Understanding the Cathode Electrochemistry of Humidified Solid‐State Lithium‐Oxygen Batteries

Lithium‐oxygen batteries (LOBs) possess a high theoretical energy density, making them potential candidates for next‐generation energy storage. However, challenges such as reactive oxygen species‐induced component degradation hinder their practical use. Inorganic solid‐state electrolytes offer an alternative to degradation‐prone aprotic electrolytes, while also protecting lithium anodes from potential atmospheric reactants. This study explores the cathode electrochemistry of solid‐state LOBs using humidified oxygen, which forms an aqueous catholyte during initial cycling, thereby improving cathode‐electrolyte contact. To quantitatively analyze the cathode electrochemistry, a ‘Humidity‐Incorporated’ Differential Electrochemical Gas Monitoring System (HiDEMS) is developed to control humidity and monitor gas consumption and evolution in real time. When studying a Li‐O 2 cell that employs a NASICON‐type Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) solid electrolyte and a porous carbon cathode, a shift in discharge products from Li 2 O 2 to LiOH is observed over repeated cycles. While Li 2 O 2 evolves O 2 during charging, LiOH oxidation leads to minimal O 2 release and increased CO 2 production, originating from oxidation of carbon electrodes. Further, dissolution of Al and P from LATP is observed, likely driven by the formation of the alkaline catholyte. The findings highlight the need for carbon‐free cathode materials and more stable solid‐state conductors to minimize side reactions and improve rechargeability in humidified solid‐state Li‐O 2 batteries.