The experimental work carried out was directed toward the synthesis of solid electrolytes that can be used in solid-state sodium ion batteries. Among the species considered (such as polymers, oxides, and sulfides), the focus was on the synthesis of sulfides, which exhibit high ionic conductivity even at low temperatures and, therefore, may be considered the most promising candidates for this use. In addition, the standard protocol for their synthesis, which is particularly complicated given their high reactivity with H2O and O2, was refined in the experimental phase. Preparation and mixing of the reagents are performed in an inert atmosphere (Ar) inside a glove box. The precursor powders are mixed with the aid of an agate mortar, and portions of the solid mixture are subjected to pressing to make pellets. After being vacuum-sealed inside a quartz tube by flame, the pellets are subjected to specific heat treatments.
Solid electrolytes belonging to the sulfide family can be defined by the following formula: NMPS in which N = Na, M = Ge, Sn, Al (of which the species Na10GeP2S12 (NGPS) represents the reference, being mirrored by the species Li10GeP2S12 (LGPS), obtained by the simple substitution of Li for Na). During the experimentation phase, some of these materials could be synthesized, whose crystal structures, degree of purity, and ionic conductivity were determined. The main results obtained are summarized below. For solid electrolytes containing Ge, the phase obtained by the simple substitution of Li with Na (Na10GeP2S12) was found to be unstable already in the previous year and therefore could not be characterized. Instead, a stable phase with different symmetry group and different stoichiometry was synthesized, corresponding to the Na11Ge2PS12 species. The Na11Sn2PS12 species was also made, which has the same stoichiometry and symmetry group as the equivalent tin-containing phase (and is characterized by a larger crystal lattice volume than the Ge-based electrolyte). Furthermore, it has been shown that in the Na11Sn2PS12 species, aluminum (Al) can replace up to 50% of Sn (Na11AlSnPS12). The crystal lattice volume and ionic conductivity tend to decrease with increasing Al content within the material. In addition, it was possible to replace Sn with silicon (Si); replacement of less than 50% of Sn leads to the formation of a new phase with formula Na11Si0.2Sn1.8PS12. With only Si and in the total absence of Sn, the formation of new solid-state electrolytes having general formula Na11.7P0.27Si2.73S12 takes place. During the present year, different types of solid electrolytes were synthesized and then used for the realization of single cells; the realization of the single cell involves the use of a sodium metal foil as the anode electrode material, while the cathode is obtained by making a solid mixture between titanium sulfide (TiS2) and the electrolyte synthesized at the RSE laboratories.