Developing high energy density batteries, such as metal–air systems, requires a good understanding of their underlying electrochemical principles. In situ characterization methods provide valuable insight into the discharge/charge mechanism of Na–O2 cells. However, previous application of soft X-ray absorption spectroscopy has been limited to ex situ studies on the discharge products of the cell. Here we report an in situ soft X-ray absorption technique for Na–O2 cells to study the formation and decomposition of the discharge products during battery cycling. Taking advantage of the elemental selectivity and chemical sensitivity of soft X-ray absorption spectroscopy, we reveal the instability of discharge products in the Na–O2 cell environment. Our results illustrate that in situ soft X-ray absorption spectroscopy is an efficient probe to study the electrochemical mechanism of alkali metal–O2 energy storage systems.
