To date, considerable efforts have been made to address the inherent problems of Li metal anode using solid electrolyte or modifying liquid electrolyte including additives, salts and/or solvents. However, despite their own advantage of good processability, the liquid electrolytes can induce the formation of the brittle and unstable enough SEI layer. Further, in order to preclude the Li metal from reacting with poisonous species such as water, O₂, and CO₂ diffusing from outside atmosphere, the use of lithium ion conducting solid electrolyte are indispensable.

 In this work, we report comprehensive investigation of degradation of protected Li electrode (PLE) in the presence of the bilayer solid electrolyte of LiPON/A-LLTO as a typical ceramic solid electrolyte in Li-O₂ cell operating in both pure O₂ gas and ambient air. The liquid electrolyte employed herein was 1 M LiTFSI in TEGDME. The evaluation includes analysis of structural and morphological change, analysis of surface chemistry change, monitoring electrochemical properties and subsequently exploring the difference in Li electrode’s degradation mechanisms in the cells with and without protection of the ceramic solid electrolyte. Furthermore, this present study supplies the circumstantial view of whole initiation and growth process of Li dendrites using optical microscope (OM). In particular, the formation process of dead Li dendrites that has barely exposed in the previous reports was also recorded.

 Obviously, with the presence of the solid electrolyte, the Li dendrites were effectively suppressed by the mechanical strength. The smooth and flat surface of the PLE was easily attained when the symmetric cell was cycled at low current density such as 0.1 mA cm^-2. In addition, in the same charge-discharge condition, the symmetric cell without solid electrolyte only operated 15 cycles, much smaller compared with the cell with solid electrolyte, 182 cycles. During the cycling process, the impedance of both cells gradually increased  with cycling. However, the degradation of the cell without solid electrolyte was severer than that of the cell with solid electrolyte. XPS spectra confirmed the presence of decomposition products of LiTFSI salt such as LiF, Li₃N, Li₂S, Li₂S₂O₄ and S_x on both tested Li electrodes after several ten cycles. However, these products of the PLE were detected with the smaller signal intensity than that of the unprotected Li electrode without the solid electrolyte. This suggests the effective suppression of electrolyte decomposition by the solid electrolyte. Hence, with the employment of LiPON/ALLTO, the Li dendrite growth and electrolyte decomposition were effectively suppressed. Accordingly, the Li-O₂ cell using the protected Li metal electrode exhibited excellent charge-discharge cycling stability with a long life span of 128 cycles under the limited capacity mode of 1000 mAh g^-1 in oxygen atmosphere and 20 cycles in air atmosphere.