Solid Li-ion electrolytes have been attracting increasing interest as a means to address the safety issues associated with flammable liquid electrolytes. In order to be viable, significantly high ionic conductivity must be exhibited. The Li rich garnet crystal structure (Li7La3Zr2O12, LLZO) has been identified as a solid electrolyte with conductivities of ~ 1 mS/cm when stabilized as the cubic polymorph. However, a tetragonal polymorph of LLZO is also possible and exhibits ~2 orders of magnitude lower ionic conductivity. The cubic phase of LLZO can be stabilized by supervalent charge doping where the excess charge is balance by Li vacancies. It has been predicted computationally by Bernstein et. al. that the critical Li vacancy concentration to stabilize the cubic phase is 0.4 � 0.5 mols per formula unit. Exploring the phase stability of LLZO is complicated by the fact that Al ions can be incorporated into the garnet from Al2O3 calcination media at high temperature and act as a dopant. Because of this, compositions which should have an insufficient number of Li vacancies from the purposefully added dopant to stabilize the cubic phase have been reported as cubic. In this report, supervalent substitution of Ta5+ for Zr4+ in the compositional range Li7-xLa3Zr2-xTa2O12 (x = 0.25, 0.5, 0.75, 1.5) was investigated. Care was taken to prevent Al uptake at high temperatures. Lab-scale x-ray diffraction was used to preliminarily characterize the phase before neutron diffraction was performed. Interestingly, the x-ray diffraction suggested the cubic polymorph was present while neutron diffraction clearly showed peak splitting, indicating the tetragonal polymorph. In order to further clarify the structure of the subcritically doped sample, SEM analysis, Raman spectroscopy, and electrochemical impedance spectroscopy with equivalent circuit modeling was performed. The cumulative results suggest that the subcritically doped composition phase segregated into a mixture of cubic and tetragonal garnet.

Tetragonal vs. cubic phase stability in Al � free Ta doped Li7La3Zr2O12 garnet Li ion solid electrolyte

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.