True Location of Insulating Byproducts in Discharge Deposits in Li–O2 Batteries

Lithium–oxygen batteries (LOBs) are next-generation rechargeable energy storage devices with a high theoretical gravimetric energy density. However, the expected energy density has not been fully achieved mainly because of high charging overvoltages. The inclusion of insulating byproducts in the discharge products has been suggested to be a critical factor for high overvoltages. However, these previous studies did not consider the growing/retreating fronts of the discharge deposits (i.e., the deposits/electrode interface or the deposits/electrolyte interface), potentially misleading conclusions. The aim of the present study is set to precisely determine the locations of insulating byproducts in individual discharge products in an LOB system, where the growing/retreating fronts have already been identified, thereby indicating the right direction for effectively reducing charging voltage. The analysis revealed the consistent presence of Li2CO3, a byproduct of decomposition of the electrolyte and/or positive electrode, inside the individual discharge products composed mainly of Li2O2, as expected from the growing/retreating fronts. The successful identification of the true locations of insulating byproducts in discharge deposits is pivotal because it can enhance our understanding of battery reactions, which can, in turn, pave the way for the development of design guidelines for advanced battery systems.