Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O_2 Battery with a Redox Mediator

摘要

Prior to the practical application of rechargeable aprotic Li-O_2 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, Li_2O_2, 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 Li_2O_2 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 Li_2O_2 particles established that both growth and decomposition proceeded at the Li_2O_2/electrolyte interface in a system containing the Br~-/Br_3~- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li_2O_2 was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li_2O_2/electrolyte interface is critical to improve the cycle life of Li-O_2 batteries.