Various
electrolyte components have been investigated with the
aim of improving the cycle life of lithium–oxygen (Li–O2) batteries. A tetraglyme-based electrolyte containing dual
anions of Br– and NO3– is a promising electrolyte system in which the cell voltage during
charging is reduced because of the redox-mediator function of the
Br–/Br3– and NO2–/NO2 couples, while the Li-metal
anode is protected by Li2O formed via the reaction between
Li metal and NO3–. To maximize the potential
of this system, the fundamental factors that limit the cycle life
should be clarified. In the present work, we used nondestructive electrochemical
impedance spectroscopy to analyze the temporal change of the charge
transfer resistances during cycles of Li–O2 batteries
with dual anions. The charge transfer resistance at the cathode was
revealed to exhibit good correlation with the reduction of the discharge
voltage. These results, combined with the results of electrode surface
inspections, revealed that irreversible accumulation of insulating
deposits such as Li2O2 and Li2CO3 on the cathode surface was a major cause of the short cycle
life. Furthermore, the analyses of the time course of the solution
resistance suggested that diminished reactivity between the redox
mediators and Li2O2 was a critical factor that
led to the irreversible accumulation of the less-reactive Li2O2 on the cathode and eventually to a shortened cycle
life. These findings indicated that increasing the reactivity between
Br3– and Li2O2 is
essentially important for improving the cycle stability of Li–O2 batteries and the reactivity can be nondestructively assessed
by tracking the dynamic changes in the solution resistance.