Implications of the unpaired spins in Li-O₂ battery chemistry and electrochemistry: A minireview

Recent experimental and theoretical reports suggest that the valence states of oxygen species residing in the Li-O₂ cell discharge products can be very complex. The Li-O₂ discharge products may consist of composite structures of Li/O₂ compounds, such as Li₂O₂, O-rich Li₂O₂, LiO₂-like superoxide, and Li₂O, which can be both crystalline and amorphous phase. Some impurities can also be present owing to undesired parasitic reactions associated with electrolyte decomposition. In particular, the observation of unpaired spins associated with Li-O₂ discharge products may be because of the oxygen-rich environment of the Li-O₂ cell, which results in reduced oxygen radical species (e.g., O₂^-, LiO₂, etc.). The presence of unpaired spins in Li-O₂ cell products can have a significant impact on electrolyte stability and cell performance (e.g., overpotential, electrical conductivity). Besides standard exsitu experimental characterization, accurate and robust insitu experimental characterizations (e.g., EPR, NMR, Raman, and FTIR spectroscopy) in combination with well-controlled materials synthesis techniques are further needed to probe the time-evolution reaction mechanism of these unpaired electron spins within the Li-O₂ cell environment for better materials understanding and future Li-O₂ battery design. Battery design: An overview is given of unpaired electron spins related to the chemical species in a Li-O₂ cell environment and their possible impact on the design and development of a practical Li-O₂ cell. The discharge products consist of composite structures of Li/O₂ compounds, such as Li₂O₂, O-rich Li₂O₂, LiO₂-like superoxide, and Li₂O (see figure).