Aluminium doped lithium lanthanum titanate (A-LLTO) powders with various excess Li2O content are synthesized using a simple citrate gel method. The obtained A-LLTO powders show an agglomerated form, composed of nano-sized particles of 20–50 nm. The morphology and conductivity of the A-LLTO ceramics are largely affected by the content of excess Li2O. The highest total ionic conductivity of 3.17 × 10−4 S cm−1 is achieved for the A-LLTO sample containing 20% excess Li2O, exhibiting a vacancy content of 6%, and a total activation energy of 0.358 eV. The A-LLTO can act as a membrane to protect lithium metal from oxygen and other contaminants diffused through the oxygen electrode part. The Li–O2 cell employing the A-LLTO solid electrolyte shows a good cycle life of longer than 100 discharge-charge cycles, under the constant capacity mode of 300 mAh g−1.