Ionic transport on different length scales: Na+ dynamics in the promising ion conductor Na3PS4 as seen by broadband conductivity and NMR spectroscopy

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Na3PS4 is a promising electrolyte for future sodium all-solid state batteries. Its readily available components make it a compelling and more sustainable alternative to recent Li-technologies. At ambient temperature, the ionic conductivity in its cubic crystal structure is in the order of 10−4 cm−1 [1]. Even though several studies focused on explaining the dynamic properties of cubic Na3PS4, the driving forces that lead to fast Na+ exchange are not yet completely clear.

Here, we synthesized nanocrystalline, defect-rich cubic Na3PS4 via a solid-state synthesis with subsequent annealing at 250 °C for 12 h. Additionally, we synthesized doped Na3PS4 to investigate the influence of foreign atoms in the crystal structure on the ionic transport properties. Ion dynamics of the powder samples were analysed using high-precision broadband impedance spectroscopy and variable-temperature, time-domain 23Na NMR spin-lattice relaxation rate measurements. We were able to separate bulk ion dynamics from electrical relaxation associated with grain boundary regions. While macroscopic transport is characterized by an activation energy of 0.36 eV, 23Na NMR indicates a much lower value of 0.18 eV, see also [2]. This discrepancy points to length-scale dependent dynamic parameters. Indeed, electric modulus spectroscopy, i.e., the analysis of resistivity peaks ρν (= M''/ω)(1/T), revealed a low-temperature activation energy of 0.13 eV, which is consistent with our result from NMR. We attribute this barrier to extremely fast local Na hopping processes constituting the basis for long-range ionic transport in Na3PS4.

[1] A. Hayashi et al., Nat. Commun., vol. 3, pp. 856-860, 2012.
[2] C. Yu et al., J. Mater. Chem. A, vol. 4, pp. 15095-15105, 2016.
Period15 Sep 2021
Event title2021 European Congress and Exhibition on Advanced Materials and Processes : EUROMAT 2021
Event typeConference
LocationVirtuell, Austria
Degree of RecognitionInternational