All-solid-state batteries have emerged as a potential alternative to traditional lithium-ion batteries due to their increased safety and energy density, enabled by solid electrolytes that exhibit fast ion conduction. While many new ion-conducting solids have been reported recently, further improvements in ionic conductivity are needed to achieve practical cell configurations such as a thick cathode design. In addition to the long-studied sulfide and oxide electrolytes, Li-ion conducting halide-based SEs have recently attracted enormous attention owing to their electrochemical, mechanical, and transport properties, compromising those of oxides and sulfides. Despite the success of Li-ion conducting halides, the research on Na-analogue remains sparse, and the reported ionic conductivities are limited. In this study, we demonstrate the significantly improved ionic conductivity in NaM⁵⁺Cl₆ (M⁵⁺ = Nb, Ta) by extending the existing structural framework of the Na-ion conducting halides Na₃M³⁺Cl₆ (M³⁺ = Y, Er).

NaM⁵⁺Cl₆, crystalizing into the monoclinic structure with the space group of P2₁/n, possesses close structural relationships with Na₃M³⁺Cl₆ (space group: P2₁/n) and Na₂M⁴⁺Cl₆ (space group: P2₁/n). The structure of Na₃M³⁺Cl₆ (space group: P2₁/n) is a heavily distorted double-perovskite structure (A₂BB’C₆: A = Na1, B = Na2, B’ = M³⁺, C = Cl) with two Na positions. With increasing the content of M⁴⁺ cations, the Na2 site becomes vacant to compensate for the charge, and the structure becomes unstable. However, a further increase in the valency of M elements to M⁵⁺, e.g., NaNbCl₆ and NaTaCl₆, stabilizes the structural framework of the monoclinic phase of Na₃M³⁺Cl₆ and Na₂ZrCl₆ again, at room temperature. Both NaNbCl₆ and NaTaCl₆ form a monoclinic structure with the space group of P2₁/n with alternating Na and vacant layers along the c-axis, and, as a result, the structural cation of M⁵⁺ is displaced from the center of the octahedra to take distance from the Na layers.

In this study, we successfully synthesized the two series of solid solutions Na_1+xNb_1−xZr_xCl₆ and Na_1+xTa_1−xZr_xCl₆ and investigated their transport properties. In both series, the ionic conductivity peaks at around x = 0.2, likely due to the enhancement in the migration entropy. However, the achievable conductivity with Ta is an order of magnitude higher due to the lower activation energy despite the almost identical unit cell sizes. We will discuss the possible reasons for this difference in the activation energy to extend it to the general design principles.