Length-Scale-Dependent Ion Dynamics in Ca-Doped Na₃PS₄

The sodium ion conductor Na3PS4 is a promising electrolyte for future all-solid-state batteries using Na+ ions as ionic charge carriers. Its readily available components make it a compelling and more sustainable alternative to recent Li-ion technologies. At ambient temperature, the ionic conductivity is in the order of 10(-4) S cm(-1), which can be optimized by adjusting doping and processing parameters. Even though several studies have focused on explaining the dynamic properties of doped and undoped Na3PS4, the driving forces that lead to fast Na+ exchange are not yet completely understood. Here, we synthesized nanocrystalline, defect-rich cubic Na3PS4 via a solid-state synthesis route and compared its properties with those of highly crystalline Ca-doped Na3-2xCaxPS4. The interconnected effects of doping and synthesis procedure on both structure and dynamic properties are investigated. X-ray diffraction reveals that the undoped samples show clear cubic and tetragonal symmetry, while for the doped samples, a phase mixture of both polymorphs is seen. High-resolution Na-23 magic angle spinning NMR spectra acquired at temperatures as low as -60 degrees C clearly reveal two different Na sites when ionic motion is partially frozen out. Ion dynamics of the powder samples were analyzed using high-precision broadband impedance spectroscopy and variable-temperature, time-domain Na-23 NMR spin-lattice relaxation rate measurements. Localized Na+ jumps detected by NMR showed higher energy barriers but faster Na+ dynamics for the Ca-doped samples. A similar trend was observed in conductivity spectroscopy with lowest activation energy for Na-ion transport in tetragonal Na3PS4 but highest attempt frequencies for the hopping motion in Ca-doped Na3PS4 with x = 0.135, making the doped sample the superior ion conductor at elevated temperatures. Our study highlights the importance of breaking down ionic transport in its elemental steps to understand the complex interplay of intrinsic and extrinsic parameters in solid electrolyte materials.