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Contemporary Li‐ion batteries are facing substantial challenges like safety and limited energy density. The development of all‐solid‐state battery cells mitigates safety hazards and allows the use of Li‐metal anodes increasing energy density. Garnet‐type solid electrolytes can be vital to achieving an all‐solid‐state cell and an understanding of the influence of its microstructure on the electrochemical performance is crucial for material and cell design. In this work the influence of grain size on the Li‐ion conductivity of Li7‐3xLa3Zr2AlxO12 (x=0.22) is presented. The synthesis and processing procedure allows changing the ceramic grain size, while maintaining the same synthesis parameters, eliminating influences of the synthesis on grain boundary composition. Field assisted sintering technology is a powerful method to obtain dense, fine‐grained ceramics with an optimal grain size of 2–3 μm, where the conductivity is double that of the counterpart (0.7 μm). A total Li‐ion conductivity of 0.43 mS cm−1 and an activation energy of 0.36 eV were achieved. The oxide‐based all‐solid‐state battery cell combining the garnet‐type electrolyte, a Li‐metal anode and a thin‐film LiCoO2 cathode was assembled and cycled at room temperature for 90 hours. This represents a proof of concept, for the application of oxide‐based electrolytes at ambient temperatures. [ABSTRACT FROM AUTHOR]