Fast Na⁺ ion dynamics in the Nb⁵⁺ bearing NaSICON Na_3+x−zNb_zZr_2−zSi_2+xP_1−xO₁₂ as probed by ²³Na NMR and conductivity spectroscopy

NaSICON-type (Na superionic conductor, Na_1+yZr₂Si_yP_3−yO₁₂, 0 < y < 3) represent a class of fast Na⁺ ion conductors with bulk ion conductivities in the mS range near room temperature. Starting from Na₃Zr₂(SiO₄)₂(PO₄), i.e., from y = 2, we investigated the change in ion transport parameter when x in Na_3+xZr₂Si_2+xP_1−xO₁₂ is increased from 0 to 0.4 and Nb⁵⁺ is substituted for Zr⁴⁺ according to Na_3+x−zNb_zZr_2−zSi_2+xP_1−xO₁₂ (z = 0.04). The later might be beneficial also to change the transport properties in or near the grain boundary regions of our polycrystalline samples. To detect such changes, we used low-temperature conductivity and electric modulus spectroscopy to differentiate between the total and the bulk (intragrain) electrical responses. However, the main purpose of the present study is to corroborate the extraordinarily high bulk ionic conductivity of approximately 7.3 mS cm⁻¹ (x = 0.4 and z = 0.04, room temperature) by diffusion-controlled ²³Na NMR spin-lattice relaxation measurements. Indeed, such experiments directly sense the ²³Na spin-fluctuations at the nuclear sites as a result of the fast intragrain motional processes. Both methods, nuclear and non-nuclear, point to a mean activation energy of 0.3 eV for Na⁺ exchange among the multiple sites in the NaSICON framework. In addition, NMR reveals local barriers characterized by activation energies as low as 0.1 eV. Altogether, from a dynamic point of view, Na_3.36Nb_0.04Zr_1.96Si_2.4P_0.6O₁₂ turned out to be indeed a very promising candidate for the breakneck development of Na-based all-solid-state battery applications.