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

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 degrees C where the conductivity is significantly affected. Combined with theoretical simulations, three transition regions during the temperature ramp to 900 degrees C were identified: (1) 200 degrees C to 350 degrees C where the air-exposed protonated Li6.4La3Zr1.4Ta0.6O12 (H-LLZTO) releases H+ and the lattice constant decreases, (2) 550 degrees C to 700 degrees C where the LLZTO surface structure becomes unstable, which leads to the formation of a La2Zr2O7 (LZO) phase, and (3) 700 degrees C to 870 degrees 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 degrees C and 900 degrees C, higher temperatures (1000 degrees 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 degrees C for 6 hours.

Automated summary

This study aims to optimize the conductivity of garnet-type solid-state electrolytes at low sintering temperatures by understanding the chemical and microstructural evolution. The results show that sintering temperature and time have a significant impact on the electrolyte's performance, and different temperature ranges result in different chemical reactions and crystal structure changes.