Insights into degradation of metallic lithium electrodes protected by a bilayer solid electrolyte based on aluminium substituted lithium lanthanum titanate in lithium-air batteries

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 Li–air batteries. 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 the 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 mA h g−1. The obtained excellent performance of the Li–air 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.