Critical Current Density Limitation of LLZO Solid Electrolyte: Microstructure vs Interface

Al-doped Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte is a promising candidate for all-solid-state lithium battery (ASSB) due to its high ionic conductivity and stability against lithium metal. Dense LLZO pellets were prepared by high-temperature sintering and a Li₃BO₃ melting agent was used to control the microstructure (grain size and grain boundary chemistry). An ionic conductivity of 0.49 mS·cm⁻¹ was measured at room temperature. The LLZO/Li interface was modified by introducing an aluminum layer. The impact of the microstructure of LLZO ceramics and the chemistry of the LLZO/Li interface were discussed by measuring the critical current density (CCD). Even though secondary phases at the grain boundary lead to an increase of the electronic conductivity, no significant influence of the microstructure on the CCD value (50 μA·cm⁻²) has been established. The low CCD value has been improved by forming an Al-Li alloy interlayer at the LLZO/Li interface, due to a better homogenization of the Li current at the interface. In parallel, the applied pressure (0.09 MPa vs. 0.4 MPa) has been studied and did impact the CCD. A value of 0.35 μA·cm⁻² was measured. These results highlight the conditions needed for keeping a good electrolyte/Li interface during the cycling of a solid state battery.