In situ stress evolution on an Au thin film cathode electrode during discharging/charging in lithium-oxygen batteries

Abstract

Li-O2 batteries have over ten times the theoretical energy density that current Li-ion batteries have, but their practical energy density is hindered by a low cycle life, rapid capacity fade, and the high overpotential needed to oxidize the primary discharge product. The primary bottleneck preventing the commercialization of Li-O2 batteries are issues related to the cathode. The slow reaction kinetics occurring during cycling lead to an irreversible accumulation of the insulating primary discharge product, lithium peroxide (Li2O2), on the cathode surface. This accumulation can lead to the passivation of the cathode surface. Many studies have focused on improving the reaction kinetics and investigating the driving forces behind the chemical instabilities of both the cathode surface and the electrolyte, but few studied have examined the mechanical implications that result from these chemical instabilities as well as the formation and decomposition of the Li2O2 discharge product. To fill this gap in knowledge, the surface deformation that occurs on the cathode surface during both the formation and removal of the Li2O2 product has been studied. The formation and decomposition of Li2O2 involves a series of complex reactions that are still being studied. Li2O2 is formed during discharging by either a solution or a surface-based reaction pathway. The dynamic chemo-mechanical changes occurring on the cathode surface will be monitored during cycling and this information will be linked to the redox potentials. To elucidate these changes occurring on the cathode surface, a new experimental technique was developed by utilizing a kSA Multi-beam optical sensor (MOS) to monitor the in-situ stress evolution that results during cycling. These stress measurements were synchronized with the electrochemical response of the electrodes during cycling. Electrolytes with different salts and solvents were used to compare the stress evolution occurring with the expected discharge reaction pathway that results. During discharging the cathode experienced stress evolution due to the formation of Li2O2, while the cathode experienced stress evolution due the removal of Li2O2 during charging. The sign and behavior of the stress shows the dependance on the electrolyte chemistry, which indicates the fundamental differences between surface versus solution-based reaction processes.