Tetragonal vs. cubic phase stability in Al – free Ta doped Li7La3Zr2O12 (LLZO)

Abstract

X-ray and neutron diffraction, Raman spectroscopy, complex impedance spectroscopy and electron microscopy were used to characterize the tetragonal vs. cubic phase stability in superionic conducting garnet-oxide electrolyte.

Abstract

Li ₇La ₃Zr ₂O ₁₂ (LLZO) garnet is attracting interest as a promising Li-ion solid electrolyte. LLZO exists in a tetragonal and cubic polymorph where the cubic phase exhibits ∼2 orders of magnitude higher Li-ion conduction. It has been suggested that a critical Li vacancy concentration (0.4–0.5 atoms per formula unit) is required to stabilize the cubic polymorph of Li ₇La ₃Zr ₂O ₁₂. This has been confirmed experimentally for Al ³⁺ doping on the Li ⁺ site. Substitution of M ⁵⁺ (M = Ta, Nb) for Zr ⁴⁺ is an alternative means to create Li vacancies and should have the same critical Li vacancy concentration, nevertheless, subcritically doped compositions (0.25 moles of Li vacancies per formula unit) have been reported as cubic. Adventitious Al, from alumina crucibles, was likely present in these studies that could have acted as a second dopant to introduce vacancies. In this work, Al-free subcritically doped (Li _6.75La ₃Zr _1.75Ta _0.25O ₁₂) and critically doped (Li _6.5La ₃Zr _1.5Ta _0.5O ₁₂) compositions are investigated. X-ray diffraction indicates that both compositions are cubic. However, upon further materials characterization, including SEM analysis, Raman spectroscopy, Electrochemical Impedance Spectroscopy, and neutron diffraction it is evident that the subcritically doped composition is a mixture of cubic and tetragonal phases. The results of this study confirm that 0.4–0.5 Li vacancies per formula unit are required to stabilize the cubic polymorph of LLZO.