Al-doped Li 7 La 3 Zr 2 O 12 (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 Li 3 BO 3 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 modi fi ed 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 signi fi cant in fl uence of the microstructure on the CCD value (50 μ A·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 μ A·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. growth in LLZO solid electrolyte is still not fully understood. to previous which study separately the impact of the microstructure of a densi fi ed LLZO pellet and the Li/LLZO interface, in this we studied the dendrite formation via the modi fi cation of the chemistry of the Li/LLZO interface and the grain boundaries of a densi fi ed LLZO pellet. The objective was to de fi ne which of the two factors is preponderant on the formation of dendrites. To do so, we modi fi ed the composition at the grain boundaries via the addition of Li 3 BO 3 and the Li/LLZO interface by adding a thin layer of Al. To discuss the formation of dendrites via the measurement of the CCD, we studied the structure and the microstructure by SEM and XRD analyses of the LLZO pellets and we have estimated the electronic conductivity at the grain boundaries by impedance spectroscopy. Our results show that the formation of dendrites seems to be mainly in fl uenced by the Li/LLZO interface chemistry. In particular, the nucleation growth processes and indirectly the dendrite formation is controlled by the chemistry and the applied pressure.