Fast Na ion transport triggered by rapid ion exchange on local length scales

The realization of green and economically friendly energy storage systems needs materials with outstanding properties. Future batteries based on Na as an abundant element take advantage of non-flammable ceramic electrolytes with very high conductivities. Na₃Zr₂(SiO₄)₂PO₄-type superionic conductors are expected to pave the way for inherently safe and sustainable all-solid-state batteries. So far, only little information has been extracted from spectroscopic measurements to clarify the origins of fast ionic hopping on the atomic length scale. Here we combined broadband conductivity spectroscopy and nuclear magnetic resonance (NMR) relaxation to study Na ion dynamics from the µm to the angstrom length scale. Spin-lattice relaxation NMR revealed a very fast Na ion exchange process in Na_3.4Sc_0.4Zr_1.6(SiO₄)₂PO₄ that is characterized by an unprecedentedly high self-diffusion coefficient of 9 × 10^-12 m²s^-1 at -10 °C. Thus, well below ambient temperature the Na ions have access to elementary diffusion processes with a mean residence time τ_NMR of only 2 ns. The underlying asymmetric diffusion-induced NMR rate peak and the corresponding conductivity isotherms measured in the MHz range reveal correlated ionic motion. Obviously, local but extremely rapid Na⁺ jumps, involving especially the transition sites in Sc-NZSP, trigger long-range ion transport and push ionic conductivity up to 2 mS/cm at room temperature.