An aluminium-doped lithium lanthanum titanate (A-LLTO) solid electrolyte was prepared using a simple citrate-gel method, and this was followed by a pelletization and the conventional sintering process. When the sintering time was varied at 1350°C for the synthesis of the A-LLTO, the A-LLTO ceramic that was sintered at 1350°C for 6h exhibited the highest ionic conductivity of 3.17×10−4 Scm−1 at 25°C. In addition, the stability and durability of the synthesized A-LLTO ceramic was tested through a one-month aqueous-solution immersion for which the pH values were varied between 0 and 14. The stability of the A-LLTO is the highest in the alkaline environment; furthermore, for its use in the aqueous-electrolyte environment, a protected lithium electrode (PLE) structure was made by combining the lithium (Li) metal, a lithium phosphorous oxynitride (LiPON) interlayer, and the A-LLTO, whereby the LiPON interlayer prevented a direct reaction between the Li metal and the A-LLTO. The Li-LiCoO2 and Li-O2 cells comprising the PLE exhibited a superior electrochemical performance when they were used in the alkaline 1M LiNO3-electrolyte environment. After 100 cycles of the charge-discharge at the 1C rate, the aqueous Li-LiCoO2 cells maintained 59.3% of the initial capacity with a coulombic efficiency of 98.3%. In addition, the aqueous Li-O2 cell operated stably for 40 cycles under the limited capacity mode of 0.5mAhcm−2. The outstanding performance of the Li-metal-based cells originates from the A-LLTO solid electrolyte, due to the latter’s high stability, ionic conductivity, and an effective suppression effect regarding the dendritic growth of the Li.