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The oxygen concentration distribution in the porous cathode of a lithium-air battery during discharge has been measured using a fine optical fiber sensor. The lithium-air battery has the highest theoretical capacity. However, for practical application, the lithium-air battery power density needs to be improved. To realize a more powerful aqueous lithium-air battery, sufficient oxygen supply into the porous cathode is required. No previous studies have measured the oxygen concentration in the porous cathode structure. In this study, platinum tetrakis pentafluorophenyl porphine (PtTFPP) was used as the oxygen indicator. When PtTFPP is exposed to excitation light, phosphorescence emission occurs, and its intensity depends on the oxygen partial pressure. Thus, the oxygen concentration can be obtained by measuring the phosphorescence intensity and using calibration data. A fine optical fiber sensor (110 μm in diameter) was constructed with PtTFPP painted on the edge. According to the experimental results, as the current density increases, the oxygen concentration in the porous cathode drastically decreases. Because of slow oxygen transport in the aqueous electrolyte and the existence of an electrolyte between the air layer and the porous cathode, sufficient oxygen is not supplied to the porous cathode. Therefore, only oxygen near the electrode surface can contribute to the discharge.

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