Singlet oxygen generation as a major cause for parasitic reactions during cycling of aprotic lithium–oxygen batteries

Abstract

Non-aqueous metal-oxygen batteries depend critically on the reversible formation/decomposition of metal oxides on cycling. Irreversible parasitic reactions cause poor rechargeability, efficiency, and cycle life and have predominantly been ascribed to the reactivity of reduced oxygen species with cell components. These species, however, cannot fully explain the side reactions. Here we show that singlet oxygen forms at the cathode of a lithium-oxygen cell during discharge and from the onset of charge, and accounts for the majority of parasitic reaction products. The amount increases during discharge, early stages of charge, and charging at higher voltages, and is enhanced by the presence of trace water. Superoxide and peroxide appear to be involved in singlet oxygen generation. Singlet oxygen traps and quenchers can reduce parasitic reactions effectively. Awareness of the highly reactive singlet oxygen in non-aqueous metal-oxygen batteries gives a rationale for future research towards achieving highly reversible cell operation.

The Long‐Term Stability of KO2 in K‐O2 Batteries

“…[16] Thedifferent chemical stability between NaO 2 and KO 2 can be related to their thermodynamic properties. [24,25] Even though both NaO 2 and KO 2 have been identified as discharge products,o nly KO 2 is thermodynamically stable.H ence,t he spontaneous disproportionation route to peroxide is prohibited in K-O 2 cells,which distinguishes them from Na-O 2 cells. [13,14] Theincreased charge overpotential triggers parasitic reactions and dramatically decreased cell efficiency.…”

Review of Electrolytes in Nonaqueous Lithium–Oxygen Batteries

“…[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Several excellent reviews have been published to summarize recent developments and our current understanding of Li-O 2 batteries, especially with respect to the cathode electrocatalytic materials. [25][26][27][28][29][30][31][32][33][34][35] These reviews have comprehensively discussed the relationship between the structure of electrocatalytic materials and the electrochemical performance of Li-O 2 batteries.…”

Defect Chemistry in Discharge Products of Li–O2 Batteries

“…The identification of active reaction intermediates is challenging due to their short lifetimes. In situ/operando characterization techniques allow in situ monitoring of Li 2 O 2 evolution, such as the local atomic structure transformation and the evolution of intermediates along with discharge and charge processes . For example, Peng et al conducted in situ surface‐enhanced Raman spectroscopy (SERS) measurements where the Li 2 O 2 ‐related peak intensity clearly increased with diminishing LiO 2 peak intensity during the charge process.…”

“…There are two widely adopted Li 2 O 2 growth models, the solution model and the surface model, that largely determine the discharge capacity and charge overpotential by controlling the structure and morphology of Li 2 O 2 . However, a series of parasitic reactions can occur during discharge and charge and lead to carbon‐based cathode decomposition and electrolyte decomposition to form irreversible carbonates, resulting in low coulombic efficiency and cyclability deterioration . These concomitant side reactions primarily arise from attacks of the active species (O 2 − , LiO 2 , and Li 2 O 2 ) .…”

“…Other charge transport pathways (e.g., grain boundaries and trapping sites) can also be created by introducing defects to achieve multichannel conduction . The presence of these defects can significantly enhance the conductivity of Li 2 O 2 and promote charge transport through Li 2 O 2 , thus increasing the cycling stability of Li–O 2 batteries . Employing other energy carriers such as LiOH and LiO 2 has been explored as an effective way to circumvent charge transport issues.…”