Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator.

Prior to the practical application of rechargeable aprotic Li-O2 batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li2O2, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li2O2 reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li2O2 particles established that both growth and decomposition proceeded at the Li2O2/electrolyte interface in a system containing the Br-/Br3- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li2O2 was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li2O2/electrolyte interface is critical to improve the cycle life of Li-O2 batteries.