Grain boundary zirconia-modified garnet solid-state electrolyte

We report a method for promoting electrochemical stability in garnet Li_6.4La₃Zr_1.4Ta_0.6O₁₂ solid-state electrolyte based on a composite two-phase oxide–oxide microstructure. Grain boundary precipitation of the controlled distribution of amorphous zirconium oxide microparticles is achieved through the addition of reactive tantalum carbide. During ambient-atmosphere sintering, the carbide decomposes through an in situ reaction, the ‘extra’ Ta substituting for Zr within the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ lattice. Density functional theory (DFT) calculations identify a thermodynamically favourable reaction path and show how substituting Ta⁵⁺ at Zr⁴⁺ sites affects the crystal structure as well as bulk ionic and electronic conductivities. Quantitative stereology highlights that zirconia also acts as a sintering aid, reducing compact porosity. Cryogenic focused-ion-beam scanning electron microscopy and fractography analysis of cycled solid-state electrolytes illustrates that near-universally observed intergranular Li-metal dendrite propagation is suppressed by the two-phase microstructure, favouring transgranular dendrites instead. Importantly, DFT demonstrates that compared with the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ surface, the zirconium oxide surface per se is less electronically conductive and does not trap excess electrons to reduce Li ions. This is a key reason for the substantial improvement in the electrochemical properties over the single-phase baseline.