Reversibility of Noble Metal-Catalyzed Aprotic Li-O2 Batteries

The aprotic Li-O-2 battery has attracted a great deal of interest because, theoretically, it can store far more energy than today's batteries. Toward unlocking the energy capabilities of this neotype energy storage system, noble metal-catalyzed high surface area carbon materials have been widely used as the 02 cathodes, and some of them exhibit excellent electrochemical performances in terms of round-trip efficiency and cycle life. However, whether these outstanding electrochemical performances are backed by the reversible formation/ decomposition of Li2O2, i.e., the desired Li-O-2 electrochemistry, remains unclear due to a lack of quantitative assays for the Li-O-2 cells. Here, noble metal (Ru and Pd)-catalyzed carbon nanotube (CNT) fabrics, prepared by magnetron sputtering, have been used as the O-2 cathode in aprotic Li-O-2 batteries. The catalyzed Li-O-2 cells exhibited considerably high round-trip efficiency and prolonged cycle life, which could match or even surpass some of the best literature results. However, a combined analysis using differential electrochemical mass spectrometry and Fourier transform infrared spectroscopy, revealed that these catalyzed Li-O-2 cells (particularly those based on Pd-CNT cathodes) did not work according to the desired Li-O-2 electrochemistry. Instead the presence of noble metal catalysts impaired the cells' reversibility, as evidenced by the decreased O-2 recovery efficiency (the ratio of the amount of O-2 evolved during recharge/that consumed in the preceding discharge) coupled with increased CO2 evolution during charging. The results reported here provide new insights into the O-2 electrochemistry in the aprotic Li-O-2 batteries containing noble metal catalysts and exemplified the importance of the quantitative assays for the Li-O-2 reactions in the course of pursuing truly rechargeable Li-O-2 batteries.