Hidden Macroscopic Degradation Behavior in Rechargeable Lithium–Oxygen Batteries under Lean Electrolyte and High Areal Capacity Conditions

Abstract

As lithium–oxygen batteries (LOBs) possess extremely high theoretical energy densities that exceed those of lithium-ion batteries, they are regarded as promising candidates for portable energy applications, including as power sources for next-generation electrical devices. However, physical degradation factors affecting the prolonged cycle of the LOBs, such as electrode volume changes and electrolyte displacement, have been rarely explored thus far, owing to the technical difficulty of applying non-destructive analytical methods to LOBs during cycling. In the present study, we employed ex situ X-ray computerized tomography (XCT), in situ internal pressure monitoring, and in situ confocal microscopy to study the physical states of the electrodes and the electrolyte in high energy density LOBs. The analysis results reveal that large irreversible volume changes occurred in both the positive electrode and the negative electrode of the LOB cells. Furthermore, it was also confirmed that the electrolyte was displaced from the pores of the positive electrode associated with the formation of Li2O2 in these pores during the discharge process. As these results present new fundamental insights into the physical behavior of LOBs during cycling conditions, the methodologies demonstrated here are therefore effective means to gain a new understanding of complex physical processes previously unexamined in LOBs, which shed light on the future design strategies of LOBs with high energy density and long cycle life.