Quantitative assessment of network depolymerization in archetypal superionic glasses and its relationship with ion conduction: A case study on Na2S-GeS2

The properties of xNa2S-(100-x)GeS2 glasses, which represent promising systems for all-solid-state batteries, are thoroughly investigated from a variety of experimental and theoretical techniques. The ionic conduction is measured as a function of composition. It reveals a powerlike behavior with a threshold composition found at low Na content. In contrast, temperature evolution suggests a typical Arrhenius behavior indicative of Na motions achieved by jumps between neighboring sites. Three particular compositions (0%, 33%, and 66% Na2S) are characterized by a combination of x-ray diffraction and density functional based molecular dynamics. Different structural properties are measured and calculated, such as structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions. The comparison with experiments reveals a rather good agreement in real and reciprocal space. The short-range order is found to consist of a base network made of GeS4/2 tetrahedra (with Ge and S coordination numbers being of about 4 and 2) that are progressively depolymerized upon Na addition that also leads to a breakdown of the ring structure. Na coordination numbers are loosely defined, especially at high Na content. Typical features of alkali-modified silicates are also found, such as the presence of channel-like dynamics, a bond-length distribution that is different between Ge and bridging or nonbridging sulfur, a distribution Qn of Ge tetrahedra having n bonding sulfur, and a decoupling at low temperatures between network species (Ge, S) and Na dynamics. However, unlike such archetypal glasses, sodium thiogermanates contain homopolar Ge-Ge bonds that are specific to Ge chalcogenides and which lead to isolated (Ge2S6)6⊖ anions at high Na content.