Modification and characterization of microporous lithosilicates Li-conductors
Zusammenfassung der Projektergebnisse
Comparison of structural and Li-dynamical properties of RUB-29 with those of other type of lithosilicates helps to better understand Li-cationic conductivity in RUB-29-type materials. In this respect, another new microporous lithosilicate, RUB-30, was of interest due to the high amount of Li. Therefore, within the frame of this project, this material was analysed and its average structure could be reported. Contrary to RUB-29, however, RUB-30 is thermally not stable above 300 °C and rapidly hydrolyzed in aqueous solution ionexchange procedures. For these reasons, characterization of the Li-conductivity in RUB-30 was not further followed. However, the structural analysis of RUB-30 convinced that RUB-29 has structural advantages for the Li conductivity against the other zeolitic lithosilicates. On the other hand, the topology of RUB-29 is very narrowly related to milarite-type minerals. Lidynamics in a Li-bearing milarite-variant, sogdianite were studied by evaluating HRNPD and single crystal A.C IS data during this project period. In fact, a conductivity at 650 °C of 4.1-10^-5 S-cm-1 parallel to the network of edge-sharing LiO4-AO6 (A = Zr, Ti, Al, Fe3+) of sogdianite is similar to that of RUB-29, but requiring much higher activation energies. The low activation energy is indicative of the ease with which charge transfer occurs in RUB-29. This may be traced back to the presence of Li2O-layers in RUB-29 providing short pathways for Li hopping. 6Li MAS NMR has reached a limit to reveal details of site-exchanging processes in crystallographically different Li sites of RUB-29. A careful evaluation of A.C. IS spectra pointed to fast motions of Li inducing high frequency relaxation resonances, in particular at low temperature ranges below 300 °C. Na-exchanged RUB-29 samples show the highest bulk conductivity observed so far. NMR and A.C. studies could be more useful when combined with structural information from X-ray and neutron diffraction data analyses. This interdisciplinary work states that the way of defect-engineering in Li2O-layers of RUB-29 can be done indirectly by controlling configurations of extra-framework constituents. Therefore, ion-exchanges with other multivalent cations such as In3+, Sn4+, Nb5+, and W6+ are worthwhile to try further investigation on RUB-29-type materials towards achieving better Li-conductivity and/or mixed ionic-electronic conductivity. The collecting and analyzing of X-ray single crystal diffraction data of RUB-29 (M = Zn, Mg, Mn, or Zr) synthesized within this project also open the way for further developments on RUB-29 type microporous Li-conductors.
