Nonaqueous lithium–air batteries
have garnered considerable
research interest over the past decade due to their extremely high
theoretical energy densities and potentially low cost. Significant
advances have been achieved both in the mechanistic understanding
of the cell reactions and in the development of effective strategies
to help realize a practical energy storage device. By drawing attention
to reports published mainly within the past 8 years, this review provides
an updated mechanistic picture of the lithium peroxide based cell
reactions and highlights key remaining challenges, including those
due to the parasitic processes occurring at the reaction product–electrolyte,
product–cathode, electrolyte–cathode, and electrolyte–anode
interfaces. We introduce the fundamental principles and critically
evaluate the effectiveness of the different strategies that have been
proposed to mitigate the various issues of this chemistry, which include
the use of solid catalysts, redox mediators, solvating additives for
oxygen reaction intermediates, gas separation membranes, etc. Recently
established cell chemistries based on the superoxide, hydroxide, and
oxide phases are also summarized and discussed.
NMR and operando gas measurements show that at low potentials, EC is dehydrogenated to VC, whereas at high potentials, EC is chemically oxidised to CO2, CO and H2O, where the water that is formed induces secondary decomposition reactions.
Understanding the mechanistic details of the superoxide induced solvent degradation, is important in the development of stable electrolytes for lithium-oxygen (Li-O2) batteries. Propylene carbonate (PC) decomposition on a model electrode surface is studied here using in situ attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The sensitivity of the SEIRAS technique to the interfacial region allows investigation of subtle changes in the interface region during electrochemical reactions. Our SEIRAS studies show that the superoxide induced ring opening reaction of PC is determined by the electrolyte cation. Computational modeling of the proposed reaction pathway of superoxide with PC revealed a large difference in the activation energy barriers when Li(+) was the countercation compared with tetraethylammonium (TEA(+)), due to the coordination of Li(+) to the carbonate functionality. While the degradation of cyclic organic carbonates during the Li-O2 battery discharge process is a well-established case, understanding these details are of significant importance toward a rational selection of the Li-O2 battery electrolytes; our work signifies the use of SEIRAS technique in this direction.
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