Pore and grain chemistry during sintering of garnet-type Li6.4La3Zr1.4Ta0.6O12 solid-state electrolytes

Abstract

Garnet-type solid-state electrolytes have significant advantages over liquid organic electrolytes but require energy-intensive sintering to achieve high density and ionic conductivity. The aim of this study is to understand the chemical and microstructural evolution towards optimizing sintering conditions to achieve good conductivity at low sintering temperatures. To this end, the pore surface chemistry, morphology, and elemental enrichment along grain boundaries are investigated using scanning electron microscopy, X-ray scattering, and thermo-gravimetric analysis at temperatures below and above 1000 °C where the conductivity is significantly affected. Combined with theoretical simulations, three transition regions during the temperature ramp to 900 °C were identified: (1) 200 °C to 350 °C where the air-exposed protonated Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (H-LLZTO) releases H⁺ and the lattice constant decreases, (2) 550 °C to 700 °C where the LLZTO surface structure becomes unstable, which leads to the formation of a La₂Zr₂O₇ (LZO) phase, and (3) 700 °C to 870 °C, where the surface Li₂CO₃ layer starts to decompose and react with the intermediate LZO phase to reform the LLZTO cubic phase. While gradual densification is observed between 750 °C and 900 °C, higher temperatures (1000 °C and above) significantly reduce the pore volume and increase the conductivity. Backscattered electron (BSE) imaging and energy dispersive spectroscopy (EDS) under cryo conditions reveals Ta enrichment and Zr depletion at grain boundaries after sintering at 1100 °C for 6 hours.