Unraveling the Unstable Nature of Tetraglyme-Based Electrolytes toward Superoxide and the Inhibitory Effect of Lithium Ions by Using In Situ Vibrational Spectroscopies

The stability of solvents is critical for the efficiency and cyclability of rechargeable aprotic lithium–oxygen (Li–O2) batteries. Here, we report a combined spectroscopic study on the stability of tetraglyme (G4), which is one of the most commonly used solvents in Li–O2 batteries, against superoxide (O2–) during oxygen reduction reaction (ORR). Based on sum-frequency generation spectroscopy characterization, we found that ORR induces significantly irreversible structural changes in G4 molecules on the electrode surface in an O2-saturated Li+-free solution. In the Li+-containing solution, however, reversibility for the structural change in G4 molecules is primarily improved. Furthermore, infrared reflectance absorption spectroscopy and surface-enhanced Raman spectroscopy measurements confirmed that G4 is extremely unstable during ORR in the Li+-free G4 solution. In addition, several decomposition products have been identified during ORR. On the other hand, the decomposition of G4 during ORR is significantly suppressed when Li+ is included in the solution. These results indicate that O2– plays a crucial role in the cathodic decomposition of the G4 solvent during ORR. The decomposition mechanism and the inhibitory effect of Li+ are discussed based on spectroscopic observations.