Probing the Origin of Overpotential for Sodium-Oxygen Batteries with Distribution of Relaxation Time

Sodium-oxygen batteries are emerging as new battery systems. Deep understanding of the origin of overpotential and the kinetic process in sodium-oxygen batteries remain challenging yet critical. We apply a method of distribution of relaxation time (DRT) to decipher the electrochemical impedance spectroscopy (EIS), allowing us to monitor the changes of different kinetic processes during the discharging and charging. The origin of the overpotential in a battery was further comprehensively investigated combining DRT analysis with differential electrochemical mass spectrometry, Raman and other characterizations. Overpotential is found to primarily stem from oxygen mass transport during discharging, and from poor solid-solid contact at the electrode surface during charging. Our work demonstrates the study of kinetic processes using DRT analysis, and suggests effective ways to improve the performances of sodium-oxygen batteries.