Structural and electrochemical characterization of sodium solid electrolyte

Abstract

In the last couple of years, a major breakthrough has been achieved as several antimony-based sodium sulfide solid electrolytes (SSEs) substituted by aliovalent elements such as W have delivered high ionic conductivities up to 41 mS/cm at 25°C. (Hayashi et al., 2019; Fuchs et al., 2020; Feng et al., 2021) These Na-defective ionic superconductors outperform even the best lithium SEs, such as Li10GeP2S12 (10 mS/cm at 25°C). To date, little is known about the synthesis of this family of materials and the transport properties of these cubic phases defined in the I-43m (no. 217) space group.We have been working on obtaining the Na2.85P0.85W0.15S4 variant (hereafter, NPWS) by single step mechanochemical synthesis. Two samples were prepared with different milling conditions: (a) one milled during 10 h at 510 rpm, and (b) one milled during 10 h at 700 rpm. The corresponding synchrotron X-ray diffractograms are displayed in Fig. 1(a), showing a crystalline phase that corresponds to a mixture of the cubic I-43m and the tetragonal P-421c phases. Additional characterisations were carried out by using scanning electron microscope (SEM) to determine the morphology of the crystalline samples. As can be seen in Fig. 1(b), a wide distribution of particle sizes can be noticed with a certain degree of agglomeration. We can also notice two types of particle morphology which can indicate the presence of two different phase, in agreement with the XRD data.Although we obtain conductivities close to 1 mS/cm, they are far below the reported values. We conclude, based on our hypotheses, that either the materials are not pure, and/or the Na ion transport through the material is synthesis dependent. An operando thermodiffraction experiment performed on the NPWS samples at D2B instrument at the Institut Laue-Langevin and ID31 at the European Synchrotron Radiation Facility help us determining the optimal temperature conditions for the formation of the NPWS crystalline phase in the I-43m space group and gave insight about the probable Na-ion transport processes as a function of the temperature