Fast Na+ ion dynamics in the Nb5+ bearing NaSICON Na3+−Nb Zr2−Si2+P1−O12 as probed by 23Na NMR and conductivity spectroscopy

NaSICON-type (Na superionic conductor, Na1+yZr2SiyP3−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 Na3+xZr2Si2+xP1−xO12 is increased from 0 to 0.4 and Nb5+ is substituted for Zr4+ according to Na3+x−zNbzZr2−zSi2+xP1−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, Na3.36Nb0.04Zr1.96Si2.4P0.6O12 turned out to be indeed a very promising candidate for the breakneck development of Na-based all-solid-state battery applications.