Abstract
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 Na3.4Sc0.4Zr1.6(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 τNMR 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.
Highlights
In the search of safe and long-lasting energy storage systems all-solid-state batteries entered the spotlight of research[1]
We present an in-depth study to explore the roots of extremely rapid 3D ion dynamics in Na3.4Sc0.4Zr1.6(SiO4)2PO4 (Sc-NZSP), which was prepared via a wet-chemical route[18]
Sc-NZSP exhibited Na ion bulk conductivities σ′ of 2 mS/cm at room temperature with activation energies ranging from 0.13 eV to 0.31 eV
Summary
In the search of safe and long-lasting energy storage systems all-solid-state batteries entered the spotlight of research[1]. It is beyond any doubt that reporting such high conductivities[4,5,6,7,8,10,25,26,27,28], especially for sodium-based batteries[19,20,21,29,30], is of enormous importance Understanding their roots, preferably with the help of theory[31,32], is crucial and would enable us to safely control their dynamic properties. For many materials, there is still no complete picture available consistently describing the interrelation of the elementary steps of hopping with long-range ion transport In many cases, it is not even clear which charge carrier and dynamic processes are responsible for the high conductivities reported. Considering the electrochemical stability of NZSP-type materials[39] this behaviour clearly opens the field for the development of powerful Na ion solid-state batteries
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