New In Operando Detection of Singlet Oxygen Inside a Li-O2 Battery

CALVO, ERNESTO J.; CÓRDOBA, DANIEL; HERNÁN B. RODRÍGUEZ

Santiago de Chile

Encuentro; 33rd Topical Meeting of the International Society of Electrochemistry; 2022

International Society of Electrochemistry

The rechargeable lithium air battery (Li-O2) with a very high energy density comparable to fosil fuels was introduced by Abraham [1]. However, the parasitic reactions of the O2 reduction products with solvent and electrolyte lead to capacity fading and high charging overpotentials. During the oxygen reduction reaction (ORR) in aprotic solvents superoxide radical anion (O2 -.) is the main one-electron reaction product, which in the presence of Li+ ions undergoes disproportionation to yield Li2O2 and O2, a fraction of which results in the excited singlet oxygen (1O2). Very reactive singlet oxygen is responsible for the spurious reactions that lead to high charging overpotential and low cycle life of the Li-O2 batteries due to solvent and electrolyte degradation [2]. Herein, we extend our previous ex-situ studies [3] and report on a new experimental set up for the in operando detection of 1O2 inside a Li-O2 battery during operation and test different physical quenchers of 1O2.Singlet oxygen detection is based on the specific reaction with the fluorescent probe 9,10-dimethylantracene (DMA) contained in the electrolyte that rapidly and selectively traps any 1O2 formed yielding the non-fluorescent endoperoxide (DMA-O2). The fluorescent decay of DMA inside the battery has been probed with a bifurcated optical fiber for the excitation at λexc. = 378 nm and fluorescent emission at λem. = 425 nm through a quartz window located at the bottom of the Li-O2 battery body.The Figure on the left depicts the time evolution of DMA (0,5mM, λexc. = 378 nm / λem. = 425 nm) fluorescent decay inside the battery during discharge at constant current in 0.5 M LiTFSI in 1-Methoxy-2-(2-methoxyethoxy) ethane diglyme, followed by open circuit (OCP) relaxation after current switch off.The Figure on the right shows the thin layer electrochemical cell and the optical path for DMA excitation and fluoresce emission inside the Li-O2 battery.In similar experiments with the same battery and electrolyte containing DMA and physical quenchers of 1O2: Respectively, 10 mM NaN3, 10 mM triphenylamine (TPA), or 10 mM tri-ethylendiamine (DABCO), a constant DMA fluorescent signal has been observed. Thus, any 1O2 formed at the O2 cathode surface during battery operation was effectively quenched extending the battery life upon cycling.The new in operando detection of 1O2 inside a Li-O2 battery is non destructive nor is it perturbing the battery operation and allows to test new strategies to mitigate the spurious reactions due to singlet oxygen in non aqueous Li-O2 batteries, and to test new quenchers with better electrochemical stability.[1]. K. M. Abraham, Z. Jiang, J. Electrochem. Soc 1996, 143, 1–5.[2]. A.Y. Tesio, W. Torres, M. Villalba, F. Davia, M. del Pozo, D. Córdoba, F.J. Williams, E.J. Calvo, ChemElectroChem. 2022 in press.[3]. D. Córdoba, H.B. Rodríguez, E. J. Calvo, ChemistrySelect 2019, 4, 12304– 12307.