Synthese, Charakterisierung und Anwendbarkeit von sulfidischen Festkörperelektrolyten

Table of Contents: The objective of this research was the synthesis of new sulfidic solid-state electrolytes, the investigation of the conductivity behavior of the materials and the characterization of this class of substances with respect to their application potential in solid-state batteries. Besides sulfide ions the even bigger selenide ions were used with the aim of maximizing conductivity during the work related to the diploma thesis. The corre-sponding investigations of the compounds Li4SnSe4 and Li4Sn2Se6 were completed during the dissertation. This approach was not pursued any further – especially due to diverse drawbacks regarding the applicability if selenium is utilized. Because of its exceptionally high conductivity for a ternary compound, Li4SnS4, which was also studied during the diploma thesis, was the source of motivation to substitute the much cheaper Sn for Ge in the superionic conductor Li10GeP2S12. With Li10SnP2S12 the synthesis of the second LGPS-type compound was accomplished, however, with only one-third of the material costs. Although Li+-ion conductivity did not reach the published record level of Li10GeP2S12 (12 mS·cm−1), it can keep up with the standard liquid electrolytes. Therefore, it is a promising material which is already commercially available under the brand name NANOMYTE® as a powder (SSE-10) or slurry (SSE-10D). Such high conductivities are not only of interest considering the prospect of application, but are also of academic interest. Thus, NMR studies are in progress in cooperation with other research groups, which will be published in the future. In addition, the maximum amount of Sn to be replaced by Si or Al without affecting the LGPS structure was systematically tested. The conductivity behavior of the resulting new compounds Li10Si0.3Sn0.7P2S12 and Li10.3Al0.3Sn0.7P2S12 was compared with the already known compounds Li10SnP2S12, Li10GeP2S12 and Li10SiP2S12, which were reproduced to this end. By means of an optimized synthesis of Li10SnP2S12, it proved possible to avoid any grain boundary resistances, consequently increasing the conductivity to 6 mS·cm–1. In the case of Li10GeP2S12, a total conductivity of 9 mS·cm–1 was attained in this work, and the grain and the grain boundary contributions were clearly separated for the first time. The conductivities of Li10SiP2S12 and Li10.3Al0.3Sn0.7P2S12 were considerably lower. Li10Si0.3Sn0.7P2S12 in contrast, exhibited a total conductivity of 8 mS·cm–1, which is almost as high as for the much more expensive Li10GeP2S12. The thesis furthermore investigated K2Hg2Se3 as a potential thermoelectric material. Naturally, the application as solid-state electrolyte in lithium-ion battery is impossible for a potassium mercurate, but the corresponding impedance analysis contributed substantially to our understanding of mixed-conducting compounds, especially with regard to the influence of the charge carrier concentration. In K2Hg2Se3, the latter could be identified as too small to ensure a sufficiently high electronic conductivity for thermoelectric applications. But by substituting Se with Te, the band gap can be reduced to such an extent that the isostructural K2Hg2Te3 exhibits a considerably higher conductivity – a publication on this topic is currently being prepared. Mixed ion-electron conductivity is also crucial for the applicability of sulfidic solid-state electrolytes. It was shown that the tetrele-containing compounds Li10GeP2S12, Li10SiP2S12 and Li10Si0.3Sn0.7P2S12 form mixed conducting interphases in contact with lithium metal. The electronic conductivity of those interphases, in the range from 10−7 S·cm−1 to 10−5 S·cm−1, is about as high as the ionic conductivity, and thus enables continuous decomposition of the electrolyte. The decomposition layers get several micrometers thick within one day. In contrast, the tetrele-free glass-ceramic 0.95 (0.8 Li2S–0.2 P2S5)–0.05 LiI, whose crystalline fraction exhibits mainly the LGPS-structure as well, forms a stable, solely Li+-ion conducting interphase with a layer thickness of about 100 nm. Its resistance, in the range of 50–100 Ω·cm2, is sufficiently low for applications in solid-state batteries with lithium as the optimal anode material. Due to their higher thermodynamic stability, oxidic solid-state electrolytes remain of interest, too, despite the lower Li+-ion conductivities. The novel compound Li3−xHxP3O9 · yDMSO was investigated in cooperation with the University of Siegen and the results are going to be published. Since several sulfidic solid-state electrolytes have achieved significantly higher Li+-ion conductivities than standard liquid electrolytes in recent years, it is now essential to optimize them regarding their interfacial properties to further advance solid-state batteries. This is going to remain a research focus in the groups of Prof. Dehnen and of Prof. Roling. In addition, volume changes during battery cycling are of critical impact, because solids can compensate them only to a very small extent. Preliminary work concerning the accompanying pressure dependence of the battery power and life-time has been conducted in the course of this dissertation and is going to be continued in the group of Prof. Roling, as well.