Crystal structures and Na-ion diffusion mechanisms of Na_3+xSc₂Si_xP_3-xO₁₂ solid electrolytes studied by neutron diffraction

The importance of exploring new solid electrolytes for all-solid state batteries has led to significant interest in NASICON-type materials. A knowledge of structural parameters that play a role in determining ionic conduction is the key to the design of improved NASICON materials. We have studied scandium-based NASICON compounds, Na_3+xSc₂Si_xP_3-xO₁₂ (x = 0.0, 0.2, 0.4, 0.8) for solid-state Na-ion batteries. AC impedance measurements show that the total conductivity of the sample increases with increasing substitution of P with Si. However, it is much reduced for x=0.8 composition. Highest Na-ion conductivity of 2.4 mS cm^-1 at 30 °C was measured in Na_3.4Sc₂Si_0.4P_2.6O₁₂ (NSS0.4P) compound. Rietveld refinement of the neutron powder diffraction patterns confirms monoclinic C2/c unit cell for the Na₃Sc₂P₃O₁₂ (NSP) but indicates a rhombohedral R3¯c unit cell for Na_3+xSc₂SixP_3-xO₁₂ (NSS0.2P, NSS0.4P, NSS0.8P) compounds. Parameters such as the area of the bottleneck region, Na-ion conduction pathways, and Na–Na hopping distances for Na-ion conduction were determined. A correlation between the conductivity and the ratio of Na site vacancy to Na site occupancy is observed. (author)