Yttrium Contenting Compositionally Complex Medium-Entropy Li-Garnet Electrolyte with Improved Ionic Conductivity

The
compositionally complex medium/high-entropy design concept
can greatly expand the categories and affect the properties of the
materials. With such a designing concept, a medium-entropy Li-garnet
electrolyte with appropriate yttrium content (formula Li6.6La3ZrNb0.3Ta0.3Hf0.3Y0.1O12) shows a record-high ionic conductivity of
∼5.7 × 10–4 S/cm, the highest reported
for any single-site substituted high/medium-entropy Li-garnet. The
assembled Li metal symmetric cells also show stable long-term cycling
(0.1 mA/cm2 for over 200 h). Neutron powder diffraction
and Rietveld refinement results indicate that a competing conduction
mechanism between (1) occupancy on high mobility of 96h sites and
(2) the associated site vacancies and the bond length requires an
appropriate content of Y for enhanced ionic conductivity. Li-ion hopping
through the bottleneck can also contribute to the conductivity. Density
functional theory and Born–Oppenheimer molecular dynamics simulations
also indicate the high mobility and number of hopping transitions
of Li ions, contributing to the high ionic conductivity.