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

The lithiation mechanism of tin nanoparticle-based negative electrodes is reported and systematically studied via operando 7Li nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) combined with ex situ 119Sn magic-angle spinning (MAS) NMR. Besides the formation of the Sn-rich phases Li2Sn5 and LiSn, also the Li-richer phase Li7Sn3 is observed in good agreement with the structural evolution of the binary Li–Sn phase diagram. However, the structural investigations using ex situ 119Sn MAS NMR clearly reveal the formation of a disordered LixSn phase with increasing lithiation, possessing the structural fingerprints of Li7Sn3 with no long-range order and a body-centered cubic (bcc) packing of Sn (from XRD). Thus, in contrast to previous studies relying on 7Li NMR only, the formation of any of the Li-rich bulk crystalline Li–Sn phases, Li13Sn5, Li5Sn2, Li7Sn2, and Li17Sn4, could not be confirmed from 119Sn MAS NMR, showing that these Li–Sn phases are not formed under electrochemical operation. From a more general point of view, our approach using ex situ 119Sn MAS NMR demonstrates the possibilities of using the heavier framework ions as reporters of the local structural environments in negative electrodes. This relies on the sensitivity of the isotropic 119Sn shift with respect to the first and second atomic coordination environments, which provides a powerful source of complementary structural information to the typically performed operando 7Li NMR and XRD measurements.