Enhancing Lithium Transport in Garnet-Type Solid Electrolyte for High-Performance All-Solid-State Batteries

Garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) is a promising solid-state-electrolyte for all-solid-state batteries; it has a good chemical stability in contact with Li metal and high ionic conductivity. However, insufficient interfacial contact and associated voids caused by poor wetting between Li metal and LLZTO can lead to a huge interface resistance and thus poor electrochemical performance including a high overpotential and limited critical current density (CCD). In addition, Li dendrites can be formed and will propagate through the LLZTO microstructure within the pores and grain boundaries, which can cause cell short circuiting. In this work, LiAlO₂ (LAO) was used as a sintering additive to improve a LLZTO microstructure with a denser structure, lower porosity, and compacted grain boundary regions. LLZTO with LAO (LLZTO-LAO) showed a higher relative density (~95%) and reduced porosity (~4.2%) compared to pure LLZTO (88% of relative density and 9.5% porosity). To calculate the ionic conductivity and activation energy of the LLZTO, a blocking electrode sputtered by gold on both side LLZTO was fabricated. LLZTO-LAO displays an improved ionic-conductivity (0.59 S cm^-1) and Li-ion activation energy (0.38 eV). In order to investigate interface resistances between LLZTO-LAO and Li metal, Li-Li symmetric electrochemical test cells were used. An enhanced interfacial resistance (72.5 Ω cm²) was demonstrated for the LLZTO-LAO cell.<br/>Li plating/stripping tests with critical current density (CCD) and cycling performance of Li symmetric cells were investigated by galvanostatic cycling experiment. While LLZTO delivers a lower CCD of 0.1 mA cm^-2, a higher CCD of 0.3 mA cm^-2 is demonstrated within the symmetric cell with LLZTO-LAO. In addition, LLZTO-LAO showed lower overpotentials than LLZTO at the various current densities. The improved CCD and overpotential of LLZTO-LAO can be attributed to LAO additives that cause an improved microstructure with a reduced porosity resulting in a good interfacial contact between electrolyte and Li metal.