A thick bulk-sized Li7La3Zr2O12 (LLZ) solid electrolyte is successfully prepared using a polymer complex method and a hot isostatic pressing (HIP) treatment. Despite the difficulties of densification, HIP-treated LLZ can be prepared as a transparent ceramic pellet because of its similar to 200 mu m primary grain size, and its conductivity is improved by its increased density (> 99%). An all-solid-state lithium battery is fabricated by using a mixed conducting ionic liquid gel to connect the bulk-sized LLZ electrolyte and a LiFePO4 (LFP) cathode. The introduction of the mixed conducting gel layer reduces the electrode/electrolyte interface resistance. The specific discharge capacity of the active cathode material is 169 mAh g(-1). (current density of 0.046 mA cm(-2)), which suggests that almost the full capacity of LFP can be utilized even under the all-solid-state cell design. The coulombic efficiency (eta c) is approximate to 1 and no side reactions are observed. The LFP/LLZ/Li cell shows a high electrochemical stability against the Li metal anode, and the cell operates for 20 cycles without dendrite growth causing a short circuit under a charge current density of 0.591 mA cm(-2) (1/2 C rate).