Enhancing the lithium-ion conductivity in Li2SrTa2-xNbxO7 (x = 0–2)

Abstract

The lithium-ion conductivity in layered oxides, Li2SrTa2O7, Li2SrTaNbO7, and Li2SrNb2O7 has been demonstrated. Neutron and X-ray diffraction experiments were used for structural characterization, and confirmed that these compounds feature the so-called Ruddlesden-Popper structure, containing bilayer stacks of TaO6 or NbO6 octahedra, separated by layers of lithium ions, consistent with a previous report on the structure of the two end members. Variable-temperature impedance spectroscopy experiments show a methodical increase in the lithium-ion conductivity: Li2SrTa2O7 < Li2SrTaNbO7 < Li2SrNb2O7. Furthermore, a hypothesis-driven strategy has been investigated by generating defects in the structure. It was hypothesized that the lithium-ion mobility could be enhanced if defects were created in the lithium layer. Therefore, a lithium-deficient analogue of the most conductive compound, Li2SrNb2O7, was synthesized. Neutron diffraction experiments indicated that the defect material, Li1.8Sr0.8La0.2Nb2O7, retains the same structural framework. Most importantly, this compound shows remarkably higher conductivity, by two orders of magnitude, compared to the parent material, Li2SrNb2O7. These findings demonstrate that defects can be utilized as an effective tool to enhance the lithium-ion mobility in solid systems.