Singlet Oxygen Formation during the Charging Process of an Aprotic Lithium–Oxygen Battery

Abstract

Aprotic lithium-oxygen (Li-O2 ) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen ((1) Δg ) is formed upon Li2 O2 oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time- and voltage-resolved in operando EPR spectroscopy in a purpose-built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long-overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li-O2 cells.

Isotopic Labeling Reveals Active Reaction Interfaces for Electrochemical Oxidation of Lithium Peroxide

“…[7b,14] Figure 3s hows the galvanostatic voltage along with OEMS gas evolution profiles for pure VC and Ru/VC electrodes.T he charge voltage of the Ru/VC electrode is about 500 mV lower than that of the pure VC electrode with similar oxygen evolution efficiency(0.76 for Ru/VC and 0.70 for VC;s ee Tables S1 and S2), confirming improved OER kinetics. 16 O 2 evolution was not detected for either cell before reaching one quarter of the capacity ( % 300 mAh g C À1 ), as shown in Figures 3b and d. In addition, 16 O 2 dominated the oxygen evolution at the later charging stage after starting to evolve at 300 mAh g C À1 . 16 O 2 evolution was not detected for either cell before reaching one quarter of the capacity ( % 300 mAh g C À1 ), as shown in Figures 3b and d. In addition, 16 O 2 dominated the oxygen evolution at the later charging stage after starting to evolve at 300 mAh g C À1 .…”

“…Figure 1s hows the 3D distribution of 18 O À ,r econstructed based on the raw data of ToF-SIMS depth scan of the discharged VC and Ru/VC electrodes.I nt he area of 20 20 mm, 18 O À shows the highest intensity at the top surface of about 5nm ( Figure 1c and f). To contrast the distribution of 18 O À versus other species,w ec ompare the change of intensity for 18 O À , Li À (exists in both Li 2 18 O 2 and Li 2 16 O 2 ), and 16 O À .Asshown in Figure S5, the 18 O À gradually decreased and almost vanished at the end of sputtering, while the Li À signal was still prominent, and can be attributed to Li in Li 2 16 O 2 .W en ote that the signal of 16 O À is overwhelming in the 3D intensity image as 16 O À could come from the PEO binder,d ecomposition products of electrolyte,a nd oxygen-containing species on carbon. When sputtered to about 175 nm from the surface,t he 18 O À signal nearly disappeared for both electrodes.T he distribu-tion of the 18 O À signal observed in the ToF-SIMS shows that the latter formed Li 2 18 O 2 is located on the top surface of the discharge product.…”

“…To ensure the order of oxygen evolution is solely related to the location of the oxygen in the Li 2 O 2 but not to other intrinsic properties of 18 O 2 that could make it evolve first, we swapped the order of oxygen during discharge,that is,wefirst discharged with 18 O 2 followed by discharge with 16 O 2 and observed the OER behavior by OEMS.C onsistently,t he latter reduced 16 O 2 (see Figure S8) was firstly detected upon charging followed by 18 O 2 (see Figure S9). [7b, 18] We further probed the oxygen evolution behaviors at al ower charging rate of 300 mAh g C À1 (TEGDME as the electrolyte solvent;s ee Figure S10) and in another solvent, 1-methylimidazole (Me-Im, donor number = 47; [3c] see Figures S11 and S12), by OEMS.S imilar to the observations in Figure 3, 18 O 2 was firstly observed followed by 16 O 2 under these two conditions (discharged in 16 O 2 followed by 18 O 2 ), and thus further supports our findings. [7b, 18] We further probed the oxygen evolution behaviors at al ower charging rate of 300 mAh g C À1 (TEGDME as the electrolyte solvent;s ee Figure S10) and in another solvent, 1-methylimidazole (Me-Im, donor number = 47; [3c] see Figures S11 and S12), by OEMS.S imilar to the observations in Figure 3, 18 O 2 was firstly observed followed by 16 O 2 under these two conditions (discharged in 16 O 2 followed by 18 O 2 ), and thus further supports our findings.…”

Prolonging the Cycle Life of a Lithium–Air Battery by Alleviating Electrolyte Degradation with a Ceramic–Carbon Composite Cathode

“…[1][2][3][4] Because Li + intercalation based batteries are gradually approaching their limits of theoreticale nergy density (< 300 Wh kg À1 ), many efforts have been devoted to developing Li metal based ones, such as NCM-Li (NCM = nickel cobalt manganese oxide), [5,6] LiÀS, [3,7] and LiÀO 2 /Li-air. [4,9] However, there are still many challenges to conquer to substantially improve its cyclability,rate capability,and tolerance of atmospheric CO 2 /H 2 Ob efore commercialization; these challenges stem from the high reactivity of activated oxygen species( such as singlet O 2 , [12] LiO 2 , [13] O 2 C À ,a nd Li 2 O 2 [14] ), [15] as well as the incorporation of solid-state discharge products, and thus, poor mass/electron transfer near unsustainable threephase interfaces. [4,9] However, there are still many challenges to conquer to substantially improve its cyclability,rate capability,and tolerance of atmospheric CO 2 /H 2 Ob efore commercialization; these challenges stem from the high reactivity of activated oxygen species( such as singlet O 2 , [12] LiO 2 , [13] O 2 C À ,a nd Li 2 O 2 [14] ), [15] as well as the incorporation of solid-state discharge products, and thus, poor mass/electron transfer near unsustainable threephase interfaces.…”

3D printed sample holder for in-operando EPR spectroscopy on high temperature polymer electrolyte fuel cells

“…Nonetheless, different designs have been shown to provide good results both for in-operando nuclear magnetic resonance (NMR) [6][7][8][9] and for electron paramagnetic resonance (EPR) [4]. For example, in-operando EPR of electrochemical cells has been performed for Nafion Ò based fuel cells [5], for lithium batteries [4,10] or for catalytic systems [11]. A set of specifications can be summarized for this purpose: The cell has to fit into the EPR resonator that offers limited space, the sample holder should not show an EPR signal that disturbs the measurement or complicates its analysis, and the sample holder must enable the functioning of the electrochemical system.…”