In Situ Stress Evolution on an Au Thin Film Cathode during Charging/Discharging of a Lithium-Oxygen Battery

Abstract

The theoretical energy density of lithium-oxygen batteries is by far the closest competitor to gasoline compared to other battery configurations. Unfortunately, the commercial implementation of this technology is hindered by the poor cycling performance, mostly due to the slow oxygen evolution/reduction kinetics. The sluggish oxygen evolution kinetics are typically more severe and cause the slow oxidation of lithium peroxide. This can cause the irreversible formation of a passivating lithium peroxide layer on the cathode surface. The interfacial dynamics due to the growth of lithium peroxide and the formation of cathode-electrolyte interface (CEI) layers remains largely unknown. It is crucial to elucidate how the nucleation of discharge products on the cathode surface affects the electrochemical performance in order to design new electrolytes and cathodes for lithium-oxygen batteries.In this study, we propose to investigate the surface stress generation associated with the formation of discharge products and CEI layers on the cathode. Thin Au films were deposited via e-beam evaporation onto a borosilicate glass substrate. These substrates were used as a cathode material. The electrolyte used was 1 M LiTFSI in DMSO, and it was saturated with oxygen prior to performing electrochemical measurements. In situ stress measurements were conducted using a multiple beam optical sensor (MOS). A custom cell was designed to monitor the physical response of the cathode during cycling while enabling optical access to the back of the substrate for stress measurements.The in situ curvature evolution on an Au coated borosilicate glass cantilever is monitored during one discharge/charge cycle at 10 µA/cm2. Tensile stress generation is observed during the formation of the lithium peroxide discharge product on the Au cathode. A large stress relaxation into the compressive regime is observed at the onset of charging. From our results, the nucleation of lithium peroxide onto the cathode surface causes tensile stress, while the oxidation of lithium peroxide causes compressive stress. Revealing the mechanical response of the charge/discharge products on the cathode will shed light into the formation mechanisms of these products, which is crucial to design both new electrolytes and cathode materials for lithium-oxygen batteries.Acknowledgement: This work was supported by the Binational Science Foundation (#2018327), and we are thankful to both Dr. Malachi Noked and Rosy for fruitful discussions.