Synthesis, crystal structure and ionic conductivity of ruddlesden-popper oxide materials: effects of ionic radii and defects on lithium-ionic conductivity.

Abstract

Layered perovskite oxides of the Ruddlesden-Popper (RP) type structure can be good lithium-ion conductors for solid electrolyte applications in all-solid-state batteries, due to the large gap separating octahedral layers which can be useful pathways for Li-ion conduction. However, little work has been done on their lithium-ion transport properties in these materials despite their interesting structural properties. This work highlights the synthesis and study of the ionic conductivities in a series of n = 2 and 3 Ruddlesden-Popper oxides, as part of an ongoing investigation in search of alternative solid electrolyte materials. Several different strategies were employed for the enhancement of the ionic conductivity in these materials. To enhance the lithium-ion mobility in Li2SrTa2O7, two hypotheses were explored: shortening Li-Li distances and inducing cation defects in the lithium layers. The first approach was carried out via the replacement of larger cations Sr2+ and Ta5+ with smaller cations La3+ and Ti4+. The resulting materials Li2La(TaTi)O7 and Li2La(NbTi)O7 are n = 2 RP materials showing higher magnitudes of conductivity than those reported for Li2SrTa2O7. Next, Li1.8La(Ta1.2Ti0.8)O7 and Li1.8La(Nb1.2Ti0.8)O7 which were made using the second strategy produced even higher values of conductivities. In addition, the two lithium-deficient materials demonstrated measurable conductivities at room temperature. A feature that was lacking in Li2La(TaTi)O7 and Li2La(NbTi)O7, as well as Li2SrTa2O7. Given the improvement in conductivity due to cation deficiencies, the role of structural deficiencies in the form of defects at various sites in the RP structure was expanded to n = 3 members in subsequent projects. This was done in the studies of materials of the series Li2-xLa2-yTi3-zNbzO10, where defects were created in inter- or intra-layer sites, i.e., A or A′. A systematic increase in conductivity that trails the degree of defects were observed in materials with deficiencies at only lithium or lanthanum sites. Further studies showed that the simultaneous incorporation of defects at A and A′ in the synthesis of Li1.9La1.9Ti2.6Nb0.4O10 greatly impactsthe conductivity in these materials, even in cases where the degree of cation-deficiency on both inter- and intra-layer sites is similar or lesser than those in compounds containing defects on only one of the two sites. These results demonstrate for the first time that ionic transport in triple-layered Ruddlesden-Popper oxides is a result of the cooperative effect of both inter- and intra-layer sites, i.e., A and A. Finally, the effects of cationic size (ionic radii) on lithium-ion mobility in RP materials were also explored