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Lithium (Li) metal can be degraded by factors such as irregular Li stripping, Li dendritic growth, and the growth of a solid electrolyte interphase (SEI) layer, finally leading to the performance deterioration of the Li-air battery. In particular, the operation of the Li-air battery in ambient air remains a considerable challenge due to the possible occurrence of parasitic reactions on Li metal with moisture and other contaminants diffusing from the outside air. In this work, a protected Li electrode (PLE) composed of Li metal covered with a bilayer lithium phosphorous oxynitride (LiPON) / aluminium substituted lithium lanthanum titanate (A-LLTO) solid electrolyte was suggested. The mechanism for the degradation of Li electrodes with and without LiPON/A-LLTO protection layer was compared using the Li symmetric cell and Li-air cell by investigating the electrochemical performance of cells and the growth of Li dendrites. The Li symmetric cell employing the LiPON/A-LLTO exhibited superior cyclability to the cell without the solid electrolyte due to the effective suppression of Li dendritic growth. Further, the aprotic Li-air cell employing the PLE showed outstanding electrochemical performance when operated in pure oxygen and even in an air atmosphere: a long life span of 128 cycles in oxygen atmosphere and 20 cycles in air atmosphere under the limited capacity mode of 1000 mAh g -1 . The obtained excellent performance of the Liair cell employing the PLE is attributed to the effective suppression of the Li dendrite growth and electrolyte decomposition with the presence of LiPON/A-LLTO. In addition, dense LiPON/A-LLTO completely protected the Li metal electrode from the penetration of oxygen, moisture, and other contaminants which can degrade the Li metal electrode.