Evidently, the number of lattice-Li + and the number of vacancies plays a crucial role for ionic conduction, so fine tuning the parameter x has been a focus of research. 118-120 A further improvement could be achieved by widening the diffusion channel through Sr 2+ -doping on the A-site. 121 Additionally, it has been found that depending on synthesis route and operating temperature poorly conducting La-rich layers are formed in the crystal structure constraining the Li + diffusion to two dimensions. This transition from the disordered to the ordered state is accompanied by a drop of one order of magnitude in conductivity. Nevertheless, by aligning the conducting planes along the measurement direction, a value of 4 • 10 -3 S/cm has been recently reported for a 13 µm single crystal. Although high resistance at the grain boundaries and low stability vs. Li metal induced by Ti 4+ /Ti 3+ redox activity have so far prevented LLTO from application, it serves as a well studied model system for ionic diffusion in perovskites. The replacement of Ti 4+ with Nb 5+ , for instance, resulting in Li x La (1-x)/3 NbO 3 (LLNO) 127 is an obvious idea which has gained renewed interest lately. As in the case of LLTO, Sr 2+ -doping 128 and exploiting A-site ordering 128,129 were done to improve the material's properties, however, the bulk conductivity of 10 -4 S/cm could hardly be increased. Exchanging the shuttle ion in LLTO from Li + to Na + and Mg 2+ was not feasible owing to the very low mobility of the respective ions in the given hosts. Still, Prussian blue analogues received the high scientific interest in their application as battery material only after their drastic performance improvement in 2015, when Goodenough and coworkers reported a rhombohedral modification of Prussian blue that occurred when the interstitial water was removed from a cubic, sodium-rich Prussian blue material in a drying process. This new kind of Prussian blue indeed shows a longer cycle life compared to the purely cubic material and a highly reversible phase transition between the modifications. 145