Dynamic Changes in Charge Transfer Resistances during Cycling of Aprotic Li–O2 Batteries

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.