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As possible electrolyte materials for all-solid-state Na-ion batteries (NIBs), scandium-substituted Na 3 Zr 2 (SiO 4 ) 2 (PO 4 ) in the structure of NASICONs (Na super-ionic conductors) have received hardly any attention so far, although among all the trivalent cations, Sc 3+ might be the most suitable substitution ion for Na 3 Zr 2 (SiO 4 ) 2 (PO 4 ) because the ionic radius of Sc 3+ (74.5 pm) is the closest to that of Zr 4+ (72.0 pm). In this study, a solution-assisted solid-state reaction (SASSR) method is described and a series of scandium-substituted Na 3 Zr 2 (SiO 4 ) 2 (PO 4 ) with the formula of Na 3+x Sc x Zr 2-x (SiO 4 ) 2 (PO 4 ) (NSZSPx, 0 ≤ x ≤ 0.6) have been prepared. This synthesis route can be applied for powder preparation on a large scale and at low cost. With increasing degrees of scandium substitution, the total conductivity of the samples also increases. An optimum total Na-ion conductivity of 4.0 × 10 -3 S cm -1 at 25 °C is achieved by Na 3.4 Sc 0.4 Zr 1.6 (SiO 4 ) 2 (PO 4 ) (NSZSP0.4), which is the best value of all reported polycrystalline Na-ion conductors. The possible reasons for such high conductivity are discussed.

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The bulk diffusion of Na in Na3.4Sc2(SiO4)0.4(PO4)2.6 was investigated by 23Na NMR relaxometry in the temperature range from 250 to 670 K. These measurements reveal fast Na diffusion with hopping rates of 3 × 108 s–1 for the Na+ ions at 350 K and activation barriers for single Na+ ion jumps of (0.20 ± 0.01) eV. From these values a diffusion coefficient of D = 6.4 × 10–12 m2/s and a Na ion conductivity of σNa = 4 mS/cm (both at 350 K) can be estimated. Measurements on two samples, one stored in air and one stored in Ar, do not show significant differences, which reveals that these NMR measurements are probing the bulk diffusion while conductivity measurements usually are also influenced by grain boundaries that can be affected by the moisture level during storage.

NASICON-based solid electrolytes with exceptionally high Na-ion conductivities are considered to enable future all solid-state Na-ion battery technologies. Despite 40 years of research the interrelation between crystal structure and Na-ion conduction is still controversially discussed and far from being fully understood. In this study, microcontact impedance spectroscopy combined with single crystal X-ray diffraction, and differential scanning calorimetry is applied to tackle the question how bulk Na-ion conductivity σbulk of sub-mm-sized flux grown Na3Sc2(PO4)3 (NSP) single crystals is influenced by supposed phase changes (α, β, and γ phase) discussed in literature. Although we found a smooth structural change at around 140 °C, which we assign to the β → γ phase transition, our conductivity data follow a single Arrhenius law from room temperature (RT) up to 220 °C. Obviously, the structural change, being mainly related to decreasing Na-ion ordering with increasing temperature, does not cause any jumps in Na-ion conductivity or any discontinuities in activation energies Ea. Bulk ion dynamics in NSP have so far rarely been documented; here, under ambient conditions, σbulk turned out to be as high as 3 × 10–4 S cm–1 at RT (Ea, bulk = 0.39 eV) when directly measured with microcontacts for individual small single crystals.

A study of the series Na 3+x Sc 2 Si x P 3-x O 12 (0 < x < 0.8) revealed very high ionic conductivity values at room temperature. The structural investigation of the substitutional disorder and position of the very mobile Na + ions in the crystal structure is the key to understanding the structure-property-chemical bonding relationships. Therefore neutron powder diffraction was carried out at 300 and 100 K on Na 3.4 Sc 2 Si 0.4 P 2.6 O 12 to refine the structural parameters and to elucidate the Na + distribution in the crystal structure.The refinement of the structure revealed that two phases are present, one rhombohedral Si-rich phase and one monoclinic Na 3 Sc 2 P 3 O 12 phase. The ratio of the two phases is 1:1 and they possess similar lattice parameters. The hopping distances of the Na + ions and the size of the bottleneck for Na + conduction were calculated and explained the high conductivity of the sample.

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