Insight into the Li Ion Dynamics in Li12Si7: Combining Field Gradient Nuclear Magnetic Resonance, One- and Two-Dimensional Magic-Angle Spinning Nuclear Magnetic Resonance, and Nuclear Magnetic Resonance Relaxometry

A comprehensive picture of the rather complex Li ion dynamics in the binary silicide Li12Si7 is presented. Long-range Li dynamics is probed by field gradient NMR methods. The obtained macroscopic tracer-diffusion coefficients are in good agreement with a jump process characterized by an activation energy of approximately 0.2 eV which was previously measured in Li-7 NMR relaxometrya microscopic method which probes ion dynamics on an atomic scale. From high-resolution magic-angle spinning (MAS) NMR, it can be concluded that 9 of the 13 crystallographically independent sites take part in this fast diffusion process. Li ions on the residual four sites are bound more tightly to the silicide Zintl anions with activation barriers ranging from 0.32 to 0.55 eV. Accordingly, the mean residence times of Li ions on these sites are considerably longer, which makes it possible to test their dynamics with 1D/2D MAS exchange NMR methods. We present a series of mixing-time-dependent 2D MAS exchange NMR measurements. The extracted Li jump rates are in very good agreement with those of dynamic processes investigated by NMR relaxometry. The data is interpreted in relation to the results of two recently published NMR studies on Li12Si7 and structural assignments based on one- and two-dimensional Si-29{Li-7} heteronuclear correlation spectroscopy. These experiments assist in attributing different ionic mobilities to the different crystallographic lithium sites, suggesting preferred ion conduction pathways. Combining all these methods and their respective results, a consistent picture of the Li ion dynamics in Li12Si7 is obtained. Overall lithium species in the vicinity of the Si-4 star unit tend to be more mobile than lithium species interacting with the Si5 ring units of the silicon framework structure.