Influence of rare-earth elements on the ionic conductivity of LATP electrolyte and its application in assembled cells

Rare earth elements (Pr, Eu, Lu) with larger ionic radius and lower electronegativity were used to dope NASICON type solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP) by solid state method. The effects of the types and the doping amounts of rare earth elements on the properties of LATP were investigated systematically. A test cell of Li4Ti5O12//Lu-LATP//LiCoO2 was assembled and the cycling performance was evaluated. The failure mechanism of the test cell was analyzed. The relative density, cell volumes, and the total ionic conductivity of LATP were increased upon doping of rare-earth elements. The grain sizes and the electronic conductivity were decreased. Among the three rare earth elements studied, Lu doped LATP achieved the highest total ionic conductivity of 3.6 x 104 S/cm, with the lowest activation energy of 0.23 ev, and the electronic conductivity was one order of magnitude lower than that of un-doped LATP. The enhancement of the total conductivity was ascribed to the synergized effect of the increase of the relative density, the enlargement of the cell volumes and the broadening of Li+ migration window owing to the larger ionic radius and the lower electronegativity of the rare earth elements. Based on the relaxation time distribution spectra, the failure mechanism of the assembled cell was attributed to the degradation of the electrolytes and the irreversible decomposition of electrodes. Schematic diagram of the mechanism of Pr/Eu/Lu doped LATPimage