Electronic properties of Ir3Li and ultra-nanocrystalline lithium superoxide formation

Abstract

Current lithium-oxygen (Li-O2) batteries suffer from large charge overpotentials related to electronic resistivity of the insulating lithium peroxide (Li2O2) discharge product. One potential solution to this challenge is the stabilization of the lithium superoxide (LiO2) discharge intermediate, which has much higher electronic conductivity compared to Li2O2. Cathodes based on small iridium (Ir) nanoparticles have been recently used in a Li-O2 battery to successfully stabilize the LiO2 product, however, the LiO2 had a short lifetime. In the previous study, researchers found that the LiO2 was stabilized on Ir3Li surfaces which were formed from Ir nanoparticles during battery operation. Little is known about the electronic properties of Ir3Li and its role in stabilizing LiO2 product formation. This work provides the first study of the electronic properties of Ir3Li, which was thermally synthesized in bulk prior to implementation on the reduced graphene oxide (rGO) cathode of a Li-O2 cell. The bulk Ir3Li was found to have comparable electrical conductivity to Ir metal, possess metal-like magnetic properties, and has an affinity towards O2 adsorption. The LiO2 discharge product formed from the Li-O2 battery discharge was characterized using Raman spectroscopy, titration, along with a comprehensive transmission electron microscopy (TEM) study. This analysis revealed the formation of ultra-nanocrystalline LiO2 particles greater than 200 nm. This result was attributed to the use of large micron sized Ir3Li particles, which could stabilize larger LiO2 particles compared to previous cathodes that utilized Ir nanoparticles that partially converted to Ir3Li during cycling. These results demonstrate that cathode properties can be modified to stabilize the bulk LiO2 discharge product, which can be useful for the further development of LiO2-based Li-O2 batteries.