During the previous year, a detailed bibliographic analysis was conducted on various categories of solid electrolytes based on lithium (Li) and sodium (Na) ions, such as polymers, oxides, and sulfides. It was demonstrated that sulfides exhibit high ionic conductivity even at low temperatures. Therefore, these materials are the most promising candidates for developing solid electrolytes for solid-state batteries. Additionally, a standard protocol for their synthesis was developed. The preparation and mixing of reagents were carried out in an inert atmosphere (Ar) inside a glove box. The precursor powders were mixed using a mortar and pestle, and portions of the solid mixture were pressed to form pellets. The pellets were vacuum-sealed inside a quartz tube using a flame and then subjected to specific thermal treatments. The protocol was tested on the species known as LGPS (Li10GeP2S12). The purity of the synthesized material was determined using X-ray diffraction (XRD). The experimental data confirmed the effectiveness of the proposed protocol.

The work carried out this year focused on synthesizing solid electrolytes starting from known lithium-based systems and then synthesizing analogous species, but with sodium ions replacing lithium and other metals replacing germanium. Solid electrolytes belonging to this family can be described by the formula NMPS, where N = Na, M = Ge, Sn, Al. During the experimentation, several of these materials were synthesized, and their crystal structures, purity, and ionic conductivity were determined. The main results are summarized below. For solid electrolytes containing Ge, the phase obtained by simply substituting Li with Na (Na10GeP2S12, NGPS) was found to be unstable and could not be characterized. In contrast, a stable phase with a different symmetry group and stoichiometry, corresponding to Na11Ge2PS12, was synthesized. This phase has the same stoichiometry and symmetry group as the equivalent phase containing tin (Na11Sn2PS12) and is characterized by a larger crystal lattice volume compared to the Ge-based electrolyte. Additionally, it has been shown that in the Na₁₁Sn₂PS₁₂ phase, aluminum (Al) can replace up to 50% of Sn (Na11AISnPS12). The crystal lattice volume and ionic conductivity tend to decrease with increasing Al content in the material. Furthermore, Sn could be replaced with silicon (Si); replacing less than 50% of Sn leads to the formation of a new phase with the formula Na11Si0.2Sn1.8PS12. With only Si, and in the complete absence of Sn, new solid electrolytes are formed with the general formula Na11.7P0.27Si2.73S12. The measurement of the ionic conductivity of these phases, which is necessary for a complete characterization of the synthesized materials, will be carried out in the next year. This characterization will allow for the selection of the most promising new sodium-based solid electrolytes to create test cells for evaluating their electrochemical performance.