electrode to 0.40 V (region III), the 119 Sn MAS NMR clearly shows the spectral finger prints of the intermetallic phase Li 7 Sn 3 as confirmed by the isotropic 119 Sn shifts at ≈1126 and ≈300 ppm, revealing that the Li–Sn phase formed at this voltage possesses on a short range order some structural features of Li 7 Sn 3 . This finding agrees with the operando 7 Li NMR data, which also showed the phase transition from LiSn to Li 7 Sn 3 in region III of the voltage profile.…”

“…Within the region II of the lithiation voltage profile of the Sn-NP-based electrodes, the 119 Sn MAS NMR spectrum at a cell voltage of 0.50 V displays, besides the 119 Sn resonances of Li 2 Sn 5 (7298 and 6839 ppm), two new resonances located at ≈5440 ppm and at ≈5960 ppm. Based on their isotropic 119 Sn shifts, these new 119 Sn resonances reveal the formation of LiSn . As discussed above, this finding can be explained with the phase transition of Li 2 Sn 5 to LiSn within region II of the lithiation profile of Sn-NP-based electrodes.…”

“…43,49 We have recently systematically synthesized all known thermodynamically stable Li x Sn phases of the binary Li−Sn intermetallic system and determined the isotropic 7 Li and 119 Sn shifts for each phase using magic-angle spinning (MAS) nuclear magnetic resonance (NMR) techniques. 50 This study showed that the isotropic 119 Sn shifts are spread over a much broader range (7300 to −200 ppm) compared to the isotropic 7 Li shifts (100 to 0 ppm), 51,52 making the Li x Sn phase assignment more straightforward due to less-pronounced NMR signal overlap. Furthermore, the 119 Sn MAS NMR experiments showed sensitivity with respect to Sn ordering (and thereby to overall crystallinity of the sample) as the 119 Sn MAS NMR spectra include: (i) narrow 119 Sn signals in accordance with the number of inequivalent Sn crystallographic sites of the pure Li x Sn phases and (ii) broadened 119 Sn resonances due to the presence of amorphous Li x Sn phases.…”

“…In 2012, Rhodes et al investigated the lithiation of pure Sn thin-film electrodes (≈5 μm thickness) by XRD methods and showed the formation of Li 2 Sn 5 , LiSn, and Li 22 Sn 5 phases, which suggests that remaining Li x Sn bulk phases (i.e., Li 7 Sn 3 , Li 2 Sn 5 , Li 13 Sn 5 , and Li 7 Sn 2 ) will not form under electrochemical operation due to a low atomic mobility at room temperature. − Recently, Lopez et al gained further insights into the lithiation mechanism of tin nanoparticle (Sn-NP)-based composite electrodes by operando and ex situ 7 Li NMR combined with pair distribution function (PDF) methods . These studies reported the observation of a number of different intermetallic phases, including Li 2 Sn 5 , LiSn, Li 7 Sn 3 , Li 13 Sn 5 , and Li 7 Sn 2 during lithiation, indicating a structural evolution of the Li x Sn phases as expected from the binary Li–Sn phase diagram, which is in contrast to previous findings reported in the literature. , We have recently systematically synthesized all known thermodynamically stable Li x Sn phases of the binary Li–Sn intermetallic system and determined the isotropic 7 Li and 119 Sn shifts for each phase using magic-angle spinning (MAS) nuclear magnetic resonance (NMR) techniques . This study showed that the isotropic 119 Sn shifts are spread over a much broader range (7300 to −200 ppm) compared to the isotropic 7 Li shifts (100 to 0 ppm), , making the Li x Sn phase assignment more straightforward due to less-pronounced NMR signal overlap.…”

Li5Sn, the Most Lithium-Rich Binary Stannide: A Combined Experimental and Computational Study

“… 18 , 23 Moreover, all known stable phases were recently studied with solid state NMR and Mössbauer spectroscopy to produce reference data for battery operando studies. 24 …”