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

Li7La3Zr2O12 (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 similar to 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 Li7La3Zr2O12. This has been confirmed experimentally for Al3+ doping on the Li+ site. Substitution of M5+ (M = Ta, Nb) for Zr4+ 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 (Li6.75La3Zr1.75Ta0.25O12) and critically doped (Li6.5La3Zr1.5Ta0.5O12) 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.